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Comparative structural analysis of inverted structures in the San Bernardo fold belt (Golfo San Jorge basin, Argentina): Inversion controls and tecto-sedimentary context of the Chubut Group
Journal Pre-proof Comparative structural analysis of inverted structures in the San Bernardo fold belt (Golfo San Jorge basin, Argentina): Inversion controls and tecto-sedimentary context of the Chubut group José Oscar Allard, Nicolás Foix, Sebastián Alberto Bueti, Federico Manuel Sánchez, María Leonor Ferreira, Mario Atencio PII:
S0895-9811(19)30414-6
DOI:
https://doi.org/10.1016/j.jsames.2019.102405
Reference:
SAMES 102405
To appear in:
Journal of South American Earth Sciences
Received Date: 7 August 2019 Revised Date:
28 October 2019
Accepted Date: 28 October 2019
Please cite this article as: Allard, José.Oscar., Foix, Nicolá., Bueti, Sebastiá.Alberto., Sánchez, F.M., Ferreira, Marí.Leonor., Atencio, M., Comparative structural analysis of inverted structures in the San Bernardo fold belt (Golfo San Jorge basin, Argentina): Inversion controls and tecto-sedimentary context of the Chubut group, Journal of South American Earth Sciences (2019), doi: https://doi.org/10.1016/ j.jsames.2019.102405. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
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COMPARATIVE STRUCTURAL ANALYSIS OF INVERTED STRUCTURES IN THE SAN
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BERNARDO FOLD BELT (GOLFO SAN JORGE BASIN, ARGENTINA): INVERSION
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CONTROLS AND TECTO-SEDIMENTARY CONTEXT OF THE CHUBUT GROUP
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José Oscar Allarda*, Nicolás Foixa,b, Sebastián Alberto Bueti a,b, Federico Manuel Sáncheza, María
6
Leonor Ferreiraa,c, Mario Atencioc
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a Universidad Nacional de la Patagonia San Juan Bosco, Ruta Nº 1 S/N, Km 4, 9005, Com. Riv., Chubut, Argentina b CONICET (Consejo Nacional de Investigaciones Científicas y Técnicas), Argentina c YPF S.A. *[email protected] (corresponding author)
12 13
ABSTRACT
14
The Golfo San Jorge Basin is one of the most important hydrocarbon-bearing basins of
15
Argentina. Its position in the distal Patagonian Broken Foreland confers a particular structural
16
architecture. Specifically, the western margin was affected by positive tectonic inversion, with
17
higher degrees of inversion at the San Bernardo Fold Belt (SBFB). This paper synthesizes the
18
comparative structural analysis of outcropped and buried inverted structures in the SBFB. The
19
study areas are regionally distributed, and the geometric characterizations integrate field
20
structural data, topographic trends, cutting records, borehole logs, and 2D-3D seismic data.
21
The geometries of the studied inversion folds are poorly vergent and can be classified as
22
asymmetric anticlines or pop-up growth folds. The architecture of the inverted fault system
23
shows: along-strike variability in reverse displacement, inherited extensional growth-strata,
24
selective inversion, oblique inversion, and heterogeneous deformation. Pre-inversion mayor
25
controls include: 1) an extensional network linked to the fabric of the pre-rift basement, 2) a
26
non-coaxial extensional phase during the early Upper Cretaceous times, and 3) a basin-scale
27
paleohigh of crystalline rocks located to the east of the SBFB. The kinematic analysis evaluates
28
the timing of the tectonic inversion phase, which is actually under discussion. Our results are
29
consistent with a tecto-sedimentary context dominated by normal fault reactivations during
30
the deposition of the basal units of the Chubut Group (Barremian-Albian), and a reverse-
31
reactivation during the record of the Laguna Palacios Formation (late Upper Cretaceous). The
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geometric-kinematic characterization of the inverted structures at the SBFB contradicts many
33
of the evidence that supports recently proposed geodynamic models for the Cretaceous
34
evolution of Southern Patagonia.
35
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Keywords: Patagonian Broken Foreland; Tectonic inversion; Inversion degree; Growth strata;
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Pop-up growth folds; 3D deformation
38 39
1. INTRODUCTION
40
Sedimentary basin research requires the integration of multiscale and multisource information
41
to obtain prosperous analysis and predictive models. In a particular way, the structural
42
framework is essential to evaluate the basin spatio-temporal evolution. Specifically, poly-phase
43
basins with positive tectonic inversion require a refined calibration in the kinematic evolution
44
of the principal structures. In the last decades, important advances have been made in the
45
understanding of the drivers and controls of positive tectonic inversion (Cooper and Williams,
46
1989; Buchanan and Buchanan, 1995; Bonini et al., 2012; Jackson et al., 2013; Reilly et al.,
47
2017; Sibson, 2017; Jagger and McClay, 2018). Nonetheless, there are still many mechanisms
48
that need more studies like selective inversion, oblique inversion, along-strike variability in the
49
deformation, and fluid migration in inverted structures. The Golfo San Jorge Basin (GSJB),
50
located in the Argentinean Patagonia, is a natural laboratory to study positive tectonic
51
inversion from outcrop and subsurface data. Specifically, the San Bernardo Fold Belt (SBFB) is a
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region of the GSJB characterized by outcropped and buried inverted structures. In this natural
53
system, the timing of the tectonic inversion is essential to evaluate trap generation as well as
54
the migration or remigration events. Despite the high-quality of the geologic information of
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the SBFB, there is no consensus about the tectonostratigraphic context of the Cretaceous oil-
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bearing sequences related to the lower Chubut Group (Aptian-Albian). Recent contributions
57
proposed contractional scenarios in the western domain of the GSJB (Gianni et al., 2015a,b,
58
2016, 2017, 2018a,b; Navarrete et al., 2015; Miller and Marino, 2019), while the traditional
59
vision is to link them to extensional-transtensional reactivation followed by thermal
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subsidence (Fitzgerald et al., 1990; Homovc et al., 1995; Peroni et al., 1995; Figari et al., 1999;
61
Rodriguez and Littke, 2001; Sylwan et al., 2011). As it is seen, both proposals are antagonistic
62
and each one has a different impact on every element of the petroleum system, so calibration
63
is required. The general aim of this work is to test the main evidence that supports the
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tectonostratigraphic contexts during the Lower to Upper Cretaceous at the SBFB. In order to
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validate one of the models, we characterized five of the most important anticlines by the
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integration of outcrop and/or subsurface information: Sierra Nevada, Sierra del Castillo,
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Península Baya, Sierra Silva, and Los Perales. Complementary, two key basin positions were
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studied: the northwestern boundary at Ferrarotti locality and the southern boundary at Sur Río
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Deseado depocenter (Fig. 1). The architecture variability of the inverted structures was used to
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address different structural controls and mechanisms poorly understood as: the influence of
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pre-inversion structural anisotropies, oblique inversion, selective inversion, heterogeneous
72
deformation, and basin-scale buttress effect. The calibration of the Cretaceous inversion phase
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in the SBFB impacts in the oil industry models and the geosciences studies in Patagonia, while
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the structural models are widely applicable to worldwide similar tectonostratigraphic
75
scenarios.
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2. GEOLOGICAL SETTING
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The GSJB is located in the center of the Argentinean Patagonia between 45° and 47°S latitude,
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and 65° and 71°W longitude, it has an extension about 180.000 km2 with one-third located
80
offshore (Fig. 1) (Rodriguez and Littke, 2001; Sylwan et al., 2011). The basin has a west-east
81
elongate geometry, their borders are to the north the North-Patagonian Region and Cañadón
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Asfalto Basin (Figari et al., 2015), and to the south the Deseado Region (Giacosa et al., 2010).
83
The western border is the South Patagonian Andes or the Patagonian Precordillera, this limit is
84
poorly-constrained and depends on the age of the sedimentary sequences and the
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depocenters considered (Fitzgerald et al., 1990; Figari et al., 2015, Miller and Marino, 2019).
86
The eastern basin limit is defined by subsurface information under the South American Atlantic
87
margin (Sylwan et al., 2011). The basin is divided into five different regions according to the
88
present structural style. The North Flank, South Flank, and Basin Center are characterized by
89
normal faults with E-W strike, where each region is dominated by dipping directions basinward
90
(Fig. 1) (Figari et al., 1999; Sylwan et al., 2011). The SBFB and Western Flank are identified by
91
positive tectonic inversion features. The former characterized by the domain of contractional
92
structures N-S oriented, and the latter by structures with NNW-SSE strikes with a lower degree
93
of inversion (Fitzgerald et al., 1990; Figari et al., 1996, 1999; Sylwan et al., 2011).
94
95 96
Figure 1. Geological map of the Golfo San Jorge Basin (GSJB) showing the studied areas at the San
97
Bernardo Fold Belt (SBFB). Surface units modified from SEGEMAR maps. Structural domains and
98
structural profiles after Figari et al. (1999) and Sylwan et al. (2011). Rose diagram shows faults strikes of
99
the SBFB classified in inverted and non-inverted faults. Data to the top of Castillo Formation, taken from
100
Figari et al. (1999). Note that bimodal distribution correlates with the structural classification.
101 102
2.1 Stratigraphy
103
The structural basement units of the GSJB are exposed in relatively small areas, their outcrops
104
are geographically distant, and its lithology exhibit a wide variety of crystalline rocks and
105
sedimentary sequences (Fig. 1). The crystalline substrate is constituted by Paleozoic to Triassic
106
intrusive and metamorphic rocks, outcropped examples of this suites are Puesto La Potranca
107
Formation and the Sierra Mora granite at the northern boundary (Giacosa, this issue).
108
Overlaying this basement are the Neopaleozoic rocks of the Tepuel Group associated to
109
marine tillites and coastal deposits (Feruglio, 1949; Ugarte, 1966; Fernández Garrasino, 1977),
110
which are affected by sparse volcanic and subvolcanic rocks of Permo-Triassic age (e.g.
111
Maliqueo Formation, Fernández Garrasino, 1977). The Liassic record continues with volcanic to
112
volcano-sedimentary units linked to continental paleoenvironments (e.g. El Córdoba
113
Formation) or sedimentary sequences dominated by shallow marine deposits with a tuff
114
record (e.g. Osta Arena Formation) (Robbiano, 1971; Fernández Garrasino, 1977; Suárez and
115
Márquez, 2007, and references therein).
116
Jurassic volcanic and volcaniclastic successions were developed overlying the structurally
117
deformed basement. These rocks are grouped into several widely accepted lithostratigraphic
118
units according to the volcanic composition, age and the geographic location, so can be
119
defined the Lonco Trapial Group and Marifil Volcanic Complex in the northern limits, whereas
120
in the southern limits the Bahía Laura Volcanic Complex (Pankhurst et al., 1998, and references
121
therein). Clavijo (1986) proposed an informal unit named Complejo Volcánico Sedimentario
122
(CVS), in which are grouped undetermined post-Liassic volcanic and volcaniclastic rocks
123
underlying the Cretaceous sedimentary record (Fig. 2). The lack of hydrocarbon accumulations
124
in those units define them as an economic basement for the oil industry (Peroni et al., 1995;
125
Rodriguez and Littke, 2001) (Fig. 2). Continues two groups that include the most important
126
units of the GSJB petroleum systems. The lower is informally known as Neocomiano and
127
formally as Las Heras Group (Lesta et al., 1980). The basal unit of this group is Pozo Anticlinal
128
Aguada Bandera Formation, which is dominated by black-shales. This unit pass in transition to
129
Pozo Cerro Guadal Formation composed by siltstones and sandstones, linked to marine and
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transitional paleoenvironments to the west and lacustrine ones to the east (Clavijo, 1986;
131
Figari et al., 1999; Miller and Marino, 2019). Both units are restricted to the subsurface, except
132
for limited outcrops at Puesto Albornoz area at the northwestern bound (Figari et al., 2015).
133
According to Clavijo (1986) and Allard et al. (2018a) these sedimentary sequences are
134
characterized by the absence of a volcanic tuff record. In disagreement, Miller and Marino
135
(2019) mentioned volcanic ashes at the base of Anticlinal Aguada Bandera Formation, but their
136
paper did not show evidence of that sedimentary record. The thicker deposits of these units
137
reach 5400 m at the Sur Río Deseado depocenter, which is located at the southern bound of
138
the GSJB (Fitzgerald et al., 1990). Clavijo (1986) interpreted this Cretaceous interval as a
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sedimentary cycle limited by regional unconformities, and later it was defined as a
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megasequence by Figari et al. (1999). Nevertheless, this regional scheme is not validated by
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Lower Cretaceous concordant seismosequences from the South Flank (Sylwan et al., 2011;
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Paredes et al., 2018) and the southern region of the SBFB (this work). The stratigraphic column
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continues with the Chubut Group defined from outcrop and subsurface information (Lesta and
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Ferello, 1972; Figari et al., 1999; Sylwan et al., 2011), with a maximum thickness estimated in
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8000 m at the Basin Center (Fitzgerald et al., 1990; Rodriguez and Littke, 2001). The oldest
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units of this group are Pozo D-129 (Lesta, 1968) and Matasiete formations (Lesta and Ferello,
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1972). The Pozo D-129 Formation is associated with an underfilled lake that accumulated the
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black-shales that are the main source rock of the basin, while the Matasiete Formation and Los
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Alazanes Member (sensu Pezzi and Medori, 1972) are the contemporaneous fluvial-alluvial
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systems that fed that paleolake (Sciutto, 1981; Paredes et al., 2007; Sylwan et al., 2011; Allard
151
et al., 2015). The Pozo D-129 Formation is constrained by microfossils to Aptian age at its
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younger sequences in the SBFB (Hechem et al., 1987). A complex puzzle of fluvial and alluvial
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deposits continues to develop in an endorheic and fluvial dominated basin (sensu Nichols,
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2012), where the proportions of tuffaceous materials respect to siliciclastic ones allow
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discriminating four units in outcrops of the SBFB (Feruglio, 1949; Lesta and Ferello, 1972):
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Castillo Formation (Albian), Bajo Barreal Formation (Coniacian-Turonian), Lago Colhué Huapi
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Formation (Turonian-Maastrichtian) (Casal et al., 2015) and Laguna Palacios Formation
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(Santonian-Maastrichtian) (Sciutto, 1981; Bridge et al., 2000). The latter is restricted to the
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western domain of the basin and it lies in angular unconformity over previous deposits
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(Sciutto, 1981; Gianni et al., 2015a). The Cenozoic record overlies with a maximum thickness of
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1000 to 1500 m at the Basin Center, represented by the intercalation of marine and
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continental sequences and multiple phases of basic intrusions and volcanic activities (Legarreta
163
and Uliana, 1994; Rodriguez and Littke, 2001; Bruni et al. 2008; Sylwan et al., 2011).
164 165
2.2. Structural and tectonic framework
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The GSJB developed over a complex and polydeformed structural basement which tectonic
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fabric and deformation events are poorly understood. Recent attempts were made by
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Navarrete et al. (2016), who described a Lower Jurassic intraplate compressional phase at the
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SBFB and Río Mayo sub-basin. Beyond the basement history, there is a general agreement to
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consider the GSJB as an intracratonic basin linked to the Gondwana break-up (Fitzgerald et al.,
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1990; Figari et al., 1999; Sylwan et al., 2011). The long lifetime of subsidence of this basin
172
responded to different tectonostratigraphic contexts. Jurassic rocks of the CVS record the early
173
phase and climax of the syn-rift (Pankhurst et al., 1998), while Neocomian fault-bounded
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basins are proposed as a posthumous phase of extension or a late syn-rift (Fitzgerald et al.,
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1990; Figari et al., 1999; Rodriguez and Littke, 2001; Sylwan et al., 2011). Recently, the
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Neocomian cycle was divided into two subcycles, the lower Anticlinal Aguada Bandera subcycle
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related to a syn-rift stage and the upper Cerro Guadal subcycle to the post-rift stage (Miller
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and Marino, 2019). The paleogeographic reconstruction defined discrete or partially connected
179
Neocomian depocenters with a better development toward the Western Flank and SBFB
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(Fitzgeral et al., 1990; Figari et al., 1995, 1999; Miller and Marino, 2019). The distribution of
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these kinematically linked depocenters shows an NNW-SSE trend that suggested a crystalline
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basement control by Clavijo (1986), Figari et al. (1999), Ramos (2015) and Paredes et al. (2018),
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but to the moment there is no clear evidence that sustains this hypothesis (Giacosa, this issue).
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During post-Barremian ages the subsidence migrated eastward, where thermal subsidence was
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the main control that developed the oval-shaped basin-fill that characterizes the GSJB (Clavijo,
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1986; Figari et al., 1999). Recently, Dávila et al. (2019) invoked dynamic subsidence as a basin-
187
scale control during Mesozoic and Cenozoic record. The western margin of the Cretaceous
188
GSJB has a controversial structural scenario. The architecture of the SBFB exposes well-
189
developed positive tectonic inversion folds, but the timing of the contractional phases is under
190
discussion. The different proposals are based on the origin of the basal sequences of the
191
Chubut Group (Pozo D-129, Matasiete, and Castillo formations), which determine two
192
incompatible tectonostratigraphic contexts. Traditional schemes consider a extensional
193
reactivation to transtensional setting or the initiation of a thermal phase (Fitzgerald et al.,
194
1990; Homovc et al., 1995; Peroni et al., 1995; Chelotti and Homovc, 1998; Figari et al., 1996,
195
1999; Rodriguez and Littke, 2001; Sylwan et al., 2011); while recent proposals take back the
196
models of Barcat et al. (1989) and Folguera and Iannzzotto (2004) and introduced a syn-
197
contractional context linked to a foreland or a broken foreland setting (Folguera and Ramos,
198
2011; Navarrete et al., 2015; Ramos, 2015; Gianni et al., 2015a,b, 2018a,b). In these scenarios,
199
the tectonic inversion proposal with synchronous extension was interpreted as a syn-orogenic
200
foreland rift by Gianni et al. (2015a,b). Contemporaneously, tectonostratigraphic studies
201
criticized specific surface and subsurface evidence of the syn-inversion scenarios (Paredes et
202
al., 2016; Bueti et al., 2017; Allard et al., 2018b). However, regional models used the
203
contractional proposals as evidence for the construction of geodynamic models for the GSJB
204
(Gianni et al., 2018a,b; Dávila et al., 2019; Miller and Marino, 2019) and neighboring regions
205
and basins (Echaurren et al., 2016; Sevignano et al., 2016; Gianni et al., 2018a,b; Horton, 2018;
206
Dávila et al., 2019). Beyond these discussions, there is consensus that the fold belt was built
207
from a polyphase tectonic inversion with specific events that have different ages, comprising
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later Upper Cretaceous (Casal et al., 2015; Gianni et al., 2015a,b; Bueti et al., 2017), Paleocene
209
(Paredes et al., 2006; Gianni et al., 2017), Miocene (Peroni et al., 1995; Rodriguez and Littke,
210
2001; Giacosa and Paredes, 2008, among others) and Quaternary (Gianni et al., 2017) phases.
211
212 213
Figure 2. Tectonostratigraphic chart of the San Bernardo Fold Belt region (Golfo San Jorge Basin). The
214
pyroclastic-volcaniclastic input is included.
215 216
3. MATERIALS AND METHODS
217
The study done was based on meso- and macroscale structural data obtained from outcrop
218
and subsurface. Base hillshade maps were constructed from SRTM dataset at 1 arc-second
219
horizontal resolution (~30 m). The topography of the outcropped anticlines was characterized
220
from DEMs using swath profiles (Telbisz et al., 2013 and references therein). Fieldwork
221
consisted of geological mapping at scales of 1:10000 to 1:1000, focused on structural
222
relationships and stratigraphic architecture. Structural measurements and strata orientation
223
were done with traditional Brunton compass and electronic clinometer, so possible errors
224
were minimized (Novakova and Pavlis, 2017). Strata attitude used in maps and sections follows
225
the convention Dip direction/Dip. The structural data were analyzed using the unlicensed
226
academic version of Rod Holcombe software GEOrient 9.5.0. Geometric and kinematic analysis
227
of seismosequences were done using an asymmetric index, which compares the thickness of
228
the analyzed interval at different positions of the structure (Mitra, 1993; Groshong, 2006;
229
Jackson et al., 2013; Allmendinger, 2015). The seismosequences were calibrated with
230
lithological regional markers redefined from the maximum detail that allowed the oil-well
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cutting record. Calibrations of formation tops proposed in well reports may have implied
232
modifications of tens to hundreds of meters (see details in Allard et al., 2018a).
233 234
4. RESULTS
235
The main inversion structures of the SBFB were previously synthesized in regional structural
236
maps like the ones in Barcat et al. (1984), Stach (1986), Fitzgerald et al. (1990), Homovc et al.
237
(1995), Peroni et al. (1995), Figari et al. (1999) and Sylwan et al. (2011). The bimodal
238
distribution of strikes of map-scale inversion folds correlates with the inverted o non-inverted
239
faults (Fig. 1), but these trends do not include the geometric characteristics and the timing of
240
their construction. The following results synthesize the mayor contributions to cover some of
241
those deficiencies. The study areas are described taking into account their geographical
242
distribution, starting at the northwestern boundary of the SBFB at the Ferrarotti area and
243
finishing at the southern boundary at the Sur Río Deseado depocenter (Fig. 1).
244
Complementary, we realized a short revision of the economic basement fabric.
245 246
4.1. Ferrarotti locality
247
The west boundary of the GSJB is poorly known, and geologic maps refer to Fernández
248
Garrasino (1977) or Stach (1986). Ferrarotti locality has the westernmost exposures of the
249
economic basement and the basal sequences of the Chubut Group in the domain of the GSJB,
250
hence these outcrops are the closest to the Andean subduction margin (Fig. 1). Recently
251
paleogeographic reconstructions defined there the northwestern margin of the Cretaceous
252
record of the GSJB (Allard et al., 2015; Figari et al., 2015). These characteristics confer to the
253
zone a key-architecture for the evaluation of the tectonostratigraphic context of the lower
254
Chubut Group levels associated with Matasiete and Pozo D-129 formations (Allard et al.,
255
2015).
256
Structural architecture. The economic basement includes Neopaleozoic rocks of the Tepuel
257
Group (Fig. 3.A), subvolcanic rocks of Maliqueo Formation, volcanic and volcaniclastic units of
258
the CVS (Fig. 3.B), and Liassic sedimentary sequences (Fig. 3.C). The Chubut Group is
259
represented by fluvial deposits with laterally-continuous green tuff levels used as marker beds
260
(Figs. 3.D,E). The structural architecture of the basement outcrops shows a folded angular
261
unconformity between the Tepuel Group and the Liassic (Fig. 3.E), and folds of different scales
262
with axis strikes with a general trend N-S, tens to hundreds of meters of amplitude and open
263
to tight geometries that occasionally reach limbs with subvertical dips (Figs. 3.C,E,F).
264
Specifically, sandstones and limestones of Liassic sequences show the smaller-scale fold trains,
265
characterized by high frequency harmonic to disharmonic geometries (Fig. 3.C). The Chubut
266
Group levels outcrop to the east of the basement outcrops defining an open syncline, gently
267
dipping toward the south (Figs. 3.D,E,F), measurements close to the basement units increase
268
the dip angles up to 30° (Figs. 3.F, 4.A).
269
270 271
Figure 3. Structural characterization of the Chubut Group and its economic basement. A, B) Southward
272
dipping anticlines developed in Tepuel Group, CVS, and Liassic deposits. C) High frequency folding in
273
Liassic levels. D) Western limb of a gentle syncline developed in Chubut Group sequences. E) Field
274
examples of the Tepuel Group-Liassic folded unconformity, Liassic fold limb with 90° of inclination, and a
275
rotated fluvial channel of the Chubut Group F) Equal-angle plot of strata attitudes (So) measured at
276
Ferrarotti study area. Note the higher deformation of the economic basement respect to the Chubut
277
Group sequences.
278 279
The subsurface relationships were studied from a paper-printed 2D seismic profile, and the
280
main features of the architecture were identified. The southward plunging syncline affecting
281
the Chubut Group levels at surface correlates with a synform pattern of strong reflectivity
282
markers (Figs. 4.A,B). These seismosequences exhibit a wedge pattern limited to the east by a
283
west-dipping fault. Their seismic reflectors at the flexural margin diverge toward the master
284
fault, proving the syn-kinematic clockwise-rotation linked to the fault activity. The strata
285
attitude of outcropped tuff levels of the Chubut Group shows an unconformity of 14° and a
286
divergence toward the east (Fig. 4.C), in correlation with the seismic wedge. The projection of
287
the basement outcrops to subsurface correlates with a wide seismic antiform suggesting a
288
first-order fold. The N-S fold system recognized in outcropped Liassic levels was used to
289
integrate outcrop and seismic data (Figs. 4.A,B). According to Pumpelly's rule, the style and
290
attitude of higher-order folds are similar to lower-order folds (Twiss and Moore, 2007; Frehner
291
and Schmid, 2016). The subsurface antiform also shows a basal unconformity with east-dipping
292
subparallel seismofacies. The latter correlates with the Tepuel Group deposits outcropped at
293
the Ferrarotti lagoon (Fig. 4.A).
294
Interpretation. The structural architecture of the lower Chubut Group is interpreted as a
295
traditional half-graben geometry (Leeder and Gawthorpe, 1987; Prosser, 1993; Williams, 1993;
296
Leeder, 2011). Furthermore, the tectonic control of a basin-margin normal fault is indirectly
297
sustained by high-accommodation fluvial sequences and axial paleocurrents (Allard et al.,
298
2015). The outcrops of the pre-Chubut Group units are associated with a paleohigh that
299
represents a large structure. Hence, the folds affecting the Liassic levels were interpreted as
300
second and third-order folds of a regional-scale feature. The paleohigh architecture was
301
reconstructed as a complex structure where high-frequency folds are associated with
302
subseismic parasitic folds. The paleohigh does not show any record of the Chubut Group, in
303
the case it was eroded, the well-developed neighboring Cretaceous depocenter can be
304
considered as a basinward migration of border-fault (Dart et al., 1995). The unconformity
305
between the economic basement and the Cretaceous sequences, and the syncline that affect
306
the Cretaceous extensional wedge defined at least three tectonic phases: 1) contraction pre-
307
Chubut Group, 2) extension syn-Chubut Group, and 3) contraction post-Chubut Group.
308
Considering the folds are laterally restricted with faults, the contractional phases were
309
interpreted as positive tectonic inversion.
310
311 312
Figure 4. Structural architecture of westernmost outcrops of the Chubut Group and its economic
313
basement at the Ferrarotti study area. A) Simplified geological/structural map showing the economic
314
basement paleohigh, which is linked to a northwest-dipping fault. Rose diagram shows axis strikes of
315
high-order folds. B) Tectonostratigraphic interpretation of a 2D seismic line. C) West limb of the syncline
316
showing east-dipping Cretaceous tuff levels. Correlation with the divergent seismic reflectors evidences
317
extensional progressive unconformities.
318 319
4.2. Sierra Nevada anticline
320
One of the largest outcropped inversion folds in the SBFB is the Sierra Nevada anticline. Its
321
height reaches 1000 m a.s.l. and it is surrounded by Eocene-Miocene basaltic plains (Bruni et
322
al. 2008). The anticline shows an erosional window that exposes almost all the Chubut Group
323
units (Sciutto and Martínez, 1996) (Figs. 5.A,B). We accessed the unmigrated paper-printed
324
version of a seismic profile across the morphostructure (Fig. 5.E). Beyond the limitations of this
325
kind of data (Herron, 2012; Jackson and Kane, 2012), the main seismic packages and structural
326
elements were perfectly identified.
327
Stratigraphic architecture. We used the exploration well YPF.Ch.SN.es-1(I) for the calibration of
328
1D subsurface architecture (Figs. 5.B,C). This well is located over Matasiete Formation levels
329
and it has a final depth of 4364 m. Upper levels are dominated by reddish mudstones, while
330
tuffaceous siltstones and tuffs cutting records were frequently identified between 521 m and
331
1200 m depth (Figs. 5.D), so the upper sequence was assigned to the basal record of the
332
Chubut Group. Black-shales were recognized in the interval 1400 m to 1641 m depth, but we
333
could not determine if they belong to Pozo D-129 or Pozo Anticlinal Aguada Bandera
334
formations. The oil-well report mentions Cyclusphaera palinozone at 2588 m depth and
335
interpreted an Early Cretaceous age (Hauterivian-Barremian), thus we inferred Neocomian
336
levels. Sequences attributed to Matasiete and Pozo D-129 formations show funnel
337
electrofacies tens of meter-thick (Fig. 5.C), which were interpreted as upward-coarsening
338
deposits cycles (Miall, 2016).
339
Using the velocity law from a well located southward, we considered an approximated one-
340
way travel time of ~2.3 km/sec in shallow sedimentary levels and ~3.2 km/sec in levels at 2300
341
m depth. According to these relationships, we estimated the lithostratigraphic units related to
342
the main seismosequences, the fold crest until to ~0.5 sec (TWT) belongs to Matasiete
343
Formation, while the asymmetric wedge until ~1.5 sec (TWT) belongs to Pozo D-129 Formation
344
and Neocomian sequences (Figs. 5.E,F). Underlying levels are dominated by intermediate to
345
basic igneous rocks, which were assigned to the economic basement (CVS) and Cenozoic
346
intrusive rocks. Regional correlation with northern outcrops of Sierra del Cerro Negro
347
supported a volcanic-volcaniclastic economic basement similar to Lonco Trapial Group (Fig. 1)
348
(Robbiano, 1971).
349
Structural architecture. Structural dips obtained from dipmeter-log data show values under
350
30°, the first 700 m depth have tens of meter intervals with increasing-decreasing dip cycles,
351
while the strata attitude between 700 and 1400 m depth is characterized by hundreds of
352
meters of uniform dips locally interrupted by anomalous trends (Fig. 5.C). At a seismic scale,
353
the architecture exposes a well-developed asymmetric fold that correlates with the surface
354
morphostructure (Figs. 5.A,B,F). The west boundary of this fold is marked by an abrupt change
355
in the seismofacies, interpreted as the Sierra Nevada main fault which is west-vergent. The
356
Lower Cretaceous asymmetric wedge has a general geometry that increases the thickness up
357
to 345 % in about 25 km (T1/T10: 3.45, Fig. 5.G). The backlimb of the Sierra Nevada anticline is
358
interrupted by a fault approximately 13 km to the east of Sierra Nevada fault (Fault "A" in Fig.
359
5.F). This is a blind structure with an upthrow to the east and downthrow to the west. The
360
spatial correlation of seismosequences defined the fault-dip is to the west, so a normal fault
361
was reconstructed (Fig. 5.H). The overlying Castillo Formation shows a faulted monocline with
362
synthetic dipping reflectors on the hanging-wall. The Lower Cretaceous seismic wedge
363
continues to the east of Fault "A" and develops a flexure over the tip of east-dipping normal
364
fault (Fault "B" in Fig. 5.F), and the basal sequences are cut by another east-dipping normal
365
fault (Fault "C" in Fig. 5.F). The northeastern extreme of the seismic profile shows lobate
366
seismofacies (Fig. 5.I), which is cut in an N-S direction by SL PBP 94-101 showing clinoforms
367
with southward down-lap stratification (Fig. 5.I).
368
Interpretation. The subsurface tectonostratigraphic architecture of Lower Cretaceous deposits
369
supports a syn-extensional wedge linked to Sierra Nevada fault, locally distort by normal faults
370
without evidence of positive tectonic inversion. The faulted monocline associated with Fault
371
"A" was interpreted as the evolution of an extensional fault-related fold (Khalil and McClay,
372
2002; Wilson et al., 2009a; Coleman et al., 2019a,b; Deng and McClay, 2019). The lobate
373
geoform is associated with transitional sedimentary sequences (Prosser, 1993; Leeder, 2011)
374
and suggests the record of an axial sedimentary system located in the flexural margin of the
375
Sierra Nevada fault. According to Allard et al. (2015) and Figari and Garcia (2018), Sierra
376
Nevada structure conformed a regional sedimentary route for the Matasiete-Pozo D-129
377
sedimentary system, so extensional axial systems correlates with that paleogeographic
378
scenario.
379
380 381
Figure 5. A) Northwestern region of the SBFB indicating the location of 2D seismic profiles. B) Simplified
382
structural map of northern Sierra Nevada anticline. C) Induction log and Dipmeter clusters. D) Cutting
383
record of Matasiete Formation showing the intercalation of green tuff deposits and reddish-mudstones.
384
Numbers indicate depth in meters. E,F) Uninterpreted and interpreted seismic profile SL PBP 94-100
385
across the Sierra Nevada anticline. G) Asymmetry indexes of cretaceous seismosequences. H) West
386
dipping fault zone interpreted from the correlation of high-impedance seismofacies. I) Lobate
387
seismofacies architecture. Arrows indicate the apparent dip direction of the seismosequences. TWT:
388
Two Way Time.
389 390
4.3. Sierra del Castillo anticline
391
The Sierra del Castillo anticline is the southern extension of the Sierra Nevada anticline (Fig.
392
6.A). It has an NNE-SSW strike and a height trend that abruptly decreases southward losing 500
393
m in 12 km, unlike Sierra Nevada range, which has an NNW-SSE strike and a subtle height
394
trend (Fig. 6.B). Sierra del Castillo range is a traditional study area of the SBFB because it has
395
excellent outcrops of the basal sequences of the Chubut Group at the Matasiete canyon (e.g.
396
Paredes et al., 2007). The structural relationships in this area have been studied since the early
397
exploration of the SBFB. Feruglio (1949) interpreted a non-faulted fold, while later studies
398
done by Stach (1986) and Paredes (2009) proposed a faulted fold. Recently, this classical
399
structural section was reinterpreted by Gianni et al. (2015a), who proposed growth-strata and
400
progressive unconformities in Cretaceous sedimentary sequences located in the forelimb. We
401
made a geometric analysis of these exposures and a new geological cross-section is proposed
402
(Fig. 6.C).
403
Structural architecture. The hanging-wall architecture shows a wide and asymmetric anticline
404
that affects the Matasiete Formation and the forelimb has overturned beddings of the lower
405
Castillo Formation (Fig. 6.D). These levels were interpreted as a high-order anticline that
406
refolded the abrupt limb of the inversion fold. Its spatial restriction to positions close to the
407
major fault and the vergence to the west suggest a normal drag fold (Grasemann et al., 2005)
408
linked to a transported fault-propagation fold (McClay, 2011). The footwall architecture has
409
major differences from previous studies; our description shows a forelimb faulted by at least
410
two retro-vergent thrusts and one reverse fault, giving a discontinuous stratigraphic column
411
(Figs. 6.C,E,F,G).
412
The southern extreme of the Sierra del Castillo range is defined by the trace of the emerge
413
Matasiete fault to the west, and by an homocline linked to the backlimb of the inversion fold
414
to the east. This structural scenario is abruptly interrupted near the Matasiete fault-tip by a
415
transverse east-plunging anticline (Fig. 6.A, 7.A). This fold is much smaller than the Sierra del
416
Castillo anticline, it is 1.5 km long, the mayor axis strike is 113°, the abrupt flank dip to the
417
southwest (So: 200°/38°), the gentle one to the northeast (So: 30°/16°) and the interlimb angle
418
is 54°.
419
Stratigraphic architecture. For our study, one of the most significant contributions is the
420
rotation of lagoons deposits contained in the paleosoils sequences of Laguna Palacios
421
Formation (Fig. 6.G). Fine tuffaceous conglomerates were identified in massive to poorly
422
stratified levels of Laguna Palacios Formation. (Fig. 6.G), which were interpreted as
423
cannibalization deposits linked to the tectonic uplift. Fossil wood fragments were exclusively
424
found in deposits above the Cenozoic basal contact (Fig. 6.F,G). This unconformity was linked
425
to the tectonic exhumation of the fossil wood rich levels of Matasiete Formation identified in
426
the Sierra del Castillo anticline by Feruglio (1949).
427
428 429
Figure 6. A) Simplified structural map of the Sierra Nevada-Matasiete fault system. Note the change in
430
the strike of the morphostructure. The satellite image shows a detail of the study area. B) Swath profile
431
showing along-strike maximum heights linked to the uplift trend of the morphostructure. C) Simplified
432
structural section transversal to Matasiete fault and Sierra del Castillo anticline. Stratigraphic thickness
433
of Matasiete and Castillo formations sensu Paredes et al. (2007) and Paredes et al. (2015), respectively.
434
D) Castillo Formation overturned beddings interpreted from fluvial architecture elements. Ch: minor
435
channel, St: trough cross-bedded sandstone, Gt/Gp: trough to planar cross-bedded conglomerate. E)
436
Architecture of the footwall of the Matasiete fault and the forelimb of Sierra del Castillo anticline. F)
437
Reverse fault (F4) cut Laguna Palacios Formation repeating its sequences. Note the progressive
438
unconformities limited by the reverse fault. G) Detail photos, color dots show the location in figure 6.F.
439
Symmetrical ripples indicate horizontal deposition linked to lagoon deposits. Tuffaceous conglomerates
440
were associated with the cannibalization of Castillo and/or Bajo Barreal formations. Fossil wood
441
fragments were interpreted as the unroofing of Matasiete Formation. Reverse fault is a discrete
442
discontinuity without a damage zone.
443 444
Interpretation. The Sierra del Castillo anticline is associated to the Matasiete fault, which
445
correlates northward with the Sierra Nevada fault. Assuming an analogous structural
446
architecture in the subsurface, the Sierra del Castillo anticline is interpreted as an inversion
447
fold of a normal fault that controlled Neocomian sequences and basal deposits of the Pozo D-
448
129-Matasiete sedimentary system. The lagoon deposits in Laguna Palacios Fm. are an
449
unquestionable horizontal marker, so their rotation supports the development of progressive
450
unconformities at the forelimb of the Sierra del Castillo anticline. The reverse fault repeats
451
some of the marker beds of Laguna Palacios Formation, so the progressive unconformities
452
were spatially restricted to the basal outcrops of that unit (Fig. 6.F). The tuffaceous
453
conglomerates and the fossil woods support unroofing sequences linked to the tectonic
454
inversion (Colombo, 1994).
455
The anticline located at the southern extreme of the range has a geometry that suggests a
456
fault-related fold linked to a northeast-dipping blind fault, confirmed by the subsurface
457
architecture in seismic line YBP95-01 (Fig. 7.B). The seismic cross-section shows the fault
458
responsible for the WNW-ESE fold has a similar dip to Matasiete fault, and the interaction
459
between these structures is a branch point. In this context, the map-view topological analysis
460
suggested the high angle intersection is an abutting fault pattern classified as Ya-node (sensu
461
Morley and Nixon, 2016). This fault was correlated with a lineament that cross-cut the
462
forelimb of Península Baya anticline (see next section).
463
464 465
Figure 7. A) Southernmost extreme of Sierra del Castillo anticline. The transverse anticline was linked to
466
the inversion of an E-W normal fault. B) Interpreted seismic profile across southern Sierra del Castillo
467
and Península Baya faults. Note the abutting W-E fault near to Matasiete fault tip. C) Swath profile
468
showing the maximum heights of Península Baya range. Note the anomalous height domain.
469 470
4.4. Península Baya anticline
471
The northwestern extreme of the Musters lake is partially compartmentalized by the Península
472
Baya anticline (Fig. 7.A). This fold is a broad morphostructure 16 km long and ~3 to ~4 km
473
wide, with low-sinuosity limits (Figs. 7.A,C, 8.A).
474
Structural architecture. The northern extreme is partially covered with Quaternary deposits,
475
but outcropped levels show the structural closure with an interference map-pattern with the
476
backlimb of the Sierra del Castillo anticline (Fig. 7.A). The southern extreme of the range is
477
completely different as it shows two straight E-W lineaments, the southernmost of them
478
determines the abrupt end of the outcropped anticline (Fig. 7.A). The anticline is composed of
479
tuffaceous levels of the Castillo Formation, and the Bajo Barreal Formation outcrops are sparse
480
(Fig. 7.A) (Stach, 1986). The central region has an asymmetric structural profile with an abrupt
481
limb to the west (dip range: 50°-80°) and a gentle one to the east (dip range: 15°-7°). The
482
forelimb is composed of concordant tuffaceous fluvial levels, measurements in tabular
483
lithofacies association show a mean So: 246°/66° (N: 12) (Figs. 8.B,C). Higher dip values up to
484
80° or overturned beddings were measured near a shortcut thrust (Figs. 8.B,C,F). A small
485
erosional-window in the forelimb exposes the Matasiete Formation overlaying the Castillo
486
Formation levels through a sharp surface (Figs. 8.C,D). This limit passes laterally to a fractured
487
and silicified tuffaceous level of the Castillo Formation (Figs. 8.D,E,F). The high resistance of
488
this level preserves the fault architecture showing a polished and striated plane with an
489
attitude 25°/50° and striations with rake -78° (Fig. 8.E), so it was classified as dip-slip
490
dominated reverse fault. The western projection of the reverse fault correlates with a west-
491
vergent anticline while the eastern extreme with a high-order symmetric anticline (Figs. 8.B,C).
492
The W-E abutting fault interpreted at the southern zone of Sierra del Castillo anticline
493
correlates with a lineament that distorts the Península Baya forelimb and cuts the trace of the
494
inferred master fault (Figs. 7.A, 8.A,B). The northern structural block of this element is the
495
upthrown side and it has associated sparse outcrops of Bajo Barreal Formation (Figs. 7.A, 8.B).
496
The subsurface architecture of Península Baya anticline is shown in the seismic profile of the
497
figure 7.B. We used the general seismic patterns to evaluate the tectonic inversion
498
architecture and the history of the previous extensional fault. Firstly, the backlimb has a
499
general concordant pattern up to 1100 msec depth. Secondly, the seismosequences C and D
500
are restricted toward the east of Península Baya fault. Specifically, these seismosequences
501
were attributed to the Matasiete-Pozo D-129 sedimentary system. This inference is based on
502
that Península Baya outcrops include the top of the Matasiete Formation (Fig. 8.C), and the
503
neighbor Sierra del Castillo anticline exposes a 650 m thick record of this unit (Fig. 6.C).
504
Interpretation. The Península Baya anticline is linked to a blind inverted fault parallel to the
505
western flank dipping to the east, which was corroborated in the seismic line YBP95-01 (Figs.
506
7.A,B). The non-andersonian dip-angle of the shortcut thrust was interpreted as a post-failure
507
rotation associated with the forelimb evolution (Coward, 1996; Jagger and McClay, 2018). This
508
hypothesis considers a neoformatted structure developed during the inversion fold
509
construction, where the shortcut fault accommodated the contraction when the main
510
inversion fault was totally or partially blocked.
511
Assuming the W-E lineament is the same structure identified at the Sierra del Castillo fault tip,
512
the local fault network has a double connected branch so it can be defined as a C-C branch
513
between a Ya-node and X-node (Sanderson and Nixon, 2015; Morley and Nixon, 2016; Peacock
514
et al., 2016; Duffy et al., 2017). Beyond this topological description, both interaction nodes
515
show structural trends. In this sense, the lineament matches with a height anomaly of the
516
Península Baya range, where the maximum height values are up to 80 m higher respect to
517
neighboring values (Fig. 7.C). Local increase of deformation is evidenced from the reverse fault,
518
a west-vergent fold, and the increase in the structural relief, suggesting a localized higher
519
degree of tectonic inversion (see Discussion).
520
The absence of seismosequences C and D toward the west of the Península Baya fault
521
evidence an important tectonic control during that interval, where only the hanging-wall
522
preserves the sedimentary record. The concordant pattern between seismosequences A and B
523
was interpreted as a homogeneous hanging-wall subsidence and subordinate to absence syn-
524
sedimentary tectonic rotation. The mentioned evidence would suggest the present fault-fold
525
geometry responds to the reverse-reactivation of a previous extensional planar and non-
526
rotational master fault (Scholz and Contreras, 1998; Twiss and Moores, 2007). According to
527
Morley (1995) and Fossen et al. (2010), this pre-inversion geometry can be linked to a graben
528
produced by approaching boundary faults. The abrupt increase in the thickness of the
529
Cretaceous sedimentary sequences was correlated with the eastern limit of the San Bernardo
530
intrabasinal high (sensu Ferello and Lesta, 1973), so it was considered as a first-order fault in
531
the mega-architecture of the SBFB. To the moment, the subsurface hard-link with Matasiete
532
fault is only speculative.
533
534 535
Figure 8. A) Satellite image of Península Baya anticline and southern extreme of Sierra del Castillo
536
anticline. B,C) Forelimb and backlimb architecture distorted by a shortcut thrust and a transverse
537
lineament. Figure 8.C includes equal-angle stereonet plot of the forelimb beds. D) Fault architecture
538
developed on silicified tuffaceous strata of the Castillo Formation E) Cataclastic breccia at fault core. The
539
detail shows a thrust plane, which rake values evidence a subordinate lateral movement component. F)
540
Overturned bedding linked to the shortcut thrust and an abutting W-E lineament.
541 542
4.5. Sierra Silva anticline
543
The Sierra Silva anticline is one of the most emblematic ranges of the SBFB situated between
544
the Musters and the Colhué Huapi lakes. Its general architecture was early characterized by
545
Moore (1960, in Ferello, 1964) and Ferello (1964), later refined by Sciutto (1981), Stach (1986)
546
and Giacosa and Paredes (2008).
547
Structural architecture. The morphostructure has an irregular shape with a west bound
548
partially curved, while the east limit may be divided into two domains, one relative straight
549
and the other conformed by several triangular-shaped outcrops (Figs. 9.A,B,C). The main fault
550
is located to the west of the forelimb as a blind fault that emerges in the southern extreme
551
(Fig. 9.C). Second-order faults with strike-slip and dip-slip movements were interpreted from
552
the lineaments that segment the topography and develop anomalous valleys (Figs. 9.A,D).
553
Lineaments a, b, c, e, and f have abutting patterns with the Sierra Silva fault trace what suggest
554
Ya-nodes (sensu Morley and Nixon, 2016), while cross-cutting relationship of lineament h
555
evidence X-node (sensu Sanderson and Nixon, 2015). The northern extreme of the Sierra Silva
556
anticline shows a double hinge geometry with a poorly developed asymmetry with limbs dips
557
under 30° (Figs. 9.A,B). The north limit of the morphostructure is the NW-SE Cerro Chenques
558
fault, which was interpreted as a sinistral fault (Sciutto, 1981; Stach, 1986, Barcat et al., 1984,
559
1989; Peroni et al., 1995; Giacosa and Paredes 2008) or an oblique inverted normal fault
560
(Allard et al., 2018b), while Gianni et al. (2015a) omitted this discontinuity (see Discussion).
561
562 563
Figure 9. A) Geological map of Sierra Silva range and schematic 1D log from YPF.Ch.SS.es-1. TVD: true
564
vertical depth. T: thickness. B) Detail of the northern extreme of Sierra Silva anticline limited by the
565
Cerro Chenques fault. C) Detail of the southern extreme of Sierra Silva anticline. Note that Sierra Silva
566
fault pass from a blind to a surface-cut fault. D) Swath profile showing the lineaments and the maximum
567
heights. Note the uplift trends. E) Field expression of the Sierra Silva fault trace. The faulted inversion
568
fold was inferred from the buried forelimb exposed near the fault tip.
569 570
The Castillo Formation shows two tabular sequences, while the Matasiete and Pozo D-129
571
formations are exposed in the fold core with a complex structural architecture defined by an
572
overturned fold and north-vergent shortcut thrusts (Figs. 9.B, 10.A). The backlimb of Sierra
573
Silva anticline is refolded as a monocline and high-order syncline and anticline (Figs. 9.B, 10.B).
574
The Cerro Chenques fault is a northeast dipping structure (Figs. 9.A,B) with important
575
differences between the structural blocks. The footwall shows the symmetric Sierra Silva
576
anticline while the hanging-wall exposes an asymmetric fold named Jerez anticline, which has
577
an abrupt limb to the east with dip values up to 75°-85° (Figs. 9.A,B, 10.A). Thus the Jerez
578
anticline is east-vergent while the Sierra Silva anticline has a poorly vergence to the west. The
579
sinistral strike-slip displacement of the Cerro Chenques fault was estimated using the inferred
580
traces of Sierra Silva and Jerez faults, showing a differential displacement with 1200 m in the
581
west-limb and 2000 m in the east-limb (Fig. 9.B).
582
583
584
Figure 10. Architecture of the northern extreme of Sierra Silva range. Structural data in profiles
585
represents mean local measurements. A) Photomosaic and structural interpretation of the front-view of
586
the Cerro Chenques fault. Equal-angle plots show the footwall has lower dispersion than the hanging-
587
wall. The Jerez anticline is an asymmetric double-hinge fold. Modified from Bueti et al. (2017) and Allard
588
et al. (2018b). B) Eastern extreme of the Sierra Silva anticline showing the backlimb architecture. Note it
589
is refolded with a monocline and a high order syncline. The monocline is interpreted as a propagation-
590
fold of the antithetic Jerez fault. Based on Bueti (2019). Location of structural profiles in Fig. 9.B.
591 592
The subsurface architecture of Sierra Silva anticline was evaluated from the calibrated column
593
of the well YPF.Ch.SS.es-1 (Fig. 9.A) and its correlation with the seismostratigraphic sequences
594
in a 2D seismic profile (Fig. 11.A,B). The YPF.Ch.SS.es-1 well shows at ~800 m depth an
595
anomalous trend in subsurface dip-log data, which was correlated with the Jerez fault (Fig.
596
11.C). The seismosequences of Pozo D-129 Formation show a thickness increase of ~34 %
597
toward Sierra Silva fault. Seismic architecture of the Cerro Chenques fault is different as it
598
shows a major anticline and a minor localized anticline, the former linked to low inversion
599
degree and the latter to oblique compression (Fig. 11.D). The strike-slip displacement
600
component of the Cerro Chenques fault correlates with outcrop-scale positive flower
601
structures at its trace (Fig. 11.E) (Paredes, 2009).
602
Interpretation. The monocline at the backlimb of the Sierra Silva anticline was interpreted as a
603
propagation fold of the Jerez fault. Thus, we interpreted a pop-up structure defined by the
604
Sierra Silva master fault and its antithetic Jerez fault. The thickness increase of Pozo D-129
605
Formation was associated with a syn-extensional wedge linked to the coaxial reactivation of
606
the Sierra Silva master fault.
607
608 609
Figure 11. A) Seismic profile across-strike to Sierra Silva anticline. Taken from Gianni et al. (2018a). B)
610
Seismic interpretation calibrated with lithostratigraphic limits defined from cutting record description.
611
Units thicknesses follow figure 9.A. Note the pop-up geometry. C) Upward trend in strata inclination
612
obtained from dipmeter log. The anomalous trend at 800 m depth was interpreted as a reverse fault. D)
613
Seismic Line 23088 showing the Cerro Cheques fault is a north-dipping normal fault with a low degree of
614
inversion. Based on Allard et al. (2018b). E) Outcrop-scale positive flower and north-vergent shortcut
615
thrusts exposed on the trace of the Cerro Chenques fault.
616 617
4.6. Los Perales anticline
618
One of the most important structural oil-traps of the subsurface SBFB is the Los Perales
619
anticline (Hechem, 2015). Published studies were done by Barcat et al. (1989), Homovc et al.
620
(1995), Figari et al. (1999), Gianni et al. (2015a,b), and Allard et al. (2018b). We revisited this
621
structure, analyzing 3D seismic data tied to regional markers (Fig. 12.A).
622
Structural architecture. Like many other structures of the SBFB, Los Perales anticline is cross-
623
cut by a series of NW-SE faults that give a partially compartmentalized anticline. General
624
architecture in across-strike profile shows a faulted-fold, which is limited in the forelimb by the
625
principal inverted fault and in the backlimb by a secondary antithetic reverse fault, what gives
626
a rounded, high-amplitude, double-hinge symmetric fold (Figs. 12.B,C). The amplitude and the
627
half wavelength of the main inversion fold changes along strike evidencing that the fold is non-
628
cylindrical. The structural map at the top of Mina del Carmen Formation (subsurface
629
equivalent to Castillo Formation) shows a fold that can be subdivided into two subparallel
630
double-plunging folds (Fig. 12.A). The seismostratigraphic characterization was done using
631
seismic profiles approximately parallel (cross-line) and perpendicular (in-lines) to the Los
632
Perales anticline and fault. Across-strike seismic profiles show the Cretaceous sequences of the
633
Chubut Group are dominated by subparallel reflectors without internal unconformities. Only
634
the bottom quarter of the Pozo D-129 sequence in the X-Line 750 has a wedge geometry that
635
narrows toward the Los Perales fault. This relative thin interval was interpreted as counter-
636
clockwise rotated on-laps (see Discussion). The asymmetric indexes between structural blocks
637
show values a little bigger than one, with thinner sequences in the footwall (Inline 750, T5/T6:
638
1.05; Inline 1234, T4/T5: 1.05); while in the hanging-wall show trends from 1.35 (Inline 1234,
639
T1/T3) to 1.03 (Inline 750, T4/T5), with decreasing thickness toward the Los Perales fault (Fig.
640
12.B,C). Along-strike Cretaceous architecture shows important geometries. The deeper
641
reflector packages were attributed to Neocomian units; they have concave-up geometry with
642
an internal pattern with seismic reflectors subparallel to gently divergent toward the central
643
part of the fault (Fig. 12.D). The Chubut Group sequences show key patterns in the along-strike
644
profile to evaluate the kinematic evolution of the structure (Fig. 12.D). Most important are the
645
thickness increase toward the fold crests (T10/T9: 1.23) and the divergence of folded
646
unconformities in that direction. This profile also shows normal faults cross-cutting the
647
anticline with a local increase of hanging-wall thickness in seismosequences of the Castillo
648
Formation (T11/T12: 1.48).
649
Interpretation. All the mentioned characteristics supported a reverse-reactivation of the Los
650
Perales fault after the deposition of the Castillo Formation. At the same time, the correlation
651
of the crest of the Los Perales anticline with the thicker thickness of the Neocomian
652
seismosequences suggests a relationship between the normal fault slip pattern and the
653
inversion degree (see Discussion).
654
A neighboring fault located 9 km to the east of the Los Perales fault shows a contrasting
655
architecture (Fig. 12.E). The fault is antithetic and divergent to the Los Perales fault, and it has
656
an upper tip located under the Chubut Group seismosequences. The Neocomian wedge
657
denotes extensional control linked to a surface-break fault with synthetic dipping units
658
(Prosser, 1993; Gawthorpe et al., 1997; Wilson et al., 2009a; Leeder, 2011; Deng and McClay,
659
2019). The overlying seismosequences abruptly develop a monocline interpreted as a forced
660
fold (Whithjack et al., 1990; Coleman et al., 2019a,b) generated by the normal fault
661
reactivation. The relationship fold amplitude/width (fold-shape-factor sensu Coleman et al.,
662
2019a) changes vertically, from a low fold-shape-factor at Pozo D-129 Formation to a high fold-
663
shape-factor at Mina del Carmen Formation. Both intervals show a similar fold width, so the
664
difference is in the fold amplitude. This geometric characteristic suggests a fold growth with
665
the width established early, and amplitude increased with ongoing normal fault slip (Coleman
666
et al., 2019a). The timing of the fold construction was defined from the seismic architecture.
667
The Pozo D-129 Formation has parallel reflectors and a tabular geometry with asymmetry
668
indexes close to 1 (T1/T2: 1.09; T2/T3: 1.08, Fig. 12.F), so a displacement quiescence period of
669
the fault was interpreted. The Castillo Formation has subtle but very important differences
670
respect the previous unit: 1) the hanging-wall sequences are up to 20 % thicker than footwall
671
ones (e.g. T5/T4: 1.2, Fig. 12.F) and 2) the monocline axis preserves the thinner thickness of
672
the unit and stratigraphic on-laps. This evidence supported a blind fault extensional
673
reactivation during the Castillo Formation deposition.
674
675 676
Figure 12. Tectonostratigraphic architecture of the Los Perales anticline. A) Time structure map (msec
677
TWT) of the top of Mina del Carmen Formation. B, C) Interpreted seismic profiles across the fold axis. D)
678
Interpreted seismic profile along the fold axis. E) Architecture of selective fault reactivation during
679
positive tectonic inversion. F) Architecture of the non-inverted fault. TWT: Two Way Time.
680 681
4.7. Sur Río Deseado depocenter
682
The south-limit of the GSJB has a long-lived depocenter, which importance was mentioned by
683
Clavijo (1986) and later described from seismic data by Fitzgerald et al. (1990). We evaluated
684
the architecture of this zone using two 2D seismic profiles, one parallel and one transversal to
685
the master fault. The seismic data was tied with two wells drilled in that area (YPF.Ch.SRD.x-2,
686
YPF.Ch.SRD.es-1), which tectonostratigraphic limits were calibrated from our cutting record
687
revision. Specifically, YPF.SC.SRD.x-2 overcomes 4000 m depth, and the interval from 2358 m
688
to bottom-depth is dominated by black-shales deposits of the Pozo Anticlinal Aguada Bandera
689
Formation. Interested readers see Allard et al. (2018a) for further details.
690
Structural architecture. The economic basement substrate was inferred in the N-S seismic
691
profile from chaotic to poorly stratified seismofacies dipping to the south, over which the
692
Neocomian reflectors show down-lap patterns toward the main fault (Fig. 13.A). The internal
693
architecture of the Neocomian wedge is complex near the master fault, with convex and
694
concave geometries, truncations and off-lap patterns, suggesting active border deposits
695
(Prosser, 1993; Gawthorpe and Leeder, 2000; Leeder, 2011; Henstra et al., 2017). The
696
asymmetry index of the wedge was estimated in 2,3, calculated from the economic basement
697
to the top of Pozo Cerro Guadal Formation (Fig. 13.A). A key structural element is a concordant
698
relationship between Pozo Cerro Guadal and the Pozo D-129 formations, and their dip
699
northward. The E-W seismic section shows subtle but not less important patterns. The main
700
structure is a gentle and wide syncline that affects the upper seismosequences of the
701
Neocomian and Pozo D-129 Formation. The internal architecture of Pozo Cerro Guadal
702
Formation has a thickness increases of ~30 % where underlying Neocomian reflectors converge
703
(T1/T2: 1.3, Fig. 13.B). Seismosequences of the Pozo D-129 Formation show relatively small
704
variations in thickness, with the maximum thickness located in the sine of the syncline, and
705
asymmetry index values from 1.2 to 1.3 (Fig. 13.B). This interval can be divided into two
706
packages, the lower much more asymmetric with values that double in the syncline sine and
707
show on-lap pattern, while the upper has a tabular geometry.
708
Interpretation. The mentioned architecture gives important tectonostratigraphic signals. The
709
opposite dip direction between the base and top of the Neocomian wedge cannot be
710
explained only with half-graben displacements patterns since the zone of maximum
711
subsidence is close to the fault (Barr, 1987; Leeder and Gawthorpe, 1987; Morley, 1995,
712
among many others). At the same time, the structural level of the deposits near the fault
713
overcoming the ones in the flexural margin evidence the differential upward-direction slip of
714
the hanging-wall and the clockwise rotation of all the reflectors packages. So, these geometries
715
were interpreted as evidence of a low-degree positive tectonic inversion of a rift-margin fault.
716
The concordant limit between the Chubut and Las Heras groups differs from the traditional
717
megasequences scheme of Figari et al. (1999). Furthermore, the intra-Neocomian
718
unconformities were linked to the local evolution of the half-graben, specifically to the
719
extensional counter-clockwise rotation of the flexural margin (Morley, 1990; Proser, 1993;
720
Leeder, 2011). Following the models of Morley (2002), the along-strike thickness trends in the
721
Pozo D-129 Formation were interpreted as a late normal fault reactivation with a fault trace
722
contraction.
723
724 725
Figure 13. Tectonostratigraphic architecture of the Sur Río Deseado depocenter. A) Seismic profile
726
across-strike the master fault. B) Seismic profile along-strike the master fault. Vertical scales in TWT:
727
Two Way Time (msec).
728 729
4.8. Economic basement
730
The heterogeneous spatial distribution and varied composition of the economic basement
731
units hinder the evaluation of its structural fabric. Recent studies analyzed 2D and 3D seismic
732
data and proposed different contractional events for the deformation of the economic
733
basement (Navarrete et al., 2015, 2016; Gianni et al., 2018b). We test the calibration of some
734
of the wells and seismic profiles used in those proposals to validate the economic basement
735
architecture. The results show important problems associated with the stratigraphic
736
calibration. A synthesis of the main observations follows.
737
Navarrete et al. (2016) studied the fabric of the pre-Cretaceous substrate at the eastern limit
738
of the SBFB, in an area located to the east of the Colhué Huapi lake. Their figure 8 shows the
739
raw information of crossline 2625 where the YPF.Ch.LA.x-2 does not reach the crystalline
740
basement seismofacies (sensu Foix et al., this issue). What is more, in their
741
tectonostratigraphic scheme, the basal sequences of the well are attributed to stratified
742
seismofacies interpreted as folded Liassic sequences. The well-report of YPF.Ch.LA.x-2 records
743
36 m of granitic rocks at the bottom depth, so the seismic calibration should have included the
744
crystalline basement. It has to be mention that the basement top in YPF.Ch.LA.x-2 correlates
745
with LA-1 A, from which a core of mica-schist was extracted, so there is no doubt of the rock
746
type (Fig. 14.A,B). A similar problem was identified at well YPF.Ch.PGS.x-1 (Fig. 14.A), which
747
according to the well-report and our cutting revision drilled at least 22 m of granitic rocks (Fig.
748
14.C), but no crystalline basement is proposed in the calibrated seismofacies shown by
749
Navarrete et al. (2016, their Fig. 6).
750
751 752
Figure 14. A) Satellite image showing the location of the revisited wells used to review the calibration of
753
basement seismofacies from previous studies. B) File of the well LA-1 where it is mentioned the
754
extraction of a core of low-grade metamorphic rock. C, D) Cutting photographs of granitic rocks and
755
black-shales used for the evaluation of the seismic basement calibrations. Numbers indicate depth in
756
meters.
757 758
The mentioned contribution also justify Upper Jurassic sequences (Lonco Trapial Group) from
759
the subsurface stratigraphy proposed in the well-report of the YPF.Ch.CBo.x-1 (Navarrete et
760
al., 2016, their Fig. 6). Our cutting calibration does not agree with the lithostratigraphic limits
761
proposed in that well-report. We redefined the units and discarded the presence of the Lonco
762
Trapial Group because we did not recognize any volcaniclastic and volcanic rocks compatible
763
with that unit (Foix et al., this issue). Taking into account the observations made, the
764
unconformities exposed in the mentioned seismic profiles of Navarrete et al. (2016) have to be
765
rethought and link them to the folding of Neocomian sequences. In that case, these
766
relationships are analogous to the Bajo Grande Unconformity, where the Baqueró Group
767
(Aptian) overlies the Bajo Grande Formation (Late Jurassic to Hauterivian) in the Deseado
768
Region (Giacosa et al., 2010; Gianni et al., 2018a,b). Further to the west, at Río Mayo sub-
769
basin, the seismic calibration using deep wells also shows problems. Navarrete et al. (2015,
770
their Fig. 8) used the 2D seismic line 7490 calibrated with the YPF.Ch.CA-x.1 and interpreted
771
the Lonco Trapial Group (Upper Jurassic). We revisited the YPF.Ch.CA.x-1 cutting record and
772
the bottom sequences were attributed to black-shales of the Neocomian Katterfeld Formation
773
(Fig. 14.D). In consequence, the seismosequences were wrongly attributed to the Lonco Trapial
774
Group, which implies the correlated seismofacies must be corrected.
775
The short revision of the study cases demonstrated the necessity of a workflow with robust
776
subsurface stratigraphic limits from cutting records previous to seismic interpretations. Until
777
new contributions revalidate the mentioned seismic architecture, those papers cannot be used
778
to evaluate the economic basement fabrics, and in consequence, neither their influence in
779
Cretaceous extensional structures (see Discussion).
780 781
5. DISCUSSION
782 783
5.1. Architecture variability and controls in tectonic inversion
784
5.1.1. Pop-Up growth folds and inherited extensional growth strata
785
The absence of a clear vergence in the SBFB was notice by Peroni et al. (1995), who referred to
786
it as an accordion system. Almost all the studies performed in this region described the
787
dominance of box-shaped anticlines (Homovc et al., 1995; Peroni et al., 1995; Figari et al.,
788
1999). All the aforementioned results show the geometric variability of inversion folds, from
789
the classic broad asymmetric anticlines to the narrow double hinge ones, and an open
790
syncline. According to the paleomagnetic study done by Somoza and Zaffarana (2008), the
791
mid-Cretaceous rocks of the SBFB show the absence of vertical-axis rotations associated with
792
tectonic inversion, so the present inversion fault traces can be considered similar to
793
Cretaceous extensional ones. From a dynamic point of view, the narrow inversion anticlines
794
cannot be associated to a lateral-strike tectonic regime as previously proposed by Barcat et al.
795
(1984) because: 1) the main structures are orthogonal to highly oblique to regional maximum
796
principal stress during the late Upper Cretaceous and Cenozoic times (Müller et al., 2016) and
797
2) folds are not linked to restrending bends or anti-dilatant oversteps. As a consequence, the
798
architecture of the SBFB has to be explained by inversion mechanisms and controls.
799
Geometries of Sierra Silva and Los Perales anticlines are partially analogous as their general
800
architectures are defined by a principal inverted fault and a secondary antithetic reverse fault.
801
Figari et al. (1999) interpreted the Los Perales antithetic fault as a reverse structure
802
neoformatted during Cenozoic contractional phase, but no geometric or kinematic justification
803
was exposed. The absence of well-developed harpoon anticlines is clearly different from
804
traditional inverted geometries (McClay and Buchanan, 1992; Granado and Ruh, 2019). The
805
nearness and overlap of master and secondary reverse faults evidence a contractional
806
transference zone similar to fold and thrust belts (Ramsay and Huber, 1987; McClay, 1992;
807
Higgins et al., 2007), but the pre-inversion history is not obvious. Neither of the two faults has
808
andersonian dip-angles near to ~30° (Sibson, 1989; Collettini and Sibson, 2001), what is
809
predictable for the main one in an inversion scenario (Sibson, 1995; Bonini et al., 2012) but
810
incompatible for an antithetic neoformatted structure, which in that case are expected to have
811
a thrust geometry under andersionian conditions (Marquez and Nogueira, 2008). The
812
tectonostratigraphic analysis done shows that the antithetic faults have no control during the
813
sedimentation of the Castillo Formation (Fig. 10) or its subsurface equivalent Mina del Carmen
814
Formation (Figs. 12.B,C). These schemes support the nucleation and growth of post-
815
sedimentary normal faults, coaxial with the master fault (Fig. 15). Later dip-linkage gave the Y-
816
shaped geometry of the normal fault system (Nicol et al., 1995), which was inherited by the
817
inversion phase. In this model, the weakness linked to the antithetic fault is associated with
818
the pre-inversion normal fault, which was resheared during tectonic inversion (Fig. 15). This
819
model differs from the Type-2 folds of Shinn (2015), which proposed reverse faults
820
neoformatted. In consequence, the pop-up geometries of inversion folds in the SBFB are
821
interpreted as pop-up growth folds in the sense of Cartwrigth (1989). The fault obliquely to the
822
Los Perales anticline is synsedimentary with Mina del Carmen Formation seismosequences, so
823
it supports a non-coaxial phase of extension during Albian times with no seismic evidence of
824
later reverse-reactivation. This kinematic evolution may be attempted to replicate in outcrops
825
of the Sierra Silva anticline trough the transverse faults. Therefore, the geometric and
826
kinematic evidence supported the selective tectonic inversion of a multiphase 3D extensional
827
fault network (see Section 5.3).
828
829 830
Figure 15. Schematic model for the construction of Sierra Silva pop-up growth fold. Compressional
831
phase reshear both high angle faults giving a pop-up geometry. BB: Bajo Barreal Formation, LCH: Lago
832
Colhué Huapi Formation.
833 834
Normal faults with multiple pulses of displacement can change from blind to emergent faults,
835
what is traduced in changes in the geometry of syn-sedimentary sequences (Gawthorpe et al.,
836
1997; Coleman et al., 2019a,b). Seismic architecture from the non-inverted fault near Los
837
Perales anticline (Fig. 12.E,F) was used to elaborate a geometric and kinematic model that
838
explains extensional inherited growth-strata geometries in inverted faults (Fig. 16). The
839
monocline generated by a normal coaxial reactivation shows ongoing deformation that folds
840
the post-extensional tabular intervals linked to fault quiescence and overlying extensional
841
growth-strata. Later tectonic inversion inherits the across-fault thickness differences, which
842
are coupled with syn-inversion growth-strata. The magnitude of the reverse-reactivation, as
843
well as the sedimentary rate, control the relationships between pre- and sin- inversion
844
geometries. During the initiation of the tectonic inversion, subtle evidence of contraction
845
mechanisms can be inferred from buttress folds or reverse drag folds that affect the syn-rift
846
wedge. This simple 2D model can be affected by previously described pop-up mechanisms,
847
increasing the complexity of the inherited extensional growth-strata and hindering its
848
recognition. Along-strike structural sections are determinants to evaluate the tectonic phase of
849
the growth-strata, being the thickness increase toward the fold crest a doubtless evidence of
850
an inherited extensional pattern.
851
852 853
Figure 16. Sequential model for inherited growth-strata architecture. Positive tectonic inversion is
854
subdivided into three stages. Initiation only expresses subtle contractional evidence like buttress folds in
855
the syn-rift wedge. Low and moderate inversion degrees are differentiated from the faulted inversion
856
fold and the occurrence of a null-point (not shown is the sketch).
857 858
5.1.2. Oblique inversion and heterogeneous deformation
859
The detailed study of the inverted structures in the SBFB allowed evaluating oblique tectonic
860
inversion and heterogeneous shortening. Both characteristics are well expressed at the
861
northern limit of the Sierra Silva anticline. The Cerro Chenques fault has ~N130° strike what
862
gives a 40° acute angle respect to the regional Upper Cretaceous to Cenozoic E-W shortening
863
direction proposed by Müller et al. (2016). This scenario justifies a stress partition into strike-
864
slip and contractional components (Letouzey et al. 1990; Bayona and Lawton, 2003; García-
865
Lasanta et al., 2018). The former shows 800 m of differential movement among the Sierra Silva
866
and Jerez faults traces (Fig. 9.B). This kinematic difference was associated with a mayor
867
shortening in the hanging-wall of the Cerro Chenques fault, which is manifested by higher
868
rotation of the forelimb of the Jerez anticline respect to the backlimb of Sierra Silva anticline.
869
This scenario was related to heterogeneous deformation. Figure 17.A shows a simplified
870
sketch, and it is compared with the expected architecture under homogeneous deformation
871
(Fig. 17.B). In this model, along-strike variability of fault strain is also expected, which should
872
have affected the fault architecture.
873
The Sierra Nevada and Sierra del Castillo anticlines were correlated to a mayor lineament
874
interpreted as a Cretaceous master normal fault, which surface expression is the Matasiete
875
fault. Along-strike change from a blind to an emergent structure evidence the modification of
876
the inversion-mechanism from propagation fold to a faulted fold, that evidence variability in
877
the degree of tectonic inversion (Fig. 17.C). The trace of the main inversion fault has a strike
878
break from NNW-SSE to N-S, suggesting collinear hard-linked normal faults segments (Jackson
879
et al., 2002; Wilson et al., 2009b; Leeder, 2011; Fossen and Rotevatn, 2016, among others).
880
That zone has the west-vergent forelimb and it is related to the unroofing of Matasiete
881
Formation during Cenozoic times, both characteristics support a local increase in the
882
deformation. At the same time, the truncation of the inversion anticline by the inverted
883
master fault denotes the inverted normal fault overcame the null-point (Cooper and Williams,
884
1989; Williams et al., 1989; Bonini et al., 2012; Reilly et al., 2017). This architecture can be
885
explained as an easier reshear linked to the fault architecture (Jackson et al., 2013), or the
886
lateral propagation toward the normal fault tip where the extensional displacement is minor,
887
so the null-point is achieved with lower reverse-reactivation (Fig. 17.C). This model can also be
888
applied to the southern extreme of the Sierra Silva anticline where the master fault emerges.
889
Southward to these morphostructures, the map-view of the Los Perales fault-fold system
890
shows the inversion anticline is linked to a sinuous fault trace and two high-order subparallel
891
braquianticlines (Figs. 12.A,D). This architecture was interpreted as a consequence of the
892
inversion of two normal faults left-lateral overlapped and hard-linked.
893
The Sur Río Deseado master fault has a strike subparallel to the Upper Cretaceous and
894
Cenozoic Andean compression, giving an unfavorable orientation for the tectonic inversion
895
(Letouzey et al., 1990). Nonetheless, the seismic architecture exposed in section 4.7 evidence
896
the reactivation with a dominance of reverse slip. Following the numerical modelling of
897
Granado and Ruh (2019), the tilting of the syn-rift half-graben supports the presence of a weak
898
inherited fault as an inversion control. In all the mentioned scenarios, high fluid-pressure
899
should have favored the reshear of the master faults (Sibson, 2017).
900
901 902
Figure 17. A,B) Alternative models for oblique inversion of a complex 3-D fault network. The
903
deformation is partitioned with a lateral component at the NNW-SSE fault. A) The heterogeneous
904
deformation implies the lateral movement combined with the rotation of the forelimb So and the
905
antithetic fault. B) The homogeneous deformation implies comparable strike-slip displacement. C)
906
Along-strike variability on inversion degree linked to inherited normal fault segments with different
907
displacement patterns. The transfer zone has a higher deformation.
908 909
5.1.3. Tectonic inversion of extensional nodes
910
The inversion architecture of complex normal fault arrays is understood from natural examples
911
(e.g. Kelly et al., 1999; Imber et al., 2005; Giacosa et al., 2010). In our case, the Lower
912
Cretaceous structures interact with a fault system with general trend W-E, linked to a non-
913
coaxial extensional phase. According to Los Perales architecture, it was syn-sedimentary to the
914
Mina del Carmen Formation, which is analogous to the results of Paredes et al. (2013) in Cerro
915
Dragón oil-field, at the eastern margin of the SBFB. The structural architecture and tectonic-
916
geomorphology of the studied ranges contributed to the comprehension of inversion
917
architecture of 3D normal faults arrays. The southern extreme of the Sierra del Castillo and
918
Península Baya anticlines allowed evaluating the response to the inversion of extensional
919
interaction nodes. The WNW-ESE normal fault with a Ya-node with Matasiete fault and an X-
920
node with Península Baya fault perturbed the inverted N-S faults dominated by dip-slip
921
displacement (Figs. 7.A, 8.A). In this case, there cannot be applied the simplistic scenario
922
where partitioned deformation is constructed from the trigonometric-vectorial decomposition
923
of the regional Z-axis (e.g. Imber et al., 2005; Giacosa et al., 2010; Navarrete et al., 2015). The
924
high oblique branch defined by the WNW-ESE lineament is dominated by dip-slip movements
925
and evidence strong along-strike variability in deformation. Its relative maximums inversion
926
degrees correlate with previous normal faults interaction zones. Assuming the structural
927
deformation is proportional to the reuse of the normal fault (Jackson et al., 2013), the
928
mentioned context suggests a localized increment of reshear linked to the inherited normal
929
fault architecture. In this sense, recent models of fault interaction mention the increase of
930
strain and coeval widening of the core and damage zones (Nixon et al., 2014; Peacock et al.,
931
2017b), what favor the fault rock development (Caine et al., 1996; Braathen et al., 2009; Choi
932
et al., 2016). All these processes should have decreased the cohesion and frictional resistance
933
of the rock after normal faulting, and in consequence, favors local inversion mechanisms and
934
focused the upward propagation of the deformation (Jackson et al., 2013). Península Baya
935
range also shows E-W lineaments at its southern extreme that crosscut all the inversion fold.
936
They have vertical uplifts of the northern blocks minor to 50 m without the distortion of the
937
backlimb structural trend. This architecture was interpreted as a relatively inactive exhumation
938
of normal faults during tectonic inversion process. A more complex array of secondary
939
transversal faults is exposed in Sierra Silva range. Its architecture with traces that finish against
940
the main fault suggests abutting faults in hanging-wall (sensu Peacock et al., 2017a), which
941
were exhumated after the tectonic inversion phase. The along-strike topographic and
942
structural trends show a clear correlation between the highest zones and the mayor transverse
943
lineaments (Fig. 9.D). This spatial relationship was interpreted as the combined effect of the
944
southward propagation of the inverted fault-fold, and an active role of the transverse faults
945
during the range uplift. The latter based on the anomalous uplift induced by the transverse
946
fault in Península Baya range (Fig. 7.C). Further fieldwork is required to evaluate the complex
947
kinematic history of this W-E fault system.
948
Recognition and comprehension of tectonic inversion of extensional nodes impact in the
949
analysis of oil-bearing structures of the SBFB, in particular, because increased strain at fault
950
interactions should have favored vertically fluid pathways that communicate reservoirs
951
(Peacock et al., 2017b, and references therein). Furthermore, inversion uplifts could combine
952
with extensional ones linked to strike-slip reactivation (Rotevatn and Peacock, 2018), favoring
953
the formation of hydrocarbon traps.
954 955
5.1.4. Selective tectonic inversion
956
Displacement transfer zones are well known in overlap thrust fault systems (McClay, 1992;
957
Higgins et al., 2007). In the case of inverted basins, the reconstruction of the pre-inversion
958
architecture is essential to evaluate the effect of the inherited faults arrays. Particularly, soft-
959
linked extensional transference zones are important in understanding the later inversion
960
(Underhill and Paterson, 1998; Konstantinovskay et al., 2007; Jackson et al., 2013; Sarhan and
961
Collier, 2018). In this scenario, overlap structures with selective inversion are almost
962
unexplored from natural examples (Kelly et al., 1999). Sibson (1995) interpreted this behavior
963
as a consequence of geometric differences in fault planes, while McClay and Buchanan (1992)
964
attributed unfavorable orientation for reactivation. The present architecture of the SBFB was
965
used to evaluate different responses of soft-link transfer zones under tectonic inversion. Pop-
966
up growth folds represent the reshear of both faults, so overlapped fault strain envelopes
967
were interpreted. The other end-member model is the selective inversion in neighboring
968
faults, where strongly non-uniform contraction localized the strain in only one fault. Examples
969
of this situation were analyzed from fault couples related to the Sierra Nevada and Los Perales
970
faults (Figs. 5.F, 12.E). In both cases, the inverted fault is located to the west, while the one
971
located to the east remains non-inverted. The latter shows an extensional architecture
972
apparently frozen, but subseismic reshear mechanisms or very low inversion degree cannot be
973
discarded. The fault-plane friction respect to the available shear stress controls the faults
974
reactivation (Lisle and Srivastava, 2004; Maerten et al., 2018). Assuming the most common
975
conditions where both faults decrease the rock resistance after extensional faulting (Handin,
976
1969; Ranalli and Yin, 1990; Twiss and More, 2007), the heterogeneous deformation
977
consumed all the local shortening using only one fault. In this context, the inverted fault
978
produced a deformation shadow, in a similar way to the stress-failure shadow proposed by
979
Dempsey et al. (2012) during the evolution of normal fault systems. Mayor inverted faults can
980
increase the strained rock volume and widens the damage zones of the inverted fault, favoring
981
fluid pathways (Caine et al., 1996; Kristensen et al., 2016; Hollinsworth et al., 2019).
982
Consequently, the increase in the fluid circulations can induce the valve-effect in reshear
983
mechanisms (Sibson, 1995; Sibson, 2017) favoring multipulse tectonic inversion processes.
984 985
5.1.5. Influence of basement fabric and distribution
986
Positive tectonic inversion implies the reactivation of previous normal faults (sensu Williams et
987
al., 1989), which history can be related to active or passive basement fabric (Morley, 1999).
988
The effects of weakness zones in the normal fault networks have been extensively studied
989
from natural examples (Morley, 1995, 1999; Duffy et al., 2005; Reeve et al., 2015; Phillips et
990
al., 2016; Collanega et al., 2019). In the case of the SBFB, Section 4.8 highlights that the actual
991
knowledge of the subsurface structural basement fabric requires major calibrations based on
992
systematic cutting record revision. Our results at the Ferrarotti study area, are still the
993
moment, the best surface analog to evaluate the basement fabric of the GSJB and its influence
994
on inverted faults. Figure 5 shows the relationship between the seismofabric and the inverted
995
faults planes, where the faults are subparallel to adjacent reflectors correlated with the
996
economic basement. This architecture suggests a weakness direction subparallel to the
997
stratification of the sedimentary basement units, analogous to merging interactions proposed
998
by Phillips et al. (2016). This study area also shows outcropped folds with half-wavelength of
999
tens to hundreds of meters in Jurassic and Paleozoic rocks with a pronounced N-S trend of the
1000
individual axis strikes (Fig. 4.A). Correlation between this robust structural trend and the N-S
1001
oriented intrabasinal paleohigh identified in the subsurface of the SBFB by Ferello and Lesta
1002
(1973) (Fig. 18) suggested a cause-effect relationship. In this way, that coincidence allows
1003
proposing that Upper Jurassic and Cretaceous extensional phases were distorted by weakness
1004
generated by fold limbs of the pre-rift units. This hypothesis is relatively unexplored, so future
1005
studies must systematically evaluate the influence of different fold dimensions (subseismic to
1006
seismic) through basement mapping, or advances techniques like 2-D convolution seismic
1007
modeling (Wrona et al., 2019) and Anisotropy of Magnetic Susceptibility (García-Lasanta et al.,
1008
2018). This hypothesis differs from the above-mentioned fabric analysis where the pre-rift
1009
weaknesses are thought as discrete foliations or plastic shear zones from the crystalline
1010
basement.
1011
The climax of the Jurassic syn-rift played a major control in the early Cretaceous extensional
1012
reactivation (e.g. Paredes et al., 2018). During the tectonic inversion phases, the resheared of
1013
master faults with high extensional displacement should have consumed the inversion with
1014
low to very low inversion ratio (sensu Williams et al., 1989), so that minor uplift favored that
1015
the inverted structure remains buried (Foix et al., 2015). This simple geometry relationship can
1016
explain the low inversion degree of Río Mayo Sub-basin (Figari et al., 1996), despite being
1017
located closer to the Andean margin.
1018
At a basin scale, the latitudinal increase on the depth of the Río Chico paleohigh (sensu
1019
Cortiñas, 1996) must have played an essential role in the construction and expression of the
1020
SBFB. Figure 18 resumes the main inverted structures from surface and subsurface of the
1021
SBFB, as well as non-inverted faults of North Flank, Basin Center, and South Flank. It is clearly
1022
shown that outcropped inversion structures are next to the metamorphic-granitic palaeohigh.
1023
We integrated this regional architecture in an inversion model where the paleohigh acted as a
1024
megascale semi-rigid element that induced a basin-scale buttress effect for the N-S faults of
1025
the SBFB (Fig. 18). At the same time, the North Flank was protected by this element, so there is
1026
no reverse-reactivation of Cretaceous structures (Figari et al., 1999). This scenario is similar to
1027
the one proposed by Shinn (2015) in the Yellow Sea subsurface.
1028
1029 1030
Figure 18. A) Structural framework of the San Bernardo Fold Belt showing the Río Chico paleohigh
1031
(Cortiñas, 1996). Map-scale faults and folds modified from Figari et al. (1999), Miller y Marino (2019),
1032
and SEGEMAR maps. Wells include the depth of the crystalline basement. Surface units of the economic
1033
basement follow references in figure 1. Note crystalline outcrops toward the northern limit of the GSJB.
1034
B) Geological section made by Ferello and Lesta (1973) showing the San Bernardo high. C) Seismic
1035
architecture of the Río Chico paleohigh.
1036 1037
5.2. Tectonostratigraphic scenario of the Chubut Group at the San Bernardo Fold Belt
1038
The current state of knowledge of the tectonostratigraphic scenario of the base of the Chubut
1039
Group (Pozo D-129, Matasiete, and Castillo formations) can be divided into two major
1040
proposals, the syn-inversion or the pre-inversion scenarios (see Introduction). The structural
1041
architecture of Ferrarotti locality is key, as its paleogeographic position is the closest to the
1042
contemporaneous Andean subduction margin, so the transmission of the Andean compression
1043
may be assumed more efficient (Heidbach et al., 2018). In consequence, the Lower Cretaceous
1044
syn-extensional context alerts the timing of the construction of other folds located to the east.
1045
In order to validate a robust structural framework, we compared in detail our geometric-
1046
kinematic results with the evidence of the syn-inversion context. The analysis of each inversion
1047
structure follows.
1048
Sierra Nevada anticline is an inversion structure that was characterized by Gianni et al. (2015a,
1049
their Fig. 10.A). They defined a subsurface architecture with a folded wedge geometry at its
1050
hinge, which was interpreted as the combination of a retro-vergent intraformational
1051
antiformal stack duplex and a vergent imbricate thrust system. Our interpretation is clearly
1052
different, the syn-rift wedge was correlated with Chubut Group and Neocomian units, and the
1053
thickness pattern was attributed to the extensional phase (Fig. 5.F). We interpreted a normal
1054
fault to the east of Sierra Nevada fault with syn-kinematic sequences of the Castillo Formation.
1055
Those authors associated this structure to a reverse fault, and the thickness anomaly in the
1056
sequences of the Castillo Formation to a folded intraformational thrust (Gianni et al., 2015a,
1057
their Fig. 10.A). It has to be highlighted that the dip direction to the east of that main fault is
1058
not consistent with the seismofacies and seismosequecences correlation (Fig. 5.H), so the
1059
reverse kinematic is unlikely. Another element that shows the geometric differences are the
1060
stratigraphic lobate geoforms (Fig. 5.I), Gianni et al. (2015a, their Fig. 10.A) proposed there a
1061
propagation fold and a shortcut thrust. To the south of Sierra Nevada anticline, the traditional
1062
surface architecture exposed at Matasiete canyon was used by Gianni et al. (2015a, their Fig.
1063
7.A) and Gianni et al. (2018a, their Fig. 8.b) as an example of growth-strata in late Early to Late
1064
Cretaceous units of the Chubut Group. Our field observations at the same outcrops are in
1065
disagreement with that proposals because the formations are faulted (Fig. 6), so the structural
1066
dips of the different structural blocks cannot be linked to growth-strata (Suppe et al., 1992;
1067
Brandes and Tanner, 2014). We consider that the progressive unconformities marked by the
1068
rotated paleosoils of Laguna Palacios Formation are the only syn-contractional evidence of the
1069
late Upper Cretaceous inversion phase.
1070
The information analyzed in Los Perales anticline contributed to important subsurface
1071
tectonostratigraphic relationships. The divergence of markers toward the central part of the
1072
Los Perales fault in an along-strike section is considered as robust evidence of an inherited
1073
pattern of an extensional reactivation during late Early Cretaceous times. The absence of
1074
compressional progressive unconformities sustains an inversion post-Castillo Formation. The
1075
thickness variations of seismosequences in the hanging-wall of the Los Perales fault are
1076
interpreted as an inherited pre-inversion sedimentary pattern linked to a monocline growth
1077
during extensional reactivation (see Section 5.1.3). Previous contributions used the mentioned
1078
asymmetry in across sections of Los Perales anticline to propose a syn-contractional context
1079
during the deposition of Pozo D-129, Matasiete and/or Castillo formations (Fig. 5 in Barcat et
1080
al., 1989; Fig. 10.B in Gianni et al., 2015a; Figs. 6.C,D in Gianni et al., 2015.b).
1081
Finally, Sierra Silva anticline deserves special analysis. The northern extreme of this fold shows
1082
tabular sequences of the Castillo and Matasiete formations in the hanging-wall of Cerro
1083
Chenques fault, which argues the absence of tectonic control during their deposition (Fig.
1084
10.A). However, Gianni et al. (2015a, their Fig. 7.B) draw growth-strata, progressive
1085
unconformities, and on-lap stratification in those outcrops from the linkage of Cretaceous
1086
sequences in the hanging-wall and footwall of the Cerro Chenques fault. At the same time,
1087
those authors also interpreted growth-strata, unconformities and dips up to 45° at the
1088
backlimb (Gianni et al., 2015a, their Fig. 7.C), but our architecture shows concordant strata and
1089
dips values under 27° (Fig. 10.B) (Allard et al., 2018b, their Fig. 5.A). The subsurface
1090
architecture was addressed in a later paper using 2D seismic data (Figs. 9.B,C in Gianni et al.,
1091
2018a). The southernmost section defines a fault-propagation fold where the fault tip is at the
1092
top of the Neocomian seismosequences (~2700 m depth), so the fault does not cut the Chubut
1093
Group reflectors. In this geometry, there is no antithetic fault and the Castillo Formation
1094
thickness reaches 1050 m in the fold crest. Our geometric analysis differ in almost everything:
1095
1) the main fault is projected upward cutting the forelimb with a tip almost at surface, this
1096
reconstruction correlates with the faulted fold interpreted from the buried forelimb at the
1097
southern limit of the anticline (Fig. 9.C,E), 2) the backlimb is cut by the Jerez fault with reverse
1098
kinematic, which was interpreted from outcrop information and dipmeter log data (Figs. 9.B,
1099
11.B,C), 3) the simplified and schematic box-shape fold reconstruction is linked to 290 m of the
1100
Castillo Formation, similar to the 340 m outcropped thickness measured by Paredes et al.
1101
(2015) in the Cerro Chenques. The figure 9.B of Gianni et al. (2018a) shows another W-E
1102
seismic profile across Sierra Silva anticline, located to the north of the previous one. Some
1103
characteristics from the published information have to be mentioned. The impedance of the
1104
reflectors of Cretaceous sequences is rarely uniform, from 650 m to 3750 m depth. The raw
1105
section shows a high impedance element crosscutting all the sequences, which is high-dipping
1106
to the east. This is extremely rare because its dip should not have favored the wave reflection
1107
and 2D seismic acquisition (Herron, 2012; Allmendinger, 2015). However, more remarkable is
1108
that this element matches exactly with the forelimb axial plane marked in a general figure. This
1109
correlation cannot be explained as the seismic record because the axial plane is a conceptual
1110
construction, so it does not exist as a reflection surface. From the geometric point of view, this
1111
seismic shows the Castillo Formation seismosequences are up to ~1500 m thick at Sierra Silva
1112
anticline, what is far superior to the record at the neighboring Cerro Chenques outcrops
1113
(Paredes et al., 2015).
1114
Taking into account the comparison made, we consider our robust evidence supports the basal
1115
units of the Chubut Group are pre-inversion units, so the expression syn-contractional rifting
1116
(sensu Gianni et al., 2015a) is only a theoretical geodynamic scenario. Until new information
1117
demonstrates the contrary, the classical tectonostratigraphic context prevails for the Early
1118
Cretaceous to lower Upper Cretaceous evolution of the SBFB. Minors calibrations may be done
1119
like: 1) define the lower Neocomian deposits as a syn-rift climax (Figs. 5.F, 13.A), 2) propose a
1120
coaxial extensional reactivation for the basal deposits of the Pozo D-129 Formation (Figs. 5.F,
1121
11.B) and 3) define a non-coaxial extension phase for the upper Castillo Formation to lower
1122
Bajo Barreal Formation (?) (Figs. 7.A,B , 9.A, 12.D,F). However, no contractional event is
1123
registered during the time spam linked to the deposition of those units.
1124
The upper units of the Chubut Group constituted by Lago Colhué Huapi and Laguna Palacios
1125
formations are linked to a different stratigraphic architecture. The former includes clasts of
1126
Castillo Formation (Casal et al., 2015), while the latter is related to low accommodation
1127
conditions tectonically induced (Allard et al., 2015), cannibalized strata and progressive
1128
unconformities (Figs. 6.F,G) (Gianni et al., 2015a). This field evidence indirectly supports a syn-
1129
inversion context at the SBFB for the upper Chubut Group, but requires further tectono-
1130
stratifraphic analysis to adjust the architecture of both units.
1131 1132
6. CONCLUSIONS
1133
This work synthesizes the compared structural analysis of outcropped and buried reverse-
1134
reactivated faults at the SBFB. The results highlight similarities and differences that adjust the
1135
geometry, kinematic evolution, and controls associated with their positive tectonic inversion.
1136
Among the most important contributions we can mention:
1137
1. The geometry of mayor inversion folds can be classified as single-fault inversion folds or
1138
pop-up growth folds. The latter implies the reverse-reactivation of secondary antithetic normal
1139
faults.
1140
2. Inversion folds have along-strike variability in the degree of inversion. Extreme cases are
1141
blind inverted faults that pass to faulted-folds toward the fault tips.
1142
3. Extensional growth-strata linked to monoclines were inherited by the tectonic inversion
1143
phase and can be confused with syn-inversion growth wedge.
1144
4. The coexistence of overlapped inverted and non-inverted subparallel faults evidence
1145
selective inversion.
1146
5. Partitioned tectonic stress can develop heterogeneous deformation associated with strike-
1147
slip displacement. In this context, a pop-up growth fold can change from poorly vergent to
1148
retro-vergent.
1149
6. Inversion of 3D extensional network favors either oblique inversion or dip-slip displacement,
1150
so deformation partitioning cannot be systematically applied. Fault nodes can correlate with
1151
increased uplift processes, which suggest topological analysis of extensional fabric is essential
1152
to evaluate the inversion architecture.
1153
7. The influence of economic basement fabric in extensional architecture and subsequent
1154
inversion should include the fold train of pre-rift sedimentary sequences. At basin-scale,
1155
crystalline basement paleohigh played a passive role as a regional buttress.
1156
The structural analysis done has evolutionary implications for the Lower Cretaceous
1157
sedimentary record at the SBFB. All the study localities, from the northwestern margin to the
1158
southern boundary of the basin, exposed robust evidence that revalidates the syn-extensional
1159
and post-extensional frameworks during the deposition of the basal sequences of the Chubut
1160
Group (Barremian-Turonian). Our results only justify syn-inversion strata in the Upper
1161
Cretaceous sequences of Laguna Palacios Formation (Santonian-Maastrichtian?). The NW-SE to
1162
W-E normal faults overprinted to the mayor inversion folds were associated to a non-coaxial
1163
extensional phase previous to the tectonic inversion.
1164 1165
7. ACKNOWLEDGMENT
1166
We want to thank YPF S.A. for the access to subsurface information and permission to publish
1167
the results. This work was partially supported by the Universidad Nacional de la Patagonia San
1168
Juan Bosco (PI-CIUNPAT-1323). The Geology Department of the UNPSJB is acknowledged for
1169
the logistic support. We thank Raul Giacosa by the critic lecture of the early version of the
1170
paper. The comments and corrections done by the reviewers Martínez and Manceda improved
1171
the final version of the contribution.
1172 1173 1174 1175 1176 1177 1178 1179 1180 1181 1182 1183 1184 1185 1186 1187 1188 1189 1190 1191 1192 1193 1194 1195
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Paredes, J.M., Foix, N., Allard, J.O., 2016. Sedimentology and alluvial architecture of the Bajo Barreal Formation (Upper Cretaceous) in the Golfo San Jorge Basin: Outcrop analogues of the richest oilbearing fluvial succession in Argentina. Marine and Petroleum Geology 72, 317-335. Paredes, J.M., Aguilar, M., Ansa, A., Giordano, S., Ledesma, M., Tejeda, S., 2018. Inherited discontinuities and fault kinematics of a multiphase, non-colinear extensional setting: Subsurface observations from the South Flank of the Golfo San Jorge basin, Patagonia. Journal of South American Earth Sciences 81, 87-107. Peacock, D.C.P., Nixon, C.W., Rotevatn, A., Sanderson, D. J., Zuluaga, L.F., 2016. Glossary of fault and other fracture networks. Journal of Structural Geology 92, 12-29. Peacock, D.C.P., Nixon, C.W., Rotevatn, A., Sanderson, D.J., Zuluaga, L.F. 2017a. Interacting faults. Journal of Structural Geology 97, 1-22. Peacock, D.C.P., Dimmen, V., Rotevatn, A., Sanderson, D.J. 2017b. A broader classification of damage zones. Journal of Structural Geology 102, 179-192. Phillips, T.B., Jackson, C.A-L., Bell, R.E., Duffy, O.B., Fossen, H., 2016. Reactivation of intrabasement structures during rifting: A case study from offshore southern Norway. Journal of Structural Geology 91, 54-73. Peroni, G., Hegedus, A., Cerdan, J., Legarreta, L., Uliana, M., Laffite, G., 1995. Hydrocarbon accumulation in an inverted segment of the Andean Foreland: San Bernardo Belt, Central Patagonia. In: Tankard, A., Suarez, R., Welsink, H. (Eds.), Petroleum Basins of South America. American Association of Petroleum Geologists Memoir, 62, 403-419. Pezzi, E.E., Medori, R.M., 1972. Proyecto Miembro Pozo LA.1 (Los Alazanes). Área Este Lago Colhué Huapi. Yacimientos Petrolíferos Fiscales, unpublished report. Prosser, S., 1993. Rift-related linked depositional systems and their seismic expression. In: G.D. Williams, and A. Dobb (Eds), Tectonics and Seismic Sequence Stratigraphy. Geological Society, London, Special Publication, 71, 35-66. Ramos, V.A., 2015. Evolución de la cuenca Golfo San Jorge: su estructuración y régimen tectónico. Revista de la Asociación Geológica Argentina 72 (1), 12-20. Ramsay, J.G., Huber, M.I., 1987. The techniques of modern structural geology: Folds and fractures (Vol. 2). Academic press, London. 700 pp. Ranalli, G., and Z.M. Yin, 1990, Critical stress difference and orientation of faults in rocks with strength anisotropies: the two dimensional case: Journal of Structural Geology, 12, 1067-1071. Reeve, M.T., Bell, R.E., Duffy, O.B., Jackson, C.A.L., Sansom, E. 2015. The growth of non-colinear normal fault systems; What can we learn from 3D seismic reflection data? Journal of Structural Geology 70, 141-155. Reilly, C., Nicol, A., Walsh, J., 2017. Importance of pre-existing fault size for the evolution of an inverted fault system. Geological Society, London, Special Publications, 439, 1, 447-463. Robbiano, J.A., 1971. Contribución al conocimiento estratigráfico de la sierra del Cerro Negro, Pampa de Agnia, provincia de Chubut, República Argentina. Revista de la Asociación Geológica Argentina, 26, 1, 41-56. Rodriguez, J.F.R., Littke, R., 2001. Petroleum generation and accumulation in the Golfo San Jorge basin, Argentina: a basin modeling study. Marine and Petroleum Geology 18, 9, 995-1028. Rotevatn A., Peacock D.C.P., 2018. Strike-slip reactivation of segmented normal faults: implications for basin structure and fluid flow. Basin Research 30, 1264-1279. Sanderson, D.J., Nixon, C.W., 2015. The use of topology in fracture network characterization. Journal of Structural Geology, 72, 55-66. Sarhan, M.A., Collier, R.E.L., 2018. Distinguishing rift-related from inversion-related anticlines: Observations from the Abu Gharadig and Gindi Basins, Western Desert, Egypt. Journal of African Earth Sciences, 145, 234-245. Savignano, E., Mazzoli, S., Arce, M., Franchini, M., Gautheron, C., Paolini, M., Zattin, M., 2016. (Un)Coupled thrust belt-foreland deformation in the northern Patagonian Andes: New insights from the Esquel-Gastre sector (41°30–43°S). Tectonics, 35, 2636-2656. SEGEMAR maps. https://sigam.segemar.gov.ar/visor/ and http://repositorio.segemar.gov.ar/ Scholz, C.H., Contreras J.C., 1998. Mechanics of continental rift architecture. Geology; 26, 11, 967-970. Sciutto, J.C., 1981. Geología del Codo del Río Senguerr, Chubut, Argentina. 8° Congreso Geológico Argentino Actas 3, 544–568, San Luis.
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Sciutto, J. C., Martínez, R. D., 1996. El Grupo Chubut en el anticlinal Sierra Nevada, Chubut, Argentina. Actas del 13° Congreso Geológico Argentino y 3° Congreso de Exploración de Hidrocarburos 1, 67-75. Shinn, Y.J., 2015. Geological structures and controls on half-graben inversion in the western Gunsan Basin, Yellow Sea. Marine and Petroleum Geology 68, 480-491. Sibson, R.H., 1989. High-angle reverse faultin in northrn New Brunswick, Canada, and its implications or fluid pressure levels. Journal of Structural Geology 11, 7, 873-877. Sibson, R.H., 1995. Selective fault reactivation during basin inversion: Potential for fluid redistribution through fault valve action. In: J.G. Buchanan and P.G. Buchanan (Eds.) Basin Inversion, Geological Society, London, Special Publication 88, 3-19. Sibson, R.H., 2017. The edge of failure: critical stress overpressure states in different tectonic regimes. In: J.P., Turner, D., Healy, R.R., Hillis, and M.J., Welch (Eds) Geomechanics and Geology, Geological Society, London, Special Publications, 458, 131-141. Somoza R., Zaffarana C.B., 2008. Mid-Cretaceous polar standstill of South America, motion of the Atlantic hotspots and the birth of the Andean cordillera. Earth and Planetary Science Letters 271, 267-277. Suárez, M., Márquez, M., 2007. A Toarcian retro-arc basin of Central Patagonia (Chubut), Argentina: Middle Jurassic closure, arc migration and tectonic setting. Andean Geology 34, 1, 63-79. Suárez, M., Márquez, M., De La Cruz, R., Navarrete, C. and Fanning, M., 2014. Cenomanian-? early Turonian minimum age of the Chubut Group, Argentina: SHRIMP U–Pb geochronology. Journal of South American Earth Sciences, 50, 67-74. Suppe, J., Chou, G.T., Hook, S., 1992. Rates of folding and faulting determined from growth strata. En: McClay, K.R., (Ed.). Thrust tectonics, 105-122. London, Chapman and Hall. Pankhurst, R.J., Leat, P.T., Sruoga, P., Rapela, C.W., M_arquez, M., Storey, B.C., Riley, T.R., 1998. The Chon Aike province of Patagonia and related rocks in West Antarctica: a silicic large igneous province. Journal of Volcanology and Geothermal Research 81, 113-136. Stach, N., 1986. Fotointerpretación de la zona comprendida entre Cerro Ferrarotti, Cañadón Grande y el Codo del Senguer". YPF internal report (unpublished). Sylwan, C., Droeven, C., Iñigo, J., Mussel, F., Padva, D., 2011. Cuenca del Golfo San Jorge. VIII Congreso de Exploración y Desarrollo de Hidrocarburos. In: Simposio Cuencas Argentinas: visión actual. Instituto Argentino de Petróleo y Gas (IAPG), Mar del Plata, 139-183. Telbisz, T., Kovács, G., Székely, B., Szabó, J., 2013. Topographic swath profile analysis: a generalization and sensitivity evaluation of a digital terrain analysis tool. Zeitschrift für Geomorphologie, 1-29. Twiss, R.J., Moores, E.M., 2007. Structural Geology. WH Freeman and Company. New York, 532 pp. Ugarte, F., 1966. La cuenca compuesta carbonífera jurásica de la Patagonia Meridional. Anales de la Universidad Nacional de la Patagonia San Juan Bosco 1, 37-68. Underhill, J.R., Paterson, S., 1998. Genesis of tectonic inversion structures: seismic evidence for the development of key structures along the Purbeck–Isle of Wight Disturbance. Journal of the Geological Society, 155, 6, 975-992. Withjack, M.O., Olson, J., Peterson, E., 1990. Experimental models of extensional forced folds. AAPG Bulletin 74, 7, 1038-1054. Williams, G.D., Powell, C.M., Cooper, M.A., 1989. Geometry and kinematics of inversion tectonics. In: M.A. Cooper and G.D. Williams (Eds), Inversion Tectonics, Geological Society, London, Special Publication 44, 3-15. Williams, G.D., 1993. Tectonics and seismic sequence stratigraphy: an introduction. In: G.D. Williams, and A. Dobb (Eds.), Tectonic and Seismic Sequence Stratigraphy. Geological Society, London, Special Publication 71, 1, 1-13. Wilson P., Gawthorpe R.L., Hodgetts D., Rarity F., Sharp I.R., 2009a. Geometry and architecture of faults in a syn-rift normal fault array: The Nukhul half-graben, Suez rift, Egypt. Journal of Structural Geology 31, 759-775. Wilson, P., Hodgetts D., Rarity F., Gawthorpe R.L., Sharp I.R., 2009b. Structural geology and 4D evolution of a half-graben: New digital outcrop modelling techniques applied to the Nukhul half-graben, Suez rift, Egypt. Journal of Structural Geology 31, 328-345. Wrona T., Fossen, H., Lecomte, I., Eide, C.H., Gawthorpe, R., 2019. Seismic Expression of Shear Zones: Insights from 2-D Convolution Seismic Modelling. EarthArXiv hppts://doi:10.31223/osf.io/5ypzg.
HIGHLIGHTS •
Inversion anticlines are analyzed from outcrop and subsurface data.
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Along-strike variability on inversion degree is highlighted.
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Pop-up growth folds and inherited extensional growth-strata are proposed.
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Basin-scale buttress effect associated with a crystalline paleohigh is discussed.
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Timing of Cretaceous tectonic inversion was calibrated.
S0895-9811(19)30414-6
DOI:
https://doi.org/10.1016/j.jsames.2019.102405
Reference:
SAMES 102405
To appear in:
Journal of South American Earth Sciences
Received Date: 7 August 2019 Revised Date:
28 October 2019
Accepted Date: 28 October 2019
Please cite this article as: Allard, José.Oscar., Foix, Nicolá., Bueti, Sebastiá.Alberto., Sánchez, F.M., Ferreira, Marí.Leonor., Atencio, M., Comparative structural analysis of inverted structures in the San Bernardo fold belt (Golfo San Jorge basin, Argentina): Inversion controls and tecto-sedimentary context of the Chubut group, Journal of South American Earth Sciences (2019), doi: https://doi.org/10.1016/ j.jsames.2019.102405. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
1
COMPARATIVE STRUCTURAL ANALYSIS OF INVERTED STRUCTURES IN THE SAN
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BERNARDO FOLD BELT (GOLFO SAN JORGE BASIN, ARGENTINA): INVERSION
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CONTROLS AND TECTO-SEDIMENTARY CONTEXT OF THE CHUBUT GROUP
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José Oscar Allarda*, Nicolás Foixa,b, Sebastián Alberto Bueti a,b, Federico Manuel Sáncheza, María
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Leonor Ferreiraa,c, Mario Atencioc
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a Universidad Nacional de la Patagonia San Juan Bosco, Ruta Nº 1 S/N, Km 4, 9005, Com. Riv., Chubut, Argentina b CONICET (Consejo Nacional de Investigaciones Científicas y Técnicas), Argentina c YPF S.A. *[email protected] (corresponding author)
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ABSTRACT
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The Golfo San Jorge Basin is one of the most important hydrocarbon-bearing basins of
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Argentina. Its position in the distal Patagonian Broken Foreland confers a particular structural
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architecture. Specifically, the western margin was affected by positive tectonic inversion, with
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higher degrees of inversion at the San Bernardo Fold Belt (SBFB). This paper synthesizes the
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comparative structural analysis of outcropped and buried inverted structures in the SBFB. The
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study areas are regionally distributed, and the geometric characterizations integrate field
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structural data, topographic trends, cutting records, borehole logs, and 2D-3D seismic data.
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The geometries of the studied inversion folds are poorly vergent and can be classified as
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asymmetric anticlines or pop-up growth folds. The architecture of the inverted fault system
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shows: along-strike variability in reverse displacement, inherited extensional growth-strata,
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selective inversion, oblique inversion, and heterogeneous deformation. Pre-inversion mayor
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controls include: 1) an extensional network linked to the fabric of the pre-rift basement, 2) a
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non-coaxial extensional phase during the early Upper Cretaceous times, and 3) a basin-scale
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paleohigh of crystalline rocks located to the east of the SBFB. The kinematic analysis evaluates
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the timing of the tectonic inversion phase, which is actually under discussion. Our results are
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consistent with a tecto-sedimentary context dominated by normal fault reactivations during
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the deposition of the basal units of the Chubut Group (Barremian-Albian), and a reverse-
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reactivation during the record of the Laguna Palacios Formation (late Upper Cretaceous). The
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geometric-kinematic characterization of the inverted structures at the SBFB contradicts many
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of the evidence that supports recently proposed geodynamic models for the Cretaceous
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evolution of Southern Patagonia.
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Keywords: Patagonian Broken Foreland; Tectonic inversion; Inversion degree; Growth strata;
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Pop-up growth folds; 3D deformation
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1. INTRODUCTION
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Sedimentary basin research requires the integration of multiscale and multisource information
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to obtain prosperous analysis and predictive models. In a particular way, the structural
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framework is essential to evaluate the basin spatio-temporal evolution. Specifically, poly-phase
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basins with positive tectonic inversion require a refined calibration in the kinematic evolution
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of the principal structures. In the last decades, important advances have been made in the
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understanding of the drivers and controls of positive tectonic inversion (Cooper and Williams,
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1989; Buchanan and Buchanan, 1995; Bonini et al., 2012; Jackson et al., 2013; Reilly et al.,
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2017; Sibson, 2017; Jagger and McClay, 2018). Nonetheless, there are still many mechanisms
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that need more studies like selective inversion, oblique inversion, along-strike variability in the
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deformation, and fluid migration in inverted structures. The Golfo San Jorge Basin (GSJB),
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located in the Argentinean Patagonia, is a natural laboratory to study positive tectonic
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inversion from outcrop and subsurface data. Specifically, the San Bernardo Fold Belt (SBFB) is a
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region of the GSJB characterized by outcropped and buried inverted structures. In this natural
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system, the timing of the tectonic inversion is essential to evaluate trap generation as well as
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the migration or remigration events. Despite the high-quality of the geologic information of
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the SBFB, there is no consensus about the tectonostratigraphic context of the Cretaceous oil-
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bearing sequences related to the lower Chubut Group (Aptian-Albian). Recent contributions
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proposed contractional scenarios in the western domain of the GSJB (Gianni et al., 2015a,b,
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2016, 2017, 2018a,b; Navarrete et al., 2015; Miller and Marino, 2019), while the traditional
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vision is to link them to extensional-transtensional reactivation followed by thermal
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subsidence (Fitzgerald et al., 1990; Homovc et al., 1995; Peroni et al., 1995; Figari et al., 1999;
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Rodriguez and Littke, 2001; Sylwan et al., 2011). As it is seen, both proposals are antagonistic
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and each one has a different impact on every element of the petroleum system, so calibration
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is required. The general aim of this work is to test the main evidence that supports the
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tectonostratigraphic contexts during the Lower to Upper Cretaceous at the SBFB. In order to
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validate one of the models, we characterized five of the most important anticlines by the
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integration of outcrop and/or subsurface information: Sierra Nevada, Sierra del Castillo,
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Península Baya, Sierra Silva, and Los Perales. Complementary, two key basin positions were
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studied: the northwestern boundary at Ferrarotti locality and the southern boundary at Sur Río
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Deseado depocenter (Fig. 1). The architecture variability of the inverted structures was used to
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address different structural controls and mechanisms poorly understood as: the influence of
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pre-inversion structural anisotropies, oblique inversion, selective inversion, heterogeneous
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deformation, and basin-scale buttress effect. The calibration of the Cretaceous inversion phase
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in the SBFB impacts in the oil industry models and the geosciences studies in Patagonia, while
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the structural models are widely applicable to worldwide similar tectonostratigraphic
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scenarios.
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2. GEOLOGICAL SETTING
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The GSJB is located in the center of the Argentinean Patagonia between 45° and 47°S latitude,
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and 65° and 71°W longitude, it has an extension about 180.000 km2 with one-third located
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offshore (Fig. 1) (Rodriguez and Littke, 2001; Sylwan et al., 2011). The basin has a west-east
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elongate geometry, their borders are to the north the North-Patagonian Region and Cañadón
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Asfalto Basin (Figari et al., 2015), and to the south the Deseado Region (Giacosa et al., 2010).
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The western border is the South Patagonian Andes or the Patagonian Precordillera, this limit is
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poorly-constrained and depends on the age of the sedimentary sequences and the
85
depocenters considered (Fitzgerald et al., 1990; Figari et al., 2015, Miller and Marino, 2019).
86
The eastern basin limit is defined by subsurface information under the South American Atlantic
87
margin (Sylwan et al., 2011). The basin is divided into five different regions according to the
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present structural style. The North Flank, South Flank, and Basin Center are characterized by
89
normal faults with E-W strike, where each region is dominated by dipping directions basinward
90
(Fig. 1) (Figari et al., 1999; Sylwan et al., 2011). The SBFB and Western Flank are identified by
91
positive tectonic inversion features. The former characterized by the domain of contractional
92
structures N-S oriented, and the latter by structures with NNW-SSE strikes with a lower degree
93
of inversion (Fitzgerald et al., 1990; Figari et al., 1996, 1999; Sylwan et al., 2011).
94
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Figure 1. Geological map of the Golfo San Jorge Basin (GSJB) showing the studied areas at the San
97
Bernardo Fold Belt (SBFB). Surface units modified from SEGEMAR maps. Structural domains and
98
structural profiles after Figari et al. (1999) and Sylwan et al. (2011). Rose diagram shows faults strikes of
99
the SBFB classified in inverted and non-inverted faults. Data to the top of Castillo Formation, taken from
100
Figari et al. (1999). Note that bimodal distribution correlates with the structural classification.
101 102
2.1 Stratigraphy
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The structural basement units of the GSJB are exposed in relatively small areas, their outcrops
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are geographically distant, and its lithology exhibit a wide variety of crystalline rocks and
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sedimentary sequences (Fig. 1). The crystalline substrate is constituted by Paleozoic to Triassic
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intrusive and metamorphic rocks, outcropped examples of this suites are Puesto La Potranca
107
Formation and the Sierra Mora granite at the northern boundary (Giacosa, this issue).
108
Overlaying this basement are the Neopaleozoic rocks of the Tepuel Group associated to
109
marine tillites and coastal deposits (Feruglio, 1949; Ugarte, 1966; Fernández Garrasino, 1977),
110
which are affected by sparse volcanic and subvolcanic rocks of Permo-Triassic age (e.g.
111
Maliqueo Formation, Fernández Garrasino, 1977). The Liassic record continues with volcanic to
112
volcano-sedimentary units linked to continental paleoenvironments (e.g. El Córdoba
113
Formation) or sedimentary sequences dominated by shallow marine deposits with a tuff
114
record (e.g. Osta Arena Formation) (Robbiano, 1971; Fernández Garrasino, 1977; Suárez and
115
Márquez, 2007, and references therein).
116
Jurassic volcanic and volcaniclastic successions were developed overlying the structurally
117
deformed basement. These rocks are grouped into several widely accepted lithostratigraphic
118
units according to the volcanic composition, age and the geographic location, so can be
119
defined the Lonco Trapial Group and Marifil Volcanic Complex in the northern limits, whereas
120
in the southern limits the Bahía Laura Volcanic Complex (Pankhurst et al., 1998, and references
121
therein). Clavijo (1986) proposed an informal unit named Complejo Volcánico Sedimentario
122
(CVS), in which are grouped undetermined post-Liassic volcanic and volcaniclastic rocks
123
underlying the Cretaceous sedimentary record (Fig. 2). The lack of hydrocarbon accumulations
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in those units define them as an economic basement for the oil industry (Peroni et al., 1995;
125
Rodriguez and Littke, 2001) (Fig. 2). Continues two groups that include the most important
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units of the GSJB petroleum systems. The lower is informally known as Neocomiano and
127
formally as Las Heras Group (Lesta et al., 1980). The basal unit of this group is Pozo Anticlinal
128
Aguada Bandera Formation, which is dominated by black-shales. This unit pass in transition to
129
Pozo Cerro Guadal Formation composed by siltstones and sandstones, linked to marine and
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transitional paleoenvironments to the west and lacustrine ones to the east (Clavijo, 1986;
131
Figari et al., 1999; Miller and Marino, 2019). Both units are restricted to the subsurface, except
132
for limited outcrops at Puesto Albornoz area at the northwestern bound (Figari et al., 2015).
133
According to Clavijo (1986) and Allard et al. (2018a) these sedimentary sequences are
134
characterized by the absence of a volcanic tuff record. In disagreement, Miller and Marino
135
(2019) mentioned volcanic ashes at the base of Anticlinal Aguada Bandera Formation, but their
136
paper did not show evidence of that sedimentary record. The thicker deposits of these units
137
reach 5400 m at the Sur Río Deseado depocenter, which is located at the southern bound of
138
the GSJB (Fitzgerald et al., 1990). Clavijo (1986) interpreted this Cretaceous interval as a
139
sedimentary cycle limited by regional unconformities, and later it was defined as a
140
megasequence by Figari et al. (1999). Nevertheless, this regional scheme is not validated by
141
Lower Cretaceous concordant seismosequences from the South Flank (Sylwan et al., 2011;
142
Paredes et al., 2018) and the southern region of the SBFB (this work). The stratigraphic column
143
continues with the Chubut Group defined from outcrop and subsurface information (Lesta and
144
Ferello, 1972; Figari et al., 1999; Sylwan et al., 2011), with a maximum thickness estimated in
145
8000 m at the Basin Center (Fitzgerald et al., 1990; Rodriguez and Littke, 2001). The oldest
146
units of this group are Pozo D-129 (Lesta, 1968) and Matasiete formations (Lesta and Ferello,
147
1972). The Pozo D-129 Formation is associated with an underfilled lake that accumulated the
148
black-shales that are the main source rock of the basin, while the Matasiete Formation and Los
149
Alazanes Member (sensu Pezzi and Medori, 1972) are the contemporaneous fluvial-alluvial
150
systems that fed that paleolake (Sciutto, 1981; Paredes et al., 2007; Sylwan et al., 2011; Allard
151
et al., 2015). The Pozo D-129 Formation is constrained by microfossils to Aptian age at its
152
younger sequences in the SBFB (Hechem et al., 1987). A complex puzzle of fluvial and alluvial
153
deposits continues to develop in an endorheic and fluvial dominated basin (sensu Nichols,
154
2012), where the proportions of tuffaceous materials respect to siliciclastic ones allow
155
discriminating four units in outcrops of the SBFB (Feruglio, 1949; Lesta and Ferello, 1972):
156
Castillo Formation (Albian), Bajo Barreal Formation (Coniacian-Turonian), Lago Colhué Huapi
157
Formation (Turonian-Maastrichtian) (Casal et al., 2015) and Laguna Palacios Formation
158
(Santonian-Maastrichtian) (Sciutto, 1981; Bridge et al., 2000). The latter is restricted to the
159
western domain of the basin and it lies in angular unconformity over previous deposits
160
(Sciutto, 1981; Gianni et al., 2015a). The Cenozoic record overlies with a maximum thickness of
161
1000 to 1500 m at the Basin Center, represented by the intercalation of marine and
162
continental sequences and multiple phases of basic intrusions and volcanic activities (Legarreta
163
and Uliana, 1994; Rodriguez and Littke, 2001; Bruni et al. 2008; Sylwan et al., 2011).
164 165
2.2. Structural and tectonic framework
166
The GSJB developed over a complex and polydeformed structural basement which tectonic
167
fabric and deformation events are poorly understood. Recent attempts were made by
168
Navarrete et al. (2016), who described a Lower Jurassic intraplate compressional phase at the
169
SBFB and Río Mayo sub-basin. Beyond the basement history, there is a general agreement to
170
consider the GSJB as an intracratonic basin linked to the Gondwana break-up (Fitzgerald et al.,
171
1990; Figari et al., 1999; Sylwan et al., 2011). The long lifetime of subsidence of this basin
172
responded to different tectonostratigraphic contexts. Jurassic rocks of the CVS record the early
173
phase and climax of the syn-rift (Pankhurst et al., 1998), while Neocomian fault-bounded
174
basins are proposed as a posthumous phase of extension or a late syn-rift (Fitzgerald et al.,
175
1990; Figari et al., 1999; Rodriguez and Littke, 2001; Sylwan et al., 2011). Recently, the
176
Neocomian cycle was divided into two subcycles, the lower Anticlinal Aguada Bandera subcycle
177
related to a syn-rift stage and the upper Cerro Guadal subcycle to the post-rift stage (Miller
178
and Marino, 2019). The paleogeographic reconstruction defined discrete or partially connected
179
Neocomian depocenters with a better development toward the Western Flank and SBFB
180
(Fitzgeral et al., 1990; Figari et al., 1995, 1999; Miller and Marino, 2019). The distribution of
181
these kinematically linked depocenters shows an NNW-SSE trend that suggested a crystalline
182
basement control by Clavijo (1986), Figari et al. (1999), Ramos (2015) and Paredes et al. (2018),
183
but to the moment there is no clear evidence that sustains this hypothesis (Giacosa, this issue).
184
During post-Barremian ages the subsidence migrated eastward, where thermal subsidence was
185
the main control that developed the oval-shaped basin-fill that characterizes the GSJB (Clavijo,
186
1986; Figari et al., 1999). Recently, Dávila et al. (2019) invoked dynamic subsidence as a basin-
187
scale control during Mesozoic and Cenozoic record. The western margin of the Cretaceous
188
GSJB has a controversial structural scenario. The architecture of the SBFB exposes well-
189
developed positive tectonic inversion folds, but the timing of the contractional phases is under
190
discussion. The different proposals are based on the origin of the basal sequences of the
191
Chubut Group (Pozo D-129, Matasiete, and Castillo formations), which determine two
192
incompatible tectonostratigraphic contexts. Traditional schemes consider a extensional
193
reactivation to transtensional setting or the initiation of a thermal phase (Fitzgerald et al.,
194
1990; Homovc et al., 1995; Peroni et al., 1995; Chelotti and Homovc, 1998; Figari et al., 1996,
195
1999; Rodriguez and Littke, 2001; Sylwan et al., 2011); while recent proposals take back the
196
models of Barcat et al. (1989) and Folguera and Iannzzotto (2004) and introduced a syn-
197
contractional context linked to a foreland or a broken foreland setting (Folguera and Ramos,
198
2011; Navarrete et al., 2015; Ramos, 2015; Gianni et al., 2015a,b, 2018a,b). In these scenarios,
199
the tectonic inversion proposal with synchronous extension was interpreted as a syn-orogenic
200
foreland rift by Gianni et al. (2015a,b). Contemporaneously, tectonostratigraphic studies
201
criticized specific surface and subsurface evidence of the syn-inversion scenarios (Paredes et
202
al., 2016; Bueti et al., 2017; Allard et al., 2018b). However, regional models used the
203
contractional proposals as evidence for the construction of geodynamic models for the GSJB
204
(Gianni et al., 2018a,b; Dávila et al., 2019; Miller and Marino, 2019) and neighboring regions
205
and basins (Echaurren et al., 2016; Sevignano et al., 2016; Gianni et al., 2018a,b; Horton, 2018;
206
Dávila et al., 2019). Beyond these discussions, there is consensus that the fold belt was built
207
from a polyphase tectonic inversion with specific events that have different ages, comprising
208
later Upper Cretaceous (Casal et al., 2015; Gianni et al., 2015a,b; Bueti et al., 2017), Paleocene
209
(Paredes et al., 2006; Gianni et al., 2017), Miocene (Peroni et al., 1995; Rodriguez and Littke,
210
2001; Giacosa and Paredes, 2008, among others) and Quaternary (Gianni et al., 2017) phases.
211
212 213
Figure 2. Tectonostratigraphic chart of the San Bernardo Fold Belt region (Golfo San Jorge Basin). The
214
pyroclastic-volcaniclastic input is included.
215 216
3. MATERIALS AND METHODS
217
The study done was based on meso- and macroscale structural data obtained from outcrop
218
and subsurface. Base hillshade maps were constructed from SRTM dataset at 1 arc-second
219
horizontal resolution (~30 m). The topography of the outcropped anticlines was characterized
220
from DEMs using swath profiles (Telbisz et al., 2013 and references therein). Fieldwork
221
consisted of geological mapping at scales of 1:10000 to 1:1000, focused on structural
222
relationships and stratigraphic architecture. Structural measurements and strata orientation
223
were done with traditional Brunton compass and electronic clinometer, so possible errors
224
were minimized (Novakova and Pavlis, 2017). Strata attitude used in maps and sections follows
225
the convention Dip direction/Dip. The structural data were analyzed using the unlicensed
226
academic version of Rod Holcombe software GEOrient 9.5.0. Geometric and kinematic analysis
227
of seismosequences were done using an asymmetric index, which compares the thickness of
228
the analyzed interval at different positions of the structure (Mitra, 1993; Groshong, 2006;
229
Jackson et al., 2013; Allmendinger, 2015). The seismosequences were calibrated with
230
lithological regional markers redefined from the maximum detail that allowed the oil-well
231
cutting record. Calibrations of formation tops proposed in well reports may have implied
232
modifications of tens to hundreds of meters (see details in Allard et al., 2018a).
233 234
4. RESULTS
235
The main inversion structures of the SBFB were previously synthesized in regional structural
236
maps like the ones in Barcat et al. (1984), Stach (1986), Fitzgerald et al. (1990), Homovc et al.
237
(1995), Peroni et al. (1995), Figari et al. (1999) and Sylwan et al. (2011). The bimodal
238
distribution of strikes of map-scale inversion folds correlates with the inverted o non-inverted
239
faults (Fig. 1), but these trends do not include the geometric characteristics and the timing of
240
their construction. The following results synthesize the mayor contributions to cover some of
241
those deficiencies. The study areas are described taking into account their geographical
242
distribution, starting at the northwestern boundary of the SBFB at the Ferrarotti area and
243
finishing at the southern boundary at the Sur Río Deseado depocenter (Fig. 1).
244
Complementary, we realized a short revision of the economic basement fabric.
245 246
4.1. Ferrarotti locality
247
The west boundary of the GSJB is poorly known, and geologic maps refer to Fernández
248
Garrasino (1977) or Stach (1986). Ferrarotti locality has the westernmost exposures of the
249
economic basement and the basal sequences of the Chubut Group in the domain of the GSJB,
250
hence these outcrops are the closest to the Andean subduction margin (Fig. 1). Recently
251
paleogeographic reconstructions defined there the northwestern margin of the Cretaceous
252
record of the GSJB (Allard et al., 2015; Figari et al., 2015). These characteristics confer to the
253
zone a key-architecture for the evaluation of the tectonostratigraphic context of the lower
254
Chubut Group levels associated with Matasiete and Pozo D-129 formations (Allard et al.,
255
2015).
256
Structural architecture. The economic basement includes Neopaleozoic rocks of the Tepuel
257
Group (Fig. 3.A), subvolcanic rocks of Maliqueo Formation, volcanic and volcaniclastic units of
258
the CVS (Fig. 3.B), and Liassic sedimentary sequences (Fig. 3.C). The Chubut Group is
259
represented by fluvial deposits with laterally-continuous green tuff levels used as marker beds
260
(Figs. 3.D,E). The structural architecture of the basement outcrops shows a folded angular
261
unconformity between the Tepuel Group and the Liassic (Fig. 3.E), and folds of different scales
262
with axis strikes with a general trend N-S, tens to hundreds of meters of amplitude and open
263
to tight geometries that occasionally reach limbs with subvertical dips (Figs. 3.C,E,F).
264
Specifically, sandstones and limestones of Liassic sequences show the smaller-scale fold trains,
265
characterized by high frequency harmonic to disharmonic geometries (Fig. 3.C). The Chubut
266
Group levels outcrop to the east of the basement outcrops defining an open syncline, gently
267
dipping toward the south (Figs. 3.D,E,F), measurements close to the basement units increase
268
the dip angles up to 30° (Figs. 3.F, 4.A).
269
270 271
Figure 3. Structural characterization of the Chubut Group and its economic basement. A, B) Southward
272
dipping anticlines developed in Tepuel Group, CVS, and Liassic deposits. C) High frequency folding in
273
Liassic levels. D) Western limb of a gentle syncline developed in Chubut Group sequences. E) Field
274
examples of the Tepuel Group-Liassic folded unconformity, Liassic fold limb with 90° of inclination, and a
275
rotated fluvial channel of the Chubut Group F) Equal-angle plot of strata attitudes (So) measured at
276
Ferrarotti study area. Note the higher deformation of the economic basement respect to the Chubut
277
Group sequences.
278 279
The subsurface relationships were studied from a paper-printed 2D seismic profile, and the
280
main features of the architecture were identified. The southward plunging syncline affecting
281
the Chubut Group levels at surface correlates with a synform pattern of strong reflectivity
282
markers (Figs. 4.A,B). These seismosequences exhibit a wedge pattern limited to the east by a
283
west-dipping fault. Their seismic reflectors at the flexural margin diverge toward the master
284
fault, proving the syn-kinematic clockwise-rotation linked to the fault activity. The strata
285
attitude of outcropped tuff levels of the Chubut Group shows an unconformity of 14° and a
286
divergence toward the east (Fig. 4.C), in correlation with the seismic wedge. The projection of
287
the basement outcrops to subsurface correlates with a wide seismic antiform suggesting a
288
first-order fold. The N-S fold system recognized in outcropped Liassic levels was used to
289
integrate outcrop and seismic data (Figs. 4.A,B). According to Pumpelly's rule, the style and
290
attitude of higher-order folds are similar to lower-order folds (Twiss and Moore, 2007; Frehner
291
and Schmid, 2016). The subsurface antiform also shows a basal unconformity with east-dipping
292
subparallel seismofacies. The latter correlates with the Tepuel Group deposits outcropped at
293
the Ferrarotti lagoon (Fig. 4.A).
294
Interpretation. The structural architecture of the lower Chubut Group is interpreted as a
295
traditional half-graben geometry (Leeder and Gawthorpe, 1987; Prosser, 1993; Williams, 1993;
296
Leeder, 2011). Furthermore, the tectonic control of a basin-margin normal fault is indirectly
297
sustained by high-accommodation fluvial sequences and axial paleocurrents (Allard et al.,
298
2015). The outcrops of the pre-Chubut Group units are associated with a paleohigh that
299
represents a large structure. Hence, the folds affecting the Liassic levels were interpreted as
300
second and third-order folds of a regional-scale feature. The paleohigh architecture was
301
reconstructed as a complex structure where high-frequency folds are associated with
302
subseismic parasitic folds. The paleohigh does not show any record of the Chubut Group, in
303
the case it was eroded, the well-developed neighboring Cretaceous depocenter can be
304
considered as a basinward migration of border-fault (Dart et al., 1995). The unconformity
305
between the economic basement and the Cretaceous sequences, and the syncline that affect
306
the Cretaceous extensional wedge defined at least three tectonic phases: 1) contraction pre-
307
Chubut Group, 2) extension syn-Chubut Group, and 3) contraction post-Chubut Group.
308
Considering the folds are laterally restricted with faults, the contractional phases were
309
interpreted as positive tectonic inversion.
310
311 312
Figure 4. Structural architecture of westernmost outcrops of the Chubut Group and its economic
313
basement at the Ferrarotti study area. A) Simplified geological/structural map showing the economic
314
basement paleohigh, which is linked to a northwest-dipping fault. Rose diagram shows axis strikes of
315
high-order folds. B) Tectonostratigraphic interpretation of a 2D seismic line. C) West limb of the syncline
316
showing east-dipping Cretaceous tuff levels. Correlation with the divergent seismic reflectors evidences
317
extensional progressive unconformities.
318 319
4.2. Sierra Nevada anticline
320
One of the largest outcropped inversion folds in the SBFB is the Sierra Nevada anticline. Its
321
height reaches 1000 m a.s.l. and it is surrounded by Eocene-Miocene basaltic plains (Bruni et
322
al. 2008). The anticline shows an erosional window that exposes almost all the Chubut Group
323
units (Sciutto and Martínez, 1996) (Figs. 5.A,B). We accessed the unmigrated paper-printed
324
version of a seismic profile across the morphostructure (Fig. 5.E). Beyond the limitations of this
325
kind of data (Herron, 2012; Jackson and Kane, 2012), the main seismic packages and structural
326
elements were perfectly identified.
327
Stratigraphic architecture. We used the exploration well YPF.Ch.SN.es-1(I) for the calibration of
328
1D subsurface architecture (Figs. 5.B,C). This well is located over Matasiete Formation levels
329
and it has a final depth of 4364 m. Upper levels are dominated by reddish mudstones, while
330
tuffaceous siltstones and tuffs cutting records were frequently identified between 521 m and
331
1200 m depth (Figs. 5.D), so the upper sequence was assigned to the basal record of the
332
Chubut Group. Black-shales were recognized in the interval 1400 m to 1641 m depth, but we
333
could not determine if they belong to Pozo D-129 or Pozo Anticlinal Aguada Bandera
334
formations. The oil-well report mentions Cyclusphaera palinozone at 2588 m depth and
335
interpreted an Early Cretaceous age (Hauterivian-Barremian), thus we inferred Neocomian
336
levels. Sequences attributed to Matasiete and Pozo D-129 formations show funnel
337
electrofacies tens of meter-thick (Fig. 5.C), which were interpreted as upward-coarsening
338
deposits cycles (Miall, 2016).
339
Using the velocity law from a well located southward, we considered an approximated one-
340
way travel time of ~2.3 km/sec in shallow sedimentary levels and ~3.2 km/sec in levels at 2300
341
m depth. According to these relationships, we estimated the lithostratigraphic units related to
342
the main seismosequences, the fold crest until to ~0.5 sec (TWT) belongs to Matasiete
343
Formation, while the asymmetric wedge until ~1.5 sec (TWT) belongs to Pozo D-129 Formation
344
and Neocomian sequences (Figs. 5.E,F). Underlying levels are dominated by intermediate to
345
basic igneous rocks, which were assigned to the economic basement (CVS) and Cenozoic
346
intrusive rocks. Regional correlation with northern outcrops of Sierra del Cerro Negro
347
supported a volcanic-volcaniclastic economic basement similar to Lonco Trapial Group (Fig. 1)
348
(Robbiano, 1971).
349
Structural architecture. Structural dips obtained from dipmeter-log data show values under
350
30°, the first 700 m depth have tens of meter intervals with increasing-decreasing dip cycles,
351
while the strata attitude between 700 and 1400 m depth is characterized by hundreds of
352
meters of uniform dips locally interrupted by anomalous trends (Fig. 5.C). At a seismic scale,
353
the architecture exposes a well-developed asymmetric fold that correlates with the surface
354
morphostructure (Figs. 5.A,B,F). The west boundary of this fold is marked by an abrupt change
355
in the seismofacies, interpreted as the Sierra Nevada main fault which is west-vergent. The
356
Lower Cretaceous asymmetric wedge has a general geometry that increases the thickness up
357
to 345 % in about 25 km (T1/T10: 3.45, Fig. 5.G). The backlimb of the Sierra Nevada anticline is
358
interrupted by a fault approximately 13 km to the east of Sierra Nevada fault (Fault "A" in Fig.
359
5.F). This is a blind structure with an upthrow to the east and downthrow to the west. The
360
spatial correlation of seismosequences defined the fault-dip is to the west, so a normal fault
361
was reconstructed (Fig. 5.H). The overlying Castillo Formation shows a faulted monocline with
362
synthetic dipping reflectors on the hanging-wall. The Lower Cretaceous seismic wedge
363
continues to the east of Fault "A" and develops a flexure over the tip of east-dipping normal
364
fault (Fault "B" in Fig. 5.F), and the basal sequences are cut by another east-dipping normal
365
fault (Fault "C" in Fig. 5.F). The northeastern extreme of the seismic profile shows lobate
366
seismofacies (Fig. 5.I), which is cut in an N-S direction by SL PBP 94-101 showing clinoforms
367
with southward down-lap stratification (Fig. 5.I).
368
Interpretation. The subsurface tectonostratigraphic architecture of Lower Cretaceous deposits
369
supports a syn-extensional wedge linked to Sierra Nevada fault, locally distort by normal faults
370
without evidence of positive tectonic inversion. The faulted monocline associated with Fault
371
"A" was interpreted as the evolution of an extensional fault-related fold (Khalil and McClay,
372
2002; Wilson et al., 2009a; Coleman et al., 2019a,b; Deng and McClay, 2019). The lobate
373
geoform is associated with transitional sedimentary sequences (Prosser, 1993; Leeder, 2011)
374
and suggests the record of an axial sedimentary system located in the flexural margin of the
375
Sierra Nevada fault. According to Allard et al. (2015) and Figari and Garcia (2018), Sierra
376
Nevada structure conformed a regional sedimentary route for the Matasiete-Pozo D-129
377
sedimentary system, so extensional axial systems correlates with that paleogeographic
378
scenario.
379
380 381
Figure 5. A) Northwestern region of the SBFB indicating the location of 2D seismic profiles. B) Simplified
382
structural map of northern Sierra Nevada anticline. C) Induction log and Dipmeter clusters. D) Cutting
383
record of Matasiete Formation showing the intercalation of green tuff deposits and reddish-mudstones.
384
Numbers indicate depth in meters. E,F) Uninterpreted and interpreted seismic profile SL PBP 94-100
385
across the Sierra Nevada anticline. G) Asymmetry indexes of cretaceous seismosequences. H) West
386
dipping fault zone interpreted from the correlation of high-impedance seismofacies. I) Lobate
387
seismofacies architecture. Arrows indicate the apparent dip direction of the seismosequences. TWT:
388
Two Way Time.
389 390
4.3. Sierra del Castillo anticline
391
The Sierra del Castillo anticline is the southern extension of the Sierra Nevada anticline (Fig.
392
6.A). It has an NNE-SSW strike and a height trend that abruptly decreases southward losing 500
393
m in 12 km, unlike Sierra Nevada range, which has an NNW-SSE strike and a subtle height
394
trend (Fig. 6.B). Sierra del Castillo range is a traditional study area of the SBFB because it has
395
excellent outcrops of the basal sequences of the Chubut Group at the Matasiete canyon (e.g.
396
Paredes et al., 2007). The structural relationships in this area have been studied since the early
397
exploration of the SBFB. Feruglio (1949) interpreted a non-faulted fold, while later studies
398
done by Stach (1986) and Paredes (2009) proposed a faulted fold. Recently, this classical
399
structural section was reinterpreted by Gianni et al. (2015a), who proposed growth-strata and
400
progressive unconformities in Cretaceous sedimentary sequences located in the forelimb. We
401
made a geometric analysis of these exposures and a new geological cross-section is proposed
402
(Fig. 6.C).
403
Structural architecture. The hanging-wall architecture shows a wide and asymmetric anticline
404
that affects the Matasiete Formation and the forelimb has overturned beddings of the lower
405
Castillo Formation (Fig. 6.D). These levels were interpreted as a high-order anticline that
406
refolded the abrupt limb of the inversion fold. Its spatial restriction to positions close to the
407
major fault and the vergence to the west suggest a normal drag fold (Grasemann et al., 2005)
408
linked to a transported fault-propagation fold (McClay, 2011). The footwall architecture has
409
major differences from previous studies; our description shows a forelimb faulted by at least
410
two retro-vergent thrusts and one reverse fault, giving a discontinuous stratigraphic column
411
(Figs. 6.C,E,F,G).
412
The southern extreme of the Sierra del Castillo range is defined by the trace of the emerge
413
Matasiete fault to the west, and by an homocline linked to the backlimb of the inversion fold
414
to the east. This structural scenario is abruptly interrupted near the Matasiete fault-tip by a
415
transverse east-plunging anticline (Fig. 6.A, 7.A). This fold is much smaller than the Sierra del
416
Castillo anticline, it is 1.5 km long, the mayor axis strike is 113°, the abrupt flank dip to the
417
southwest (So: 200°/38°), the gentle one to the northeast (So: 30°/16°) and the interlimb angle
418
is 54°.
419
Stratigraphic architecture. For our study, one of the most significant contributions is the
420
rotation of lagoons deposits contained in the paleosoils sequences of Laguna Palacios
421
Formation (Fig. 6.G). Fine tuffaceous conglomerates were identified in massive to poorly
422
stratified levels of Laguna Palacios Formation. (Fig. 6.G), which were interpreted as
423
cannibalization deposits linked to the tectonic uplift. Fossil wood fragments were exclusively
424
found in deposits above the Cenozoic basal contact (Fig. 6.F,G). This unconformity was linked
425
to the tectonic exhumation of the fossil wood rich levels of Matasiete Formation identified in
426
the Sierra del Castillo anticline by Feruglio (1949).
427
428 429
Figure 6. A) Simplified structural map of the Sierra Nevada-Matasiete fault system. Note the change in
430
the strike of the morphostructure. The satellite image shows a detail of the study area. B) Swath profile
431
showing along-strike maximum heights linked to the uplift trend of the morphostructure. C) Simplified
432
structural section transversal to Matasiete fault and Sierra del Castillo anticline. Stratigraphic thickness
433
of Matasiete and Castillo formations sensu Paredes et al. (2007) and Paredes et al. (2015), respectively.
434
D) Castillo Formation overturned beddings interpreted from fluvial architecture elements. Ch: minor
435
channel, St: trough cross-bedded sandstone, Gt/Gp: trough to planar cross-bedded conglomerate. E)
436
Architecture of the footwall of the Matasiete fault and the forelimb of Sierra del Castillo anticline. F)
437
Reverse fault (F4) cut Laguna Palacios Formation repeating its sequences. Note the progressive
438
unconformities limited by the reverse fault. G) Detail photos, color dots show the location in figure 6.F.
439
Symmetrical ripples indicate horizontal deposition linked to lagoon deposits. Tuffaceous conglomerates
440
were associated with the cannibalization of Castillo and/or Bajo Barreal formations. Fossil wood
441
fragments were interpreted as the unroofing of Matasiete Formation. Reverse fault is a discrete
442
discontinuity without a damage zone.
443 444
Interpretation. The Sierra del Castillo anticline is associated to the Matasiete fault, which
445
correlates northward with the Sierra Nevada fault. Assuming an analogous structural
446
architecture in the subsurface, the Sierra del Castillo anticline is interpreted as an inversion
447
fold of a normal fault that controlled Neocomian sequences and basal deposits of the Pozo D-
448
129-Matasiete sedimentary system. The lagoon deposits in Laguna Palacios Fm. are an
449
unquestionable horizontal marker, so their rotation supports the development of progressive
450
unconformities at the forelimb of the Sierra del Castillo anticline. The reverse fault repeats
451
some of the marker beds of Laguna Palacios Formation, so the progressive unconformities
452
were spatially restricted to the basal outcrops of that unit (Fig. 6.F). The tuffaceous
453
conglomerates and the fossil woods support unroofing sequences linked to the tectonic
454
inversion (Colombo, 1994).
455
The anticline located at the southern extreme of the range has a geometry that suggests a
456
fault-related fold linked to a northeast-dipping blind fault, confirmed by the subsurface
457
architecture in seismic line YBP95-01 (Fig. 7.B). The seismic cross-section shows the fault
458
responsible for the WNW-ESE fold has a similar dip to Matasiete fault, and the interaction
459
between these structures is a branch point. In this context, the map-view topological analysis
460
suggested the high angle intersection is an abutting fault pattern classified as Ya-node (sensu
461
Morley and Nixon, 2016). This fault was correlated with a lineament that cross-cut the
462
forelimb of Península Baya anticline (see next section).
463
464 465
Figure 7. A) Southernmost extreme of Sierra del Castillo anticline. The transverse anticline was linked to
466
the inversion of an E-W normal fault. B) Interpreted seismic profile across southern Sierra del Castillo
467
and Península Baya faults. Note the abutting W-E fault near to Matasiete fault tip. C) Swath profile
468
showing the maximum heights of Península Baya range. Note the anomalous height domain.
469 470
4.4. Península Baya anticline
471
The northwestern extreme of the Musters lake is partially compartmentalized by the Península
472
Baya anticline (Fig. 7.A). This fold is a broad morphostructure 16 km long and ~3 to ~4 km
473
wide, with low-sinuosity limits (Figs. 7.A,C, 8.A).
474
Structural architecture. The northern extreme is partially covered with Quaternary deposits,
475
but outcropped levels show the structural closure with an interference map-pattern with the
476
backlimb of the Sierra del Castillo anticline (Fig. 7.A). The southern extreme of the range is
477
completely different as it shows two straight E-W lineaments, the southernmost of them
478
determines the abrupt end of the outcropped anticline (Fig. 7.A). The anticline is composed of
479
tuffaceous levels of the Castillo Formation, and the Bajo Barreal Formation outcrops are sparse
480
(Fig. 7.A) (Stach, 1986). The central region has an asymmetric structural profile with an abrupt
481
limb to the west (dip range: 50°-80°) and a gentle one to the east (dip range: 15°-7°). The
482
forelimb is composed of concordant tuffaceous fluvial levels, measurements in tabular
483
lithofacies association show a mean So: 246°/66° (N: 12) (Figs. 8.B,C). Higher dip values up to
484
80° or overturned beddings were measured near a shortcut thrust (Figs. 8.B,C,F). A small
485
erosional-window in the forelimb exposes the Matasiete Formation overlaying the Castillo
486
Formation levels through a sharp surface (Figs. 8.C,D). This limit passes laterally to a fractured
487
and silicified tuffaceous level of the Castillo Formation (Figs. 8.D,E,F). The high resistance of
488
this level preserves the fault architecture showing a polished and striated plane with an
489
attitude 25°/50° and striations with rake -78° (Fig. 8.E), so it was classified as dip-slip
490
dominated reverse fault. The western projection of the reverse fault correlates with a west-
491
vergent anticline while the eastern extreme with a high-order symmetric anticline (Figs. 8.B,C).
492
The W-E abutting fault interpreted at the southern zone of Sierra del Castillo anticline
493
correlates with a lineament that distorts the Península Baya forelimb and cuts the trace of the
494
inferred master fault (Figs. 7.A, 8.A,B). The northern structural block of this element is the
495
upthrown side and it has associated sparse outcrops of Bajo Barreal Formation (Figs. 7.A, 8.B).
496
The subsurface architecture of Península Baya anticline is shown in the seismic profile of the
497
figure 7.B. We used the general seismic patterns to evaluate the tectonic inversion
498
architecture and the history of the previous extensional fault. Firstly, the backlimb has a
499
general concordant pattern up to 1100 msec depth. Secondly, the seismosequences C and D
500
are restricted toward the east of Península Baya fault. Specifically, these seismosequences
501
were attributed to the Matasiete-Pozo D-129 sedimentary system. This inference is based on
502
that Península Baya outcrops include the top of the Matasiete Formation (Fig. 8.C), and the
503
neighbor Sierra del Castillo anticline exposes a 650 m thick record of this unit (Fig. 6.C).
504
Interpretation. The Península Baya anticline is linked to a blind inverted fault parallel to the
505
western flank dipping to the east, which was corroborated in the seismic line YBP95-01 (Figs.
506
7.A,B). The non-andersonian dip-angle of the shortcut thrust was interpreted as a post-failure
507
rotation associated with the forelimb evolution (Coward, 1996; Jagger and McClay, 2018). This
508
hypothesis considers a neoformatted structure developed during the inversion fold
509
construction, where the shortcut fault accommodated the contraction when the main
510
inversion fault was totally or partially blocked.
511
Assuming the W-E lineament is the same structure identified at the Sierra del Castillo fault tip,
512
the local fault network has a double connected branch so it can be defined as a C-C branch
513
between a Ya-node and X-node (Sanderson and Nixon, 2015; Morley and Nixon, 2016; Peacock
514
et al., 2016; Duffy et al., 2017). Beyond this topological description, both interaction nodes
515
show structural trends. In this sense, the lineament matches with a height anomaly of the
516
Península Baya range, where the maximum height values are up to 80 m higher respect to
517
neighboring values (Fig. 7.C). Local increase of deformation is evidenced from the reverse fault,
518
a west-vergent fold, and the increase in the structural relief, suggesting a localized higher
519
degree of tectonic inversion (see Discussion).
520
The absence of seismosequences C and D toward the west of the Península Baya fault
521
evidence an important tectonic control during that interval, where only the hanging-wall
522
preserves the sedimentary record. The concordant pattern between seismosequences A and B
523
was interpreted as a homogeneous hanging-wall subsidence and subordinate to absence syn-
524
sedimentary tectonic rotation. The mentioned evidence would suggest the present fault-fold
525
geometry responds to the reverse-reactivation of a previous extensional planar and non-
526
rotational master fault (Scholz and Contreras, 1998; Twiss and Moores, 2007). According to
527
Morley (1995) and Fossen et al. (2010), this pre-inversion geometry can be linked to a graben
528
produced by approaching boundary faults. The abrupt increase in the thickness of the
529
Cretaceous sedimentary sequences was correlated with the eastern limit of the San Bernardo
530
intrabasinal high (sensu Ferello and Lesta, 1973), so it was considered as a first-order fault in
531
the mega-architecture of the SBFB. To the moment, the subsurface hard-link with Matasiete
532
fault is only speculative.
533
534 535
Figure 8. A) Satellite image of Península Baya anticline and southern extreme of Sierra del Castillo
536
anticline. B,C) Forelimb and backlimb architecture distorted by a shortcut thrust and a transverse
537
lineament. Figure 8.C includes equal-angle stereonet plot of the forelimb beds. D) Fault architecture
538
developed on silicified tuffaceous strata of the Castillo Formation E) Cataclastic breccia at fault core. The
539
detail shows a thrust plane, which rake values evidence a subordinate lateral movement component. F)
540
Overturned bedding linked to the shortcut thrust and an abutting W-E lineament.
541 542
4.5. Sierra Silva anticline
543
The Sierra Silva anticline is one of the most emblematic ranges of the SBFB situated between
544
the Musters and the Colhué Huapi lakes. Its general architecture was early characterized by
545
Moore (1960, in Ferello, 1964) and Ferello (1964), later refined by Sciutto (1981), Stach (1986)
546
and Giacosa and Paredes (2008).
547
Structural architecture. The morphostructure has an irregular shape with a west bound
548
partially curved, while the east limit may be divided into two domains, one relative straight
549
and the other conformed by several triangular-shaped outcrops (Figs. 9.A,B,C). The main fault
550
is located to the west of the forelimb as a blind fault that emerges in the southern extreme
551
(Fig. 9.C). Second-order faults with strike-slip and dip-slip movements were interpreted from
552
the lineaments that segment the topography and develop anomalous valleys (Figs. 9.A,D).
553
Lineaments a, b, c, e, and f have abutting patterns with the Sierra Silva fault trace what suggest
554
Ya-nodes (sensu Morley and Nixon, 2016), while cross-cutting relationship of lineament h
555
evidence X-node (sensu Sanderson and Nixon, 2015). The northern extreme of the Sierra Silva
556
anticline shows a double hinge geometry with a poorly developed asymmetry with limbs dips
557
under 30° (Figs. 9.A,B). The north limit of the morphostructure is the NW-SE Cerro Chenques
558
fault, which was interpreted as a sinistral fault (Sciutto, 1981; Stach, 1986, Barcat et al., 1984,
559
1989; Peroni et al., 1995; Giacosa and Paredes 2008) or an oblique inverted normal fault
560
(Allard et al., 2018b), while Gianni et al. (2015a) omitted this discontinuity (see Discussion).
561
562 563
Figure 9. A) Geological map of Sierra Silva range and schematic 1D log from YPF.Ch.SS.es-1. TVD: true
564
vertical depth. T: thickness. B) Detail of the northern extreme of Sierra Silva anticline limited by the
565
Cerro Chenques fault. C) Detail of the southern extreme of Sierra Silva anticline. Note that Sierra Silva
566
fault pass from a blind to a surface-cut fault. D) Swath profile showing the lineaments and the maximum
567
heights. Note the uplift trends. E) Field expression of the Sierra Silva fault trace. The faulted inversion
568
fold was inferred from the buried forelimb exposed near the fault tip.
569 570
The Castillo Formation shows two tabular sequences, while the Matasiete and Pozo D-129
571
formations are exposed in the fold core with a complex structural architecture defined by an
572
overturned fold and north-vergent shortcut thrusts (Figs. 9.B, 10.A). The backlimb of Sierra
573
Silva anticline is refolded as a monocline and high-order syncline and anticline (Figs. 9.B, 10.B).
574
The Cerro Chenques fault is a northeast dipping structure (Figs. 9.A,B) with important
575
differences between the structural blocks. The footwall shows the symmetric Sierra Silva
576
anticline while the hanging-wall exposes an asymmetric fold named Jerez anticline, which has
577
an abrupt limb to the east with dip values up to 75°-85° (Figs. 9.A,B, 10.A). Thus the Jerez
578
anticline is east-vergent while the Sierra Silva anticline has a poorly vergence to the west. The
579
sinistral strike-slip displacement of the Cerro Chenques fault was estimated using the inferred
580
traces of Sierra Silva and Jerez faults, showing a differential displacement with 1200 m in the
581
west-limb and 2000 m in the east-limb (Fig. 9.B).
582
583
584
Figure 10. Architecture of the northern extreme of Sierra Silva range. Structural data in profiles
585
represents mean local measurements. A) Photomosaic and structural interpretation of the front-view of
586
the Cerro Chenques fault. Equal-angle plots show the footwall has lower dispersion than the hanging-
587
wall. The Jerez anticline is an asymmetric double-hinge fold. Modified from Bueti et al. (2017) and Allard
588
et al. (2018b). B) Eastern extreme of the Sierra Silva anticline showing the backlimb architecture. Note it
589
is refolded with a monocline and a high order syncline. The monocline is interpreted as a propagation-
590
fold of the antithetic Jerez fault. Based on Bueti (2019). Location of structural profiles in Fig. 9.B.
591 592
The subsurface architecture of Sierra Silva anticline was evaluated from the calibrated column
593
of the well YPF.Ch.SS.es-1 (Fig. 9.A) and its correlation with the seismostratigraphic sequences
594
in a 2D seismic profile (Fig. 11.A,B). The YPF.Ch.SS.es-1 well shows at ~800 m depth an
595
anomalous trend in subsurface dip-log data, which was correlated with the Jerez fault (Fig.
596
11.C). The seismosequences of Pozo D-129 Formation show a thickness increase of ~34 %
597
toward Sierra Silva fault. Seismic architecture of the Cerro Chenques fault is different as it
598
shows a major anticline and a minor localized anticline, the former linked to low inversion
599
degree and the latter to oblique compression (Fig. 11.D). The strike-slip displacement
600
component of the Cerro Chenques fault correlates with outcrop-scale positive flower
601
structures at its trace (Fig. 11.E) (Paredes, 2009).
602
Interpretation. The monocline at the backlimb of the Sierra Silva anticline was interpreted as a
603
propagation fold of the Jerez fault. Thus, we interpreted a pop-up structure defined by the
604
Sierra Silva master fault and its antithetic Jerez fault. The thickness increase of Pozo D-129
605
Formation was associated with a syn-extensional wedge linked to the coaxial reactivation of
606
the Sierra Silva master fault.
607
608 609
Figure 11. A) Seismic profile across-strike to Sierra Silva anticline. Taken from Gianni et al. (2018a). B)
610
Seismic interpretation calibrated with lithostratigraphic limits defined from cutting record description.
611
Units thicknesses follow figure 9.A. Note the pop-up geometry. C) Upward trend in strata inclination
612
obtained from dipmeter log. The anomalous trend at 800 m depth was interpreted as a reverse fault. D)
613
Seismic Line 23088 showing the Cerro Cheques fault is a north-dipping normal fault with a low degree of
614
inversion. Based on Allard et al. (2018b). E) Outcrop-scale positive flower and north-vergent shortcut
615
thrusts exposed on the trace of the Cerro Chenques fault.
616 617
4.6. Los Perales anticline
618
One of the most important structural oil-traps of the subsurface SBFB is the Los Perales
619
anticline (Hechem, 2015). Published studies were done by Barcat et al. (1989), Homovc et al.
620
(1995), Figari et al. (1999), Gianni et al. (2015a,b), and Allard et al. (2018b). We revisited this
621
structure, analyzing 3D seismic data tied to regional markers (Fig. 12.A).
622
Structural architecture. Like many other structures of the SBFB, Los Perales anticline is cross-
623
cut by a series of NW-SE faults that give a partially compartmentalized anticline. General
624
architecture in across-strike profile shows a faulted-fold, which is limited in the forelimb by the
625
principal inverted fault and in the backlimb by a secondary antithetic reverse fault, what gives
626
a rounded, high-amplitude, double-hinge symmetric fold (Figs. 12.B,C). The amplitude and the
627
half wavelength of the main inversion fold changes along strike evidencing that the fold is non-
628
cylindrical. The structural map at the top of Mina del Carmen Formation (subsurface
629
equivalent to Castillo Formation) shows a fold that can be subdivided into two subparallel
630
double-plunging folds (Fig. 12.A). The seismostratigraphic characterization was done using
631
seismic profiles approximately parallel (cross-line) and perpendicular (in-lines) to the Los
632
Perales anticline and fault. Across-strike seismic profiles show the Cretaceous sequences of the
633
Chubut Group are dominated by subparallel reflectors without internal unconformities. Only
634
the bottom quarter of the Pozo D-129 sequence in the X-Line 750 has a wedge geometry that
635
narrows toward the Los Perales fault. This relative thin interval was interpreted as counter-
636
clockwise rotated on-laps (see Discussion). The asymmetric indexes between structural blocks
637
show values a little bigger than one, with thinner sequences in the footwall (Inline 750, T5/T6:
638
1.05; Inline 1234, T4/T5: 1.05); while in the hanging-wall show trends from 1.35 (Inline 1234,
639
T1/T3) to 1.03 (Inline 750, T4/T5), with decreasing thickness toward the Los Perales fault (Fig.
640
12.B,C). Along-strike Cretaceous architecture shows important geometries. The deeper
641
reflector packages were attributed to Neocomian units; they have concave-up geometry with
642
an internal pattern with seismic reflectors subparallel to gently divergent toward the central
643
part of the fault (Fig. 12.D). The Chubut Group sequences show key patterns in the along-strike
644
profile to evaluate the kinematic evolution of the structure (Fig. 12.D). Most important are the
645
thickness increase toward the fold crests (T10/T9: 1.23) and the divergence of folded
646
unconformities in that direction. This profile also shows normal faults cross-cutting the
647
anticline with a local increase of hanging-wall thickness in seismosequences of the Castillo
648
Formation (T11/T12: 1.48).
649
Interpretation. All the mentioned characteristics supported a reverse-reactivation of the Los
650
Perales fault after the deposition of the Castillo Formation. At the same time, the correlation
651
of the crest of the Los Perales anticline with the thicker thickness of the Neocomian
652
seismosequences suggests a relationship between the normal fault slip pattern and the
653
inversion degree (see Discussion).
654
A neighboring fault located 9 km to the east of the Los Perales fault shows a contrasting
655
architecture (Fig. 12.E). The fault is antithetic and divergent to the Los Perales fault, and it has
656
an upper tip located under the Chubut Group seismosequences. The Neocomian wedge
657
denotes extensional control linked to a surface-break fault with synthetic dipping units
658
(Prosser, 1993; Gawthorpe et al., 1997; Wilson et al., 2009a; Leeder, 2011; Deng and McClay,
659
2019). The overlying seismosequences abruptly develop a monocline interpreted as a forced
660
fold (Whithjack et al., 1990; Coleman et al., 2019a,b) generated by the normal fault
661
reactivation. The relationship fold amplitude/width (fold-shape-factor sensu Coleman et al.,
662
2019a) changes vertically, from a low fold-shape-factor at Pozo D-129 Formation to a high fold-
663
shape-factor at Mina del Carmen Formation. Both intervals show a similar fold width, so the
664
difference is in the fold amplitude. This geometric characteristic suggests a fold growth with
665
the width established early, and amplitude increased with ongoing normal fault slip (Coleman
666
et al., 2019a). The timing of the fold construction was defined from the seismic architecture.
667
The Pozo D-129 Formation has parallel reflectors and a tabular geometry with asymmetry
668
indexes close to 1 (T1/T2: 1.09; T2/T3: 1.08, Fig. 12.F), so a displacement quiescence period of
669
the fault was interpreted. The Castillo Formation has subtle but very important differences
670
respect the previous unit: 1) the hanging-wall sequences are up to 20 % thicker than footwall
671
ones (e.g. T5/T4: 1.2, Fig. 12.F) and 2) the monocline axis preserves the thinner thickness of
672
the unit and stratigraphic on-laps. This evidence supported a blind fault extensional
673
reactivation during the Castillo Formation deposition.
674
675 676
Figure 12. Tectonostratigraphic architecture of the Los Perales anticline. A) Time structure map (msec
677
TWT) of the top of Mina del Carmen Formation. B, C) Interpreted seismic profiles across the fold axis. D)
678
Interpreted seismic profile along the fold axis. E) Architecture of selective fault reactivation during
679
positive tectonic inversion. F) Architecture of the non-inverted fault. TWT: Two Way Time.
680 681
4.7. Sur Río Deseado depocenter
682
The south-limit of the GSJB has a long-lived depocenter, which importance was mentioned by
683
Clavijo (1986) and later described from seismic data by Fitzgerald et al. (1990). We evaluated
684
the architecture of this zone using two 2D seismic profiles, one parallel and one transversal to
685
the master fault. The seismic data was tied with two wells drilled in that area (YPF.Ch.SRD.x-2,
686
YPF.Ch.SRD.es-1), which tectonostratigraphic limits were calibrated from our cutting record
687
revision. Specifically, YPF.SC.SRD.x-2 overcomes 4000 m depth, and the interval from 2358 m
688
to bottom-depth is dominated by black-shales deposits of the Pozo Anticlinal Aguada Bandera
689
Formation. Interested readers see Allard et al. (2018a) for further details.
690
Structural architecture. The economic basement substrate was inferred in the N-S seismic
691
profile from chaotic to poorly stratified seismofacies dipping to the south, over which the
692
Neocomian reflectors show down-lap patterns toward the main fault (Fig. 13.A). The internal
693
architecture of the Neocomian wedge is complex near the master fault, with convex and
694
concave geometries, truncations and off-lap patterns, suggesting active border deposits
695
(Prosser, 1993; Gawthorpe and Leeder, 2000; Leeder, 2011; Henstra et al., 2017). The
696
asymmetry index of the wedge was estimated in 2,3, calculated from the economic basement
697
to the top of Pozo Cerro Guadal Formation (Fig. 13.A). A key structural element is a concordant
698
relationship between Pozo Cerro Guadal and the Pozo D-129 formations, and their dip
699
northward. The E-W seismic section shows subtle but not less important patterns. The main
700
structure is a gentle and wide syncline that affects the upper seismosequences of the
701
Neocomian and Pozo D-129 Formation. The internal architecture of Pozo Cerro Guadal
702
Formation has a thickness increases of ~30 % where underlying Neocomian reflectors converge
703
(T1/T2: 1.3, Fig. 13.B). Seismosequences of the Pozo D-129 Formation show relatively small
704
variations in thickness, with the maximum thickness located in the sine of the syncline, and
705
asymmetry index values from 1.2 to 1.3 (Fig. 13.B). This interval can be divided into two
706
packages, the lower much more asymmetric with values that double in the syncline sine and
707
show on-lap pattern, while the upper has a tabular geometry.
708
Interpretation. The mentioned architecture gives important tectonostratigraphic signals. The
709
opposite dip direction between the base and top of the Neocomian wedge cannot be
710
explained only with half-graben displacements patterns since the zone of maximum
711
subsidence is close to the fault (Barr, 1987; Leeder and Gawthorpe, 1987; Morley, 1995,
712
among many others). At the same time, the structural level of the deposits near the fault
713
overcoming the ones in the flexural margin evidence the differential upward-direction slip of
714
the hanging-wall and the clockwise rotation of all the reflectors packages. So, these geometries
715
were interpreted as evidence of a low-degree positive tectonic inversion of a rift-margin fault.
716
The concordant limit between the Chubut and Las Heras groups differs from the traditional
717
megasequences scheme of Figari et al. (1999). Furthermore, the intra-Neocomian
718
unconformities were linked to the local evolution of the half-graben, specifically to the
719
extensional counter-clockwise rotation of the flexural margin (Morley, 1990; Proser, 1993;
720
Leeder, 2011). Following the models of Morley (2002), the along-strike thickness trends in the
721
Pozo D-129 Formation were interpreted as a late normal fault reactivation with a fault trace
722
contraction.
723
724 725
Figure 13. Tectonostratigraphic architecture of the Sur Río Deseado depocenter. A) Seismic profile
726
across-strike the master fault. B) Seismic profile along-strike the master fault. Vertical scales in TWT:
727
Two Way Time (msec).
728 729
4.8. Economic basement
730
The heterogeneous spatial distribution and varied composition of the economic basement
731
units hinder the evaluation of its structural fabric. Recent studies analyzed 2D and 3D seismic
732
data and proposed different contractional events for the deformation of the economic
733
basement (Navarrete et al., 2015, 2016; Gianni et al., 2018b). We test the calibration of some
734
of the wells and seismic profiles used in those proposals to validate the economic basement
735
architecture. The results show important problems associated with the stratigraphic
736
calibration. A synthesis of the main observations follows.
737
Navarrete et al. (2016) studied the fabric of the pre-Cretaceous substrate at the eastern limit
738
of the SBFB, in an area located to the east of the Colhué Huapi lake. Their figure 8 shows the
739
raw information of crossline 2625 where the YPF.Ch.LA.x-2 does not reach the crystalline
740
basement seismofacies (sensu Foix et al., this issue). What is more, in their
741
tectonostratigraphic scheme, the basal sequences of the well are attributed to stratified
742
seismofacies interpreted as folded Liassic sequences. The well-report of YPF.Ch.LA.x-2 records
743
36 m of granitic rocks at the bottom depth, so the seismic calibration should have included the
744
crystalline basement. It has to be mention that the basement top in YPF.Ch.LA.x-2 correlates
745
with LA-1 A, from which a core of mica-schist was extracted, so there is no doubt of the rock
746
type (Fig. 14.A,B). A similar problem was identified at well YPF.Ch.PGS.x-1 (Fig. 14.A), which
747
according to the well-report and our cutting revision drilled at least 22 m of granitic rocks (Fig.
748
14.C), but no crystalline basement is proposed in the calibrated seismofacies shown by
749
Navarrete et al. (2016, their Fig. 6).
750
751 752
Figure 14. A) Satellite image showing the location of the revisited wells used to review the calibration of
753
basement seismofacies from previous studies. B) File of the well LA-1 where it is mentioned the
754
extraction of a core of low-grade metamorphic rock. C, D) Cutting photographs of granitic rocks and
755
black-shales used for the evaluation of the seismic basement calibrations. Numbers indicate depth in
756
meters.
757 758
The mentioned contribution also justify Upper Jurassic sequences (Lonco Trapial Group) from
759
the subsurface stratigraphy proposed in the well-report of the YPF.Ch.CBo.x-1 (Navarrete et
760
al., 2016, their Fig. 6). Our cutting calibration does not agree with the lithostratigraphic limits
761
proposed in that well-report. We redefined the units and discarded the presence of the Lonco
762
Trapial Group because we did not recognize any volcaniclastic and volcanic rocks compatible
763
with that unit (Foix et al., this issue). Taking into account the observations made, the
764
unconformities exposed in the mentioned seismic profiles of Navarrete et al. (2016) have to be
765
rethought and link them to the folding of Neocomian sequences. In that case, these
766
relationships are analogous to the Bajo Grande Unconformity, where the Baqueró Group
767
(Aptian) overlies the Bajo Grande Formation (Late Jurassic to Hauterivian) in the Deseado
768
Region (Giacosa et al., 2010; Gianni et al., 2018a,b). Further to the west, at Río Mayo sub-
769
basin, the seismic calibration using deep wells also shows problems. Navarrete et al. (2015,
770
their Fig. 8) used the 2D seismic line 7490 calibrated with the YPF.Ch.CA-x.1 and interpreted
771
the Lonco Trapial Group (Upper Jurassic). We revisited the YPF.Ch.CA.x-1 cutting record and
772
the bottom sequences were attributed to black-shales of the Neocomian Katterfeld Formation
773
(Fig. 14.D). In consequence, the seismosequences were wrongly attributed to the Lonco Trapial
774
Group, which implies the correlated seismofacies must be corrected.
775
The short revision of the study cases demonstrated the necessity of a workflow with robust
776
subsurface stratigraphic limits from cutting records previous to seismic interpretations. Until
777
new contributions revalidate the mentioned seismic architecture, those papers cannot be used
778
to evaluate the economic basement fabrics, and in consequence, neither their influence in
779
Cretaceous extensional structures (see Discussion).
780 781
5. DISCUSSION
782 783
5.1. Architecture variability and controls in tectonic inversion
784
5.1.1. Pop-Up growth folds and inherited extensional growth strata
785
The absence of a clear vergence in the SBFB was notice by Peroni et al. (1995), who referred to
786
it as an accordion system. Almost all the studies performed in this region described the
787
dominance of box-shaped anticlines (Homovc et al., 1995; Peroni et al., 1995; Figari et al.,
788
1999). All the aforementioned results show the geometric variability of inversion folds, from
789
the classic broad asymmetric anticlines to the narrow double hinge ones, and an open
790
syncline. According to the paleomagnetic study done by Somoza and Zaffarana (2008), the
791
mid-Cretaceous rocks of the SBFB show the absence of vertical-axis rotations associated with
792
tectonic inversion, so the present inversion fault traces can be considered similar to
793
Cretaceous extensional ones. From a dynamic point of view, the narrow inversion anticlines
794
cannot be associated to a lateral-strike tectonic regime as previously proposed by Barcat et al.
795
(1984) because: 1) the main structures are orthogonal to highly oblique to regional maximum
796
principal stress during the late Upper Cretaceous and Cenozoic times (Müller et al., 2016) and
797
2) folds are not linked to restrending bends or anti-dilatant oversteps. As a consequence, the
798
architecture of the SBFB has to be explained by inversion mechanisms and controls.
799
Geometries of Sierra Silva and Los Perales anticlines are partially analogous as their general
800
architectures are defined by a principal inverted fault and a secondary antithetic reverse fault.
801
Figari et al. (1999) interpreted the Los Perales antithetic fault as a reverse structure
802
neoformatted during Cenozoic contractional phase, but no geometric or kinematic justification
803
was exposed. The absence of well-developed harpoon anticlines is clearly different from
804
traditional inverted geometries (McClay and Buchanan, 1992; Granado and Ruh, 2019). The
805
nearness and overlap of master and secondary reverse faults evidence a contractional
806
transference zone similar to fold and thrust belts (Ramsay and Huber, 1987; McClay, 1992;
807
Higgins et al., 2007), but the pre-inversion history is not obvious. Neither of the two faults has
808
andersonian dip-angles near to ~30° (Sibson, 1989; Collettini and Sibson, 2001), what is
809
predictable for the main one in an inversion scenario (Sibson, 1995; Bonini et al., 2012) but
810
incompatible for an antithetic neoformatted structure, which in that case are expected to have
811
a thrust geometry under andersionian conditions (Marquez and Nogueira, 2008). The
812
tectonostratigraphic analysis done shows that the antithetic faults have no control during the
813
sedimentation of the Castillo Formation (Fig. 10) or its subsurface equivalent Mina del Carmen
814
Formation (Figs. 12.B,C). These schemes support the nucleation and growth of post-
815
sedimentary normal faults, coaxial with the master fault (Fig. 15). Later dip-linkage gave the Y-
816
shaped geometry of the normal fault system (Nicol et al., 1995), which was inherited by the
817
inversion phase. In this model, the weakness linked to the antithetic fault is associated with
818
the pre-inversion normal fault, which was resheared during tectonic inversion (Fig. 15). This
819
model differs from the Type-2 folds of Shinn (2015), which proposed reverse faults
820
neoformatted. In consequence, the pop-up geometries of inversion folds in the SBFB are
821
interpreted as pop-up growth folds in the sense of Cartwrigth (1989). The fault obliquely to the
822
Los Perales anticline is synsedimentary with Mina del Carmen Formation seismosequences, so
823
it supports a non-coaxial phase of extension during Albian times with no seismic evidence of
824
later reverse-reactivation. This kinematic evolution may be attempted to replicate in outcrops
825
of the Sierra Silva anticline trough the transverse faults. Therefore, the geometric and
826
kinematic evidence supported the selective tectonic inversion of a multiphase 3D extensional
827
fault network (see Section 5.3).
828
829 830
Figure 15. Schematic model for the construction of Sierra Silva pop-up growth fold. Compressional
831
phase reshear both high angle faults giving a pop-up geometry. BB: Bajo Barreal Formation, LCH: Lago
832
Colhué Huapi Formation.
833 834
Normal faults with multiple pulses of displacement can change from blind to emergent faults,
835
what is traduced in changes in the geometry of syn-sedimentary sequences (Gawthorpe et al.,
836
1997; Coleman et al., 2019a,b). Seismic architecture from the non-inverted fault near Los
837
Perales anticline (Fig. 12.E,F) was used to elaborate a geometric and kinematic model that
838
explains extensional inherited growth-strata geometries in inverted faults (Fig. 16). The
839
monocline generated by a normal coaxial reactivation shows ongoing deformation that folds
840
the post-extensional tabular intervals linked to fault quiescence and overlying extensional
841
growth-strata. Later tectonic inversion inherits the across-fault thickness differences, which
842
are coupled with syn-inversion growth-strata. The magnitude of the reverse-reactivation, as
843
well as the sedimentary rate, control the relationships between pre- and sin- inversion
844
geometries. During the initiation of the tectonic inversion, subtle evidence of contraction
845
mechanisms can be inferred from buttress folds or reverse drag folds that affect the syn-rift
846
wedge. This simple 2D model can be affected by previously described pop-up mechanisms,
847
increasing the complexity of the inherited extensional growth-strata and hindering its
848
recognition. Along-strike structural sections are determinants to evaluate the tectonic phase of
849
the growth-strata, being the thickness increase toward the fold crest a doubtless evidence of
850
an inherited extensional pattern.
851
852 853
Figure 16. Sequential model for inherited growth-strata architecture. Positive tectonic inversion is
854
subdivided into three stages. Initiation only expresses subtle contractional evidence like buttress folds in
855
the syn-rift wedge. Low and moderate inversion degrees are differentiated from the faulted inversion
856
fold and the occurrence of a null-point (not shown is the sketch).
857 858
5.1.2. Oblique inversion and heterogeneous deformation
859
The detailed study of the inverted structures in the SBFB allowed evaluating oblique tectonic
860
inversion and heterogeneous shortening. Both characteristics are well expressed at the
861
northern limit of the Sierra Silva anticline. The Cerro Chenques fault has ~N130° strike what
862
gives a 40° acute angle respect to the regional Upper Cretaceous to Cenozoic E-W shortening
863
direction proposed by Müller et al. (2016). This scenario justifies a stress partition into strike-
864
slip and contractional components (Letouzey et al. 1990; Bayona and Lawton, 2003; García-
865
Lasanta et al., 2018). The former shows 800 m of differential movement among the Sierra Silva
866
and Jerez faults traces (Fig. 9.B). This kinematic difference was associated with a mayor
867
shortening in the hanging-wall of the Cerro Chenques fault, which is manifested by higher
868
rotation of the forelimb of the Jerez anticline respect to the backlimb of Sierra Silva anticline.
869
This scenario was related to heterogeneous deformation. Figure 17.A shows a simplified
870
sketch, and it is compared with the expected architecture under homogeneous deformation
871
(Fig. 17.B). In this model, along-strike variability of fault strain is also expected, which should
872
have affected the fault architecture.
873
The Sierra Nevada and Sierra del Castillo anticlines were correlated to a mayor lineament
874
interpreted as a Cretaceous master normal fault, which surface expression is the Matasiete
875
fault. Along-strike change from a blind to an emergent structure evidence the modification of
876
the inversion-mechanism from propagation fold to a faulted fold, that evidence variability in
877
the degree of tectonic inversion (Fig. 17.C). The trace of the main inversion fault has a strike
878
break from NNW-SSE to N-S, suggesting collinear hard-linked normal faults segments (Jackson
879
et al., 2002; Wilson et al., 2009b; Leeder, 2011; Fossen and Rotevatn, 2016, among others).
880
That zone has the west-vergent forelimb and it is related to the unroofing of Matasiete
881
Formation during Cenozoic times, both characteristics support a local increase in the
882
deformation. At the same time, the truncation of the inversion anticline by the inverted
883
master fault denotes the inverted normal fault overcame the null-point (Cooper and Williams,
884
1989; Williams et al., 1989; Bonini et al., 2012; Reilly et al., 2017). This architecture can be
885
explained as an easier reshear linked to the fault architecture (Jackson et al., 2013), or the
886
lateral propagation toward the normal fault tip where the extensional displacement is minor,
887
so the null-point is achieved with lower reverse-reactivation (Fig. 17.C). This model can also be
888
applied to the southern extreme of the Sierra Silva anticline where the master fault emerges.
889
Southward to these morphostructures, the map-view of the Los Perales fault-fold system
890
shows the inversion anticline is linked to a sinuous fault trace and two high-order subparallel
891
braquianticlines (Figs. 12.A,D). This architecture was interpreted as a consequence of the
892
inversion of two normal faults left-lateral overlapped and hard-linked.
893
The Sur Río Deseado master fault has a strike subparallel to the Upper Cretaceous and
894
Cenozoic Andean compression, giving an unfavorable orientation for the tectonic inversion
895
(Letouzey et al., 1990). Nonetheless, the seismic architecture exposed in section 4.7 evidence
896
the reactivation with a dominance of reverse slip. Following the numerical modelling of
897
Granado and Ruh (2019), the tilting of the syn-rift half-graben supports the presence of a weak
898
inherited fault as an inversion control. In all the mentioned scenarios, high fluid-pressure
899
should have favored the reshear of the master faults (Sibson, 2017).
900
901 902
Figure 17. A,B) Alternative models for oblique inversion of a complex 3-D fault network. The
903
deformation is partitioned with a lateral component at the NNW-SSE fault. A) The heterogeneous
904
deformation implies the lateral movement combined with the rotation of the forelimb So and the
905
antithetic fault. B) The homogeneous deformation implies comparable strike-slip displacement. C)
906
Along-strike variability on inversion degree linked to inherited normal fault segments with different
907
displacement patterns. The transfer zone has a higher deformation.
908 909
5.1.3. Tectonic inversion of extensional nodes
910
The inversion architecture of complex normal fault arrays is understood from natural examples
911
(e.g. Kelly et al., 1999; Imber et al., 2005; Giacosa et al., 2010). In our case, the Lower
912
Cretaceous structures interact with a fault system with general trend W-E, linked to a non-
913
coaxial extensional phase. According to Los Perales architecture, it was syn-sedimentary to the
914
Mina del Carmen Formation, which is analogous to the results of Paredes et al. (2013) in Cerro
915
Dragón oil-field, at the eastern margin of the SBFB. The structural architecture and tectonic-
916
geomorphology of the studied ranges contributed to the comprehension of inversion
917
architecture of 3D normal faults arrays. The southern extreme of the Sierra del Castillo and
918
Península Baya anticlines allowed evaluating the response to the inversion of extensional
919
interaction nodes. The WNW-ESE normal fault with a Ya-node with Matasiete fault and an X-
920
node with Península Baya fault perturbed the inverted N-S faults dominated by dip-slip
921
displacement (Figs. 7.A, 8.A). In this case, there cannot be applied the simplistic scenario
922
where partitioned deformation is constructed from the trigonometric-vectorial decomposition
923
of the regional Z-axis (e.g. Imber et al., 2005; Giacosa et al., 2010; Navarrete et al., 2015). The
924
high oblique branch defined by the WNW-ESE lineament is dominated by dip-slip movements
925
and evidence strong along-strike variability in deformation. Its relative maximums inversion
926
degrees correlate with previous normal faults interaction zones. Assuming the structural
927
deformation is proportional to the reuse of the normal fault (Jackson et al., 2013), the
928
mentioned context suggests a localized increment of reshear linked to the inherited normal
929
fault architecture. In this sense, recent models of fault interaction mention the increase of
930
strain and coeval widening of the core and damage zones (Nixon et al., 2014; Peacock et al.,
931
2017b), what favor the fault rock development (Caine et al., 1996; Braathen et al., 2009; Choi
932
et al., 2016). All these processes should have decreased the cohesion and frictional resistance
933
of the rock after normal faulting, and in consequence, favors local inversion mechanisms and
934
focused the upward propagation of the deformation (Jackson et al., 2013). Península Baya
935
range also shows E-W lineaments at its southern extreme that crosscut all the inversion fold.
936
They have vertical uplifts of the northern blocks minor to 50 m without the distortion of the
937
backlimb structural trend. This architecture was interpreted as a relatively inactive exhumation
938
of normal faults during tectonic inversion process. A more complex array of secondary
939
transversal faults is exposed in Sierra Silva range. Its architecture with traces that finish against
940
the main fault suggests abutting faults in hanging-wall (sensu Peacock et al., 2017a), which
941
were exhumated after the tectonic inversion phase. The along-strike topographic and
942
structural trends show a clear correlation between the highest zones and the mayor transverse
943
lineaments (Fig. 9.D). This spatial relationship was interpreted as the combined effect of the
944
southward propagation of the inverted fault-fold, and an active role of the transverse faults
945
during the range uplift. The latter based on the anomalous uplift induced by the transverse
946
fault in Península Baya range (Fig. 7.C). Further fieldwork is required to evaluate the complex
947
kinematic history of this W-E fault system.
948
Recognition and comprehension of tectonic inversion of extensional nodes impact in the
949
analysis of oil-bearing structures of the SBFB, in particular, because increased strain at fault
950
interactions should have favored vertically fluid pathways that communicate reservoirs
951
(Peacock et al., 2017b, and references therein). Furthermore, inversion uplifts could combine
952
with extensional ones linked to strike-slip reactivation (Rotevatn and Peacock, 2018), favoring
953
the formation of hydrocarbon traps.
954 955
5.1.4. Selective tectonic inversion
956
Displacement transfer zones are well known in overlap thrust fault systems (McClay, 1992;
957
Higgins et al., 2007). In the case of inverted basins, the reconstruction of the pre-inversion
958
architecture is essential to evaluate the effect of the inherited faults arrays. Particularly, soft-
959
linked extensional transference zones are important in understanding the later inversion
960
(Underhill and Paterson, 1998; Konstantinovskay et al., 2007; Jackson et al., 2013; Sarhan and
961
Collier, 2018). In this scenario, overlap structures with selective inversion are almost
962
unexplored from natural examples (Kelly et al., 1999). Sibson (1995) interpreted this behavior
963
as a consequence of geometric differences in fault planes, while McClay and Buchanan (1992)
964
attributed unfavorable orientation for reactivation. The present architecture of the SBFB was
965
used to evaluate different responses of soft-link transfer zones under tectonic inversion. Pop-
966
up growth folds represent the reshear of both faults, so overlapped fault strain envelopes
967
were interpreted. The other end-member model is the selective inversion in neighboring
968
faults, where strongly non-uniform contraction localized the strain in only one fault. Examples
969
of this situation were analyzed from fault couples related to the Sierra Nevada and Los Perales
970
faults (Figs. 5.F, 12.E). In both cases, the inverted fault is located to the west, while the one
971
located to the east remains non-inverted. The latter shows an extensional architecture
972
apparently frozen, but subseismic reshear mechanisms or very low inversion degree cannot be
973
discarded. The fault-plane friction respect to the available shear stress controls the faults
974
reactivation (Lisle and Srivastava, 2004; Maerten et al., 2018). Assuming the most common
975
conditions where both faults decrease the rock resistance after extensional faulting (Handin,
976
1969; Ranalli and Yin, 1990; Twiss and More, 2007), the heterogeneous deformation
977
consumed all the local shortening using only one fault. In this context, the inverted fault
978
produced a deformation shadow, in a similar way to the stress-failure shadow proposed by
979
Dempsey et al. (2012) during the evolution of normal fault systems. Mayor inverted faults can
980
increase the strained rock volume and widens the damage zones of the inverted fault, favoring
981
fluid pathways (Caine et al., 1996; Kristensen et al., 2016; Hollinsworth et al., 2019).
982
Consequently, the increase in the fluid circulations can induce the valve-effect in reshear
983
mechanisms (Sibson, 1995; Sibson, 2017) favoring multipulse tectonic inversion processes.
984 985
5.1.5. Influence of basement fabric and distribution
986
Positive tectonic inversion implies the reactivation of previous normal faults (sensu Williams et
987
al., 1989), which history can be related to active or passive basement fabric (Morley, 1999).
988
The effects of weakness zones in the normal fault networks have been extensively studied
989
from natural examples (Morley, 1995, 1999; Duffy et al., 2005; Reeve et al., 2015; Phillips et
990
al., 2016; Collanega et al., 2019). In the case of the SBFB, Section 4.8 highlights that the actual
991
knowledge of the subsurface structural basement fabric requires major calibrations based on
992
systematic cutting record revision. Our results at the Ferrarotti study area, are still the
993
moment, the best surface analog to evaluate the basement fabric of the GSJB and its influence
994
on inverted faults. Figure 5 shows the relationship between the seismofabric and the inverted
995
faults planes, where the faults are subparallel to adjacent reflectors correlated with the
996
economic basement. This architecture suggests a weakness direction subparallel to the
997
stratification of the sedimentary basement units, analogous to merging interactions proposed
998
by Phillips et al. (2016). This study area also shows outcropped folds with half-wavelength of
999
tens to hundreds of meters in Jurassic and Paleozoic rocks with a pronounced N-S trend of the
1000
individual axis strikes (Fig. 4.A). Correlation between this robust structural trend and the N-S
1001
oriented intrabasinal paleohigh identified in the subsurface of the SBFB by Ferello and Lesta
1002
(1973) (Fig. 18) suggested a cause-effect relationship. In this way, that coincidence allows
1003
proposing that Upper Jurassic and Cretaceous extensional phases were distorted by weakness
1004
generated by fold limbs of the pre-rift units. This hypothesis is relatively unexplored, so future
1005
studies must systematically evaluate the influence of different fold dimensions (subseismic to
1006
seismic) through basement mapping, or advances techniques like 2-D convolution seismic
1007
modeling (Wrona et al., 2019) and Anisotropy of Magnetic Susceptibility (García-Lasanta et al.,
1008
2018). This hypothesis differs from the above-mentioned fabric analysis where the pre-rift
1009
weaknesses are thought as discrete foliations or plastic shear zones from the crystalline
1010
basement.
1011
The climax of the Jurassic syn-rift played a major control in the early Cretaceous extensional
1012
reactivation (e.g. Paredes et al., 2018). During the tectonic inversion phases, the resheared of
1013
master faults with high extensional displacement should have consumed the inversion with
1014
low to very low inversion ratio (sensu Williams et al., 1989), so that minor uplift favored that
1015
the inverted structure remains buried (Foix et al., 2015). This simple geometry relationship can
1016
explain the low inversion degree of Río Mayo Sub-basin (Figari et al., 1996), despite being
1017
located closer to the Andean margin.
1018
At a basin scale, the latitudinal increase on the depth of the Río Chico paleohigh (sensu
1019
Cortiñas, 1996) must have played an essential role in the construction and expression of the
1020
SBFB. Figure 18 resumes the main inverted structures from surface and subsurface of the
1021
SBFB, as well as non-inverted faults of North Flank, Basin Center, and South Flank. It is clearly
1022
shown that outcropped inversion structures are next to the metamorphic-granitic palaeohigh.
1023
We integrated this regional architecture in an inversion model where the paleohigh acted as a
1024
megascale semi-rigid element that induced a basin-scale buttress effect for the N-S faults of
1025
the SBFB (Fig. 18). At the same time, the North Flank was protected by this element, so there is
1026
no reverse-reactivation of Cretaceous structures (Figari et al., 1999). This scenario is similar to
1027
the one proposed by Shinn (2015) in the Yellow Sea subsurface.
1028
1029 1030
Figure 18. A) Structural framework of the San Bernardo Fold Belt showing the Río Chico paleohigh
1031
(Cortiñas, 1996). Map-scale faults and folds modified from Figari et al. (1999), Miller y Marino (2019),
1032
and SEGEMAR maps. Wells include the depth of the crystalline basement. Surface units of the economic
1033
basement follow references in figure 1. Note crystalline outcrops toward the northern limit of the GSJB.
1034
B) Geological section made by Ferello and Lesta (1973) showing the San Bernardo high. C) Seismic
1035
architecture of the Río Chico paleohigh.
1036 1037
5.2. Tectonostratigraphic scenario of the Chubut Group at the San Bernardo Fold Belt
1038
The current state of knowledge of the tectonostratigraphic scenario of the base of the Chubut
1039
Group (Pozo D-129, Matasiete, and Castillo formations) can be divided into two major
1040
proposals, the syn-inversion or the pre-inversion scenarios (see Introduction). The structural
1041
architecture of Ferrarotti locality is key, as its paleogeographic position is the closest to the
1042
contemporaneous Andean subduction margin, so the transmission of the Andean compression
1043
may be assumed more efficient (Heidbach et al., 2018). In consequence, the Lower Cretaceous
1044
syn-extensional context alerts the timing of the construction of other folds located to the east.
1045
In order to validate a robust structural framework, we compared in detail our geometric-
1046
kinematic results with the evidence of the syn-inversion context. The analysis of each inversion
1047
structure follows.
1048
Sierra Nevada anticline is an inversion structure that was characterized by Gianni et al. (2015a,
1049
their Fig. 10.A). They defined a subsurface architecture with a folded wedge geometry at its
1050
hinge, which was interpreted as the combination of a retro-vergent intraformational
1051
antiformal stack duplex and a vergent imbricate thrust system. Our interpretation is clearly
1052
different, the syn-rift wedge was correlated with Chubut Group and Neocomian units, and the
1053
thickness pattern was attributed to the extensional phase (Fig. 5.F). We interpreted a normal
1054
fault to the east of Sierra Nevada fault with syn-kinematic sequences of the Castillo Formation.
1055
Those authors associated this structure to a reverse fault, and the thickness anomaly in the
1056
sequences of the Castillo Formation to a folded intraformational thrust (Gianni et al., 2015a,
1057
their Fig. 10.A). It has to be highlighted that the dip direction to the east of that main fault is
1058
not consistent with the seismofacies and seismosequecences correlation (Fig. 5.H), so the
1059
reverse kinematic is unlikely. Another element that shows the geometric differences are the
1060
stratigraphic lobate geoforms (Fig. 5.I), Gianni et al. (2015a, their Fig. 10.A) proposed there a
1061
propagation fold and a shortcut thrust. To the south of Sierra Nevada anticline, the traditional
1062
surface architecture exposed at Matasiete canyon was used by Gianni et al. (2015a, their Fig.
1063
7.A) and Gianni et al. (2018a, their Fig. 8.b) as an example of growth-strata in late Early to Late
1064
Cretaceous units of the Chubut Group. Our field observations at the same outcrops are in
1065
disagreement with that proposals because the formations are faulted (Fig. 6), so the structural
1066
dips of the different structural blocks cannot be linked to growth-strata (Suppe et al., 1992;
1067
Brandes and Tanner, 2014). We consider that the progressive unconformities marked by the
1068
rotated paleosoils of Laguna Palacios Formation are the only syn-contractional evidence of the
1069
late Upper Cretaceous inversion phase.
1070
The information analyzed in Los Perales anticline contributed to important subsurface
1071
tectonostratigraphic relationships. The divergence of markers toward the central part of the
1072
Los Perales fault in an along-strike section is considered as robust evidence of an inherited
1073
pattern of an extensional reactivation during late Early Cretaceous times. The absence of
1074
compressional progressive unconformities sustains an inversion post-Castillo Formation. The
1075
thickness variations of seismosequences in the hanging-wall of the Los Perales fault are
1076
interpreted as an inherited pre-inversion sedimentary pattern linked to a monocline growth
1077
during extensional reactivation (see Section 5.1.3). Previous contributions used the mentioned
1078
asymmetry in across sections of Los Perales anticline to propose a syn-contractional context
1079
during the deposition of Pozo D-129, Matasiete and/or Castillo formations (Fig. 5 in Barcat et
1080
al., 1989; Fig. 10.B in Gianni et al., 2015a; Figs. 6.C,D in Gianni et al., 2015.b).
1081
Finally, Sierra Silva anticline deserves special analysis. The northern extreme of this fold shows
1082
tabular sequences of the Castillo and Matasiete formations in the hanging-wall of Cerro
1083
Chenques fault, which argues the absence of tectonic control during their deposition (Fig.
1084
10.A). However, Gianni et al. (2015a, their Fig. 7.B) draw growth-strata, progressive
1085
unconformities, and on-lap stratification in those outcrops from the linkage of Cretaceous
1086
sequences in the hanging-wall and footwall of the Cerro Chenques fault. At the same time,
1087
those authors also interpreted growth-strata, unconformities and dips up to 45° at the
1088
backlimb (Gianni et al., 2015a, their Fig. 7.C), but our architecture shows concordant strata and
1089
dips values under 27° (Fig. 10.B) (Allard et al., 2018b, their Fig. 5.A). The subsurface
1090
architecture was addressed in a later paper using 2D seismic data (Figs. 9.B,C in Gianni et al.,
1091
2018a). The southernmost section defines a fault-propagation fold where the fault tip is at the
1092
top of the Neocomian seismosequences (~2700 m depth), so the fault does not cut the Chubut
1093
Group reflectors. In this geometry, there is no antithetic fault and the Castillo Formation
1094
thickness reaches 1050 m in the fold crest. Our geometric analysis differ in almost everything:
1095
1) the main fault is projected upward cutting the forelimb with a tip almost at surface, this
1096
reconstruction correlates with the faulted fold interpreted from the buried forelimb at the
1097
southern limit of the anticline (Fig. 9.C,E), 2) the backlimb is cut by the Jerez fault with reverse
1098
kinematic, which was interpreted from outcrop information and dipmeter log data (Figs. 9.B,
1099
11.B,C), 3) the simplified and schematic box-shape fold reconstruction is linked to 290 m of the
1100
Castillo Formation, similar to the 340 m outcropped thickness measured by Paredes et al.
1101
(2015) in the Cerro Chenques. The figure 9.B of Gianni et al. (2018a) shows another W-E
1102
seismic profile across Sierra Silva anticline, located to the north of the previous one. Some
1103
characteristics from the published information have to be mentioned. The impedance of the
1104
reflectors of Cretaceous sequences is rarely uniform, from 650 m to 3750 m depth. The raw
1105
section shows a high impedance element crosscutting all the sequences, which is high-dipping
1106
to the east. This is extremely rare because its dip should not have favored the wave reflection
1107
and 2D seismic acquisition (Herron, 2012; Allmendinger, 2015). However, more remarkable is
1108
that this element matches exactly with the forelimb axial plane marked in a general figure. This
1109
correlation cannot be explained as the seismic record because the axial plane is a conceptual
1110
construction, so it does not exist as a reflection surface. From the geometric point of view, this
1111
seismic shows the Castillo Formation seismosequences are up to ~1500 m thick at Sierra Silva
1112
anticline, what is far superior to the record at the neighboring Cerro Chenques outcrops
1113
(Paredes et al., 2015).
1114
Taking into account the comparison made, we consider our robust evidence supports the basal
1115
units of the Chubut Group are pre-inversion units, so the expression syn-contractional rifting
1116
(sensu Gianni et al., 2015a) is only a theoretical geodynamic scenario. Until new information
1117
demonstrates the contrary, the classical tectonostratigraphic context prevails for the Early
1118
Cretaceous to lower Upper Cretaceous evolution of the SBFB. Minors calibrations may be done
1119
like: 1) define the lower Neocomian deposits as a syn-rift climax (Figs. 5.F, 13.A), 2) propose a
1120
coaxial extensional reactivation for the basal deposits of the Pozo D-129 Formation (Figs. 5.F,
1121
11.B) and 3) define a non-coaxial extension phase for the upper Castillo Formation to lower
1122
Bajo Barreal Formation (?) (Figs. 7.A,B , 9.A, 12.D,F). However, no contractional event is
1123
registered during the time spam linked to the deposition of those units.
1124
The upper units of the Chubut Group constituted by Lago Colhué Huapi and Laguna Palacios
1125
formations are linked to a different stratigraphic architecture. The former includes clasts of
1126
Castillo Formation (Casal et al., 2015), while the latter is related to low accommodation
1127
conditions tectonically induced (Allard et al., 2015), cannibalized strata and progressive
1128
unconformities (Figs. 6.F,G) (Gianni et al., 2015a). This field evidence indirectly supports a syn-
1129
inversion context at the SBFB for the upper Chubut Group, but requires further tectono-
1130
stratifraphic analysis to adjust the architecture of both units.
1131 1132
6. CONCLUSIONS
1133
This work synthesizes the compared structural analysis of outcropped and buried reverse-
1134
reactivated faults at the SBFB. The results highlight similarities and differences that adjust the
1135
geometry, kinematic evolution, and controls associated with their positive tectonic inversion.
1136
Among the most important contributions we can mention:
1137
1. The geometry of mayor inversion folds can be classified as single-fault inversion folds or
1138
pop-up growth folds. The latter implies the reverse-reactivation of secondary antithetic normal
1139
faults.
1140
2. Inversion folds have along-strike variability in the degree of inversion. Extreme cases are
1141
blind inverted faults that pass to faulted-folds toward the fault tips.
1142
3. Extensional growth-strata linked to monoclines were inherited by the tectonic inversion
1143
phase and can be confused with syn-inversion growth wedge.
1144
4. The coexistence of overlapped inverted and non-inverted subparallel faults evidence
1145
selective inversion.
1146
5. Partitioned tectonic stress can develop heterogeneous deformation associated with strike-
1147
slip displacement. In this context, a pop-up growth fold can change from poorly vergent to
1148
retro-vergent.
1149
6. Inversion of 3D extensional network favors either oblique inversion or dip-slip displacement,
1150
so deformation partitioning cannot be systematically applied. Fault nodes can correlate with
1151
increased uplift processes, which suggest topological analysis of extensional fabric is essential
1152
to evaluate the inversion architecture.
1153
7. The influence of economic basement fabric in extensional architecture and subsequent
1154
inversion should include the fold train of pre-rift sedimentary sequences. At basin-scale,
1155
crystalline basement paleohigh played a passive role as a regional buttress.
1156
The structural analysis done has evolutionary implications for the Lower Cretaceous
1157
sedimentary record at the SBFB. All the study localities, from the northwestern margin to the
1158
southern boundary of the basin, exposed robust evidence that revalidates the syn-extensional
1159
and post-extensional frameworks during the deposition of the basal sequences of the Chubut
1160
Group (Barremian-Turonian). Our results only justify syn-inversion strata in the Upper
1161
Cretaceous sequences of Laguna Palacios Formation (Santonian-Maastrichtian?). The NW-SE to
1162
W-E normal faults overprinted to the mayor inversion folds were associated to a non-coaxial
1163
extensional phase previous to the tectonic inversion.
1164 1165
7. ACKNOWLEDGMENT
1166
We want to thank YPF S.A. for the access to subsurface information and permission to publish
1167
the results. This work was partially supported by the Universidad Nacional de la Patagonia San
1168
Juan Bosco (PI-CIUNPAT-1323). The Geology Department of the UNPSJB is acknowledged for
1169
the logistic support. We thank Raul Giacosa by the critic lecture of the early version of the
1170
paper. The comments and corrections done by the reviewers Martínez and Manceda improved
1171
the final version of the contribution.
1172 1173 1174 1175 1176 1177 1178 1179 1180 1181 1182 1183 1184 1185 1186 1187 1188 1189 1190 1191 1192 1193 1194 1195
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HIGHLIGHTS •
Inversion anticlines are analyzed from outcrop and subsurface data.
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Along-strike variability on inversion degree is highlighted.
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Pop-up growth folds and inherited extensional growth-strata are proposed.
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Basin-scale buttress effect associated with a crystalline paleohigh is discussed.
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Timing of Cretaceous tectonic inversion was calibrated.