Folding, thrusting and development of push-up structures during the Miocene tectonic inversion of the Austral Basin, Southern Patagonian Andes (50°S)

    Folding, thrusting and development of push-up structures during the Miocene tectonic inversion of the Austral Basin, Southern Patagonian Andes (50S) Henrique Zerfass, Victor A. Ramos, Matias C. Ghiglione, Maximiliano Naipauer, Hugo J. Belotti, Isabela O. Carmo PII: DOI: Reference:

S0040-1951(17)30010-0 doi:10.1016/j.tecto.2017.01.010 TECTO 127382

To appear in:

Tectonophysics

Received date: Revised date: Accepted date:

18 February 2016 6 January 2017 12 January 2017

Please cite this article as: Zerfass, Henrique, Ramos, Victor A., Ghiglione, Matias C., Naipauer, Maximiliano, Belotti, Hugo J., Carmo, Isabela O., Folding, thrusting and development of push-up structures during the Miocene tectonic inversion of the Austral Basin, Southern Patagonian Andes (50S), Tectonophysics (2017), doi:10.1016/j.tecto.2017.01.010

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ACCEPTED MANUSCRIPT Folding, thrusting and development of push-up structures during the Miocene

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tectonic inversion of the Austral Basin, Southern Patagonian Andes (50ºS)

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Henrique Zerfass1, Victor A. Ramos2, Matias C. Ghiglione2, Maximiliano Naipauer2, Hugo J. Belotti3, Isabela O. Carmo4

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1 – Corresponding Author: Petróleo Brasileiro S.A., Universidade Petrobras – Rua Ulysses Guimarães 565 – 20211-225 – Rio de Janeiro – RJ – Brazil

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([email protected])

2 – Instituto de Estudios Andinos Don Pablo Groeber (IDEAN - Universidad de

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Buenos Aires, CONICET), Buenos Aires, Argentina.

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3 – Petrobras Argentina S.A., E&P, Buenos Aires, Argentina. Now Pampa Energía S.A.

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4–Petróleo Brasileiro S.A., Centro de Pesquisa e Desenvolvimento Leopoldo Miguez de Mello (CENPES), Rio de Janeiro – RJ – Brazil

Abstract

Rift and post-rift sections of the Austral Basin in the Southern Patagonian fold and thrust belt were examined in outcrop and in 2D seismic reflection sections to evaluate geometric and kinematic aspects of the extension and subsequent inversion. The syn-

ACCEPTED MANUSCRIPT rift section is composed of dacitic lava flows of the El Quemado Complex (Jurassic), the lowermost unit of the basin. The coastal sandstones of the Springhill Formation

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(Tithonian-Berriasian) and marine shales of the Río Mayer Formation (Berriasian-

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Albian) are thought to be sag units. However, field data suggest that these units are

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also part of the syn-rift successions, as evidenced by extensional growth strata in the lower levels of the Río Mayer Formation. A dacitic flow that is texturally and

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compositionally similar to those of the syn-rift phase and that is termed here the Río Guanaco dacite is interfingered with the basal strata of the Río Mayer Formation and

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has a U-Pb zircon crystallization age of c. 141 Ma (Berriasian). The outcropping section was inverted by compression in the early Miocene, according to structural and

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isotopic data of adjacent areas, producing a broad anticline with inverted and fossil extensional faults as well as newly generated thrusts. At the seismic scale, most of the

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extensional master faults present negligible reverse slip, and shortening was accommodated in hanging wall push-up structures. This situation is interpreted as a

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result of the competent metamorphic basement and volcanic syn-rift section, inducing frictional lock-up of the master faults.

Keywords Patagonian Andes, Austral Basin, tectonic inversion, push-up structures

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1. Introduction

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The inversion of extensional structures and syn-rift stratigraphic sections under

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compressional regimes is important to evaluate the history of a basin and in many

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cases has implication to hydrocarbon exploration. Compression produces positive features, such as fault-propagation and fault-bend folds, harpoon and push-up or pop-

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up structures (Mitra, 1993; McClay, 1995; Pace and Calamita, 2013). The process of inversion also influences the tectonic thermal history of a basin, producing uplift and

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exhumation of buried strata (Bonini et al., 2012).

The selective reactivation of normal faults under compressional stresses is a theme

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that has been debated in the last years (e.g. Bonini et al., 2012; Mescua and Giambiagi, 2012; Pace and Calamita, 2013). There is a complex interplay of different factors, such

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as fault geometry relative to the compressional stress field, rheology of the fault zone and lithology of the wall rocks. New insights from different geological settings are

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necessary to better discuss the theme. The Austral-Magallanes Basin is located in the foreland of the Southern Patagonian Andes (Fig. 1) and is one of the main oil and gas producer basins of Argentina and Chile. It displays good examples of the tectonic inversion of Jurassic rifts during Late Cretaceous and Cenozoic Andean compression. Regional studies performed by many authors have described the role of positive inversion in the development of the fold and thrust belt of the Patagonian Andes (e.g., Kraemer, 1998; Kraemer et al., 2002; Ghiglione et al., 2009; Fosdick et al., 2011, 2013; Giacosa et al., 2012). In addition, the physical modeling of positive inversion by Likerman et al. (2013) focused on the

ACCEPTED MANUSCRIPT transfer zones as important structures inherited from the rift phase. Inversion processes produced structural traps for hydrocarbons and influenced the history of maturation

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and migration (Rodríguez et al., 2008). The mechanical behavior of the brittle

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structures under inversion needs to be discussed in detail. However, the knowledge of

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the factors that play a role in the reactivation of faults or, alternatively, the displacement of conjugate or newly formed faults is an open question. This

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information is relevant to hydrocarbon exploration because the positive features produced during inversion can be located near the master fault or displaced laterally.

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The field study and the seismic reflection data analysis presented here provide some insights into this issue.

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In addition, the timing of the rift-sag transition in the Austral-Magallanes basin, notably the minimum age of the rift-related volcanism, is an issue that is revisited (e.g.

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Malkowski et al., 2015). This interval was studied in detail, and field data and U-Pb zircon ages from a newly recognized subaqueous lava flow or shallow sill in the Río

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Guanaco area (Santa Cruz Province, Argentina, figs. 1, 4, 5) are here presented to better constrain the age of the volcanism and associated extensional deformation.

2. Geotectonic evolution The Austral Basin is limited by basement blocks and rocks of the Patagonian Batholith along its western and southern active border (Fig. 1). The basement rocks are Late Paleozoic metasedimentary rocks of the Bahía de la Lancha and Río Lácteo formations (Kraemer, 1998; Hervé et al., 2008; Giacosa et al., 2012). The 150 Ma Patagonian Batholith is composed of calc-alkaline granitoids (Hervé et al., 2007), with

ACCEPTED MANUSCRIPT evidence of widespread Early Cretaceous magmatic activity (cf. Gonzalez Guillot, 2016). Towards the craton interior of Patagonia, the Austral Basin is limited by the

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Deseado Massif and its offshore continuation, the Río Chico arch, a structural high

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with a stable upper Proterozoic – lower Paleozoic granitic-gneissic basement

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(Pankhurst et al., 2003, Fig. 1). The Patagonian fold and thrust belt (Fig. 1) is associated with thick Upper Cretaceous-Cenozoic foreland deposits (Wilson, 1991;

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Kraemer, 1998; Giacosa et al., 2012; Barberón et al., 2015; Ghiglione et al., 2016a). The basement and internal domain are dominated by the inversion of rift basins,

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whereas the external domain presents shallow detachments producing thrust sheets and related folds (Kraemer, 1998; Ghiglione et al., 2014).

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The history of the Austral Basin started in the Jurassic, during a rift phase in Southwestern Gondwana related to opening of the Weddell Sea and the very early

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movements that would give rise to the South Atlantic Ocean (Uliana and Biddle, 1988; Rodríguez et al., 2008). This process culminated with the development of the Rocas

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Verdes back- arc basin (Fig. 2), in which bimodal magmatism and rifting occurred between at least 152 and 142 Ma (Calderón et al., 2007; Malkowski et al., 2015). These events were concomitant with the production of oceanic crust (Dalziel et al., 1974) and the Sarmiento Ophiolite Complex (Stern and De Wit, 2003), whose obduction was accommodated along the Canal de las Montañas shear zone (Calderón et al., 2012). In the continental intraplate setting, a series of half-grabens with a northnorthwestern direction were formed (Fig. 2). These rift depocenters were filled with intermediate and acidic lava flows, pyroclastics and lacustrine sediments, which are

ACCEPTED MANUSCRIPT represented by the El Quemado Complex (coeval to the Tobífera Formation of Chile), the basal (syn-rift) unit of the Austral Basin (Thomas, 1949; Dott et al., 1982; Wilson,

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1991; Kraemer, 1998; Kraemer et al., 2002; Rodríguez et al., 2008; Barbeau et al.,

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2009; Giacosa et al., 2012; Fig. 3). The final rifting stages were coeval with an

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important marine transgression, which is represented by the Springhill Formation, composed of fluvial and coastal quartzitic sandstones (Thomas, 1949), and the black

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shales and marls of the Río Mayer Formation, deposited in an inner to outer shelf environment (Arbe, 2002; Richiano et al., 2012; Richiano, 2014). The Springhill

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Formation (Tithonian – Valanginian) constitutes the most important reservoir in the basin (Riccardi, 1971; Kielbowicz et al., 1983; Rodríguez et al., 2008; Schwarz et al.,

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2011, Fig. 3). The Lower Cretaceous Río Mayer Formation is the main source rock (Arbe, 2002; Rodríguez et al., 2008; Hollis et al., 2009; Richiano et al., 2012;

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Richiano, 2014) and is equivalent to the Zapata Formation in Chile (Katz 1963; Scott 1966; Natland et al. 1974; Winn and Dott 1979; Dott et al. 1982; Wilson 1991), with

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detrital zircons indicating a maximum Hauterivian age of sedimentation (Calderón et al., 2007).

Ricchiano et al. (2012) and Ricchiano (2014) divided the Río Mayer Formation into three informal levels (lower, middle and upper; Fig. 3), which are important to the relative chronology of the tectonic processes. The lower level, with a BerriasianValanginian age (Kraemer and Riccardi, 1997; Ricchiano, 2014), is composed of organic-rich shales. The middle level is composed of marls with a ValanginianHauterivian age (Aguirre-Urreta, 2002; Richiano, 2014). The upper level consists of a succession of organic matter-rich shales, where the organic matter is composed mainly

ACCEPTED MANUSCRIPT of continental detritus (Ricchiano et al., 2012; Richiano, 2014); its age falls in the Hauterivian-Albian interval (Kraemer and Riccardi, 1997; Aguirre-Urreta, 2002; Perez

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Loinaze et al., 2012; Richiano, 2014). According to Belotti et al. (2013), the shale

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levels are potential non-conventional reservoirs and constitute an exploratory frontier

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in the basin.

The transition between the rift and sag phases is subject of controversy. The

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geometry of the beds of the El Quemado Complex has been deformed into halfgrabens, which can be clearly seen in outcrops (e.g., Kraemer, 1998) and seismic

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reflection sections (Likerman et al., 2013; Ghiglione et al., 2014). In contrast, the Springhill and Río Mayer formations expand the limits of the fault-bounded

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depocenters in the basin as a whole and can be considered sag deposits. However, as will be discussed later, the final manifestations of extensional faulting lasted until the

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time of deposition of the lower level of the Río Mayer Formation, at least in the study area (Kraemer and Riccardi, 1997).

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The onset of the first compressional phase occurred close to the Lower-Upper Cretaceous boundary, when the drifting phase in the South Atlantic was established and the subduction regime of the Pacific margin changed to positive roll-back (Katz, 1973; Dalziel et al., 1974; Dalziel, 1981; Stern et al., 1991; Ramos and Ghiglione, 2008; Malkowski et al., 2016). At this latitude, deposits related to this phase are represented by the Cerro Toro Formation (Ghiglione et al., 2014; 2015), which is upper Albian to lower Campanian in age, based on a fossil assemblage of ammonites and innoceramids (Arbe and Hechem, 1984; Kraemer and Riccardi, 1997) and can be divided into two members, CT1 and CT2. According to Ghiglione et al. (2014), the

ACCEPTED MANUSCRIPT basal member (CT1), which is upper Albian to lower Coniacian in age (Arbe and Hechem, 1984; Kraemer and Riccardi, 1997; Riccardi, 2002) (Fig. 3), is the

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stratigraphic record of the first thrusting phase, equivalent to the Punta Barrosa

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Formation of Última Esperanza in Chile. The CT2 member is Coniacian–Santonian in

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age (Kraemer and Riccardi, 1997; Kraemer, 1998). Upper Cretaceous foreland deposition is also represented by the overlying Alta Vista and Anita formations, which

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are Campanian in age (Fig. 3) (Kraemer, 1998; Arbe, 2002; Rodríguez et al., 2008). During the Eocene, another compressional phase took place in the region and

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was associated with an increasing convergence velocity and a change in the subduction direction towards the northeast (Pardo Casas and Molnar, 1987; Somoza and Ghidella,

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2005; 2012). In the study region, the inner fold and thrust belt was uplifted during this period and exhibited a thick-skinned style (Kraemer, 1998; Kraemer et al., 2002;

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Giacosa et al., 2012). The basal units deposited during this foreland stage are represented by the Man Aike and Río Leona formations (Fig. 3) (Russo et al., 1980;

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Malumián and Nañez, 1988; Malumián, 1990; Rodríguez et al., 2008; Fosdick et al., 2014). Those foreland deposits spanning the middle Eocene to early Miocene rest unconformably on top of the marine Maastrichtian Calafate Formation, bracketing a Paleocene-lower Eocene hiatus (Camacho et al., 2000; Malumián et al., 2000) related to the Eocene deformational event (Marenssi et al., 2002). The present configuration of the Southern Patagonian Andes began to develop in the early Neogene, during the last compressional phase, giving rise to the main distinctive characteristics when compared with other segments of the orogen. The Chile Ridge, which divides the fast Nazca plate with a subduction rate of 10 cm/year

ACCEPTED MANUSCRIPT (Somoza and Ghidella, 2012) and the slow Antarctica plate with a subduction rate of 2–2.5 cm/year (De Mets et al., 1994; Sdrolias and Müller, 2006), is colliding with the

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South American plate at 46º 30´S (Forsythe et al.. 1986; Gorring et al., 1997; Bourgois

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and Michaud, 2002; Lagabrielle et al., 2004). The speed difference produces a still-

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opening slab window since approximately 17–18 Ma (see Ghiglione et al., 2016a for a review), when the Nazca–Antarctic ridge collided with South America at 53°–54°S

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(Breitsprecher and Thorkelson, 2008). Examples of these phenomena include (i) conspicuous plateau basalts (with OIB chemical affinities) in the foreland setting

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(Gorring et al., 1997), (ii) a wide fold and thrust belt (Ramos, 1989) and (iii) the exhumation of the Jurassic-Cenozoic magmatic arc and the Patagonian Batholith

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(Ramos, 1982, 1989; 2005; Ramos and Kay, 1992; Murdie et al., 1993, Fig. 1). Subsidence in response to tectonic loading produced the flooding of the shallow

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marine "Patagoniense" transgression during the latest Oligocene to early Miocene (Malumián, 2002; Cuitiño and Scasso, 2010; Cuitiño et al., 2012; 2013; Parras et al.,

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2012). Deformation and consequent uplift of the fold and thrust belt in the early Miocene triggered a regression and the east-flowing rivers of the Santa Cruz Formation (Marenssi et al., 2005; Blisniuk et al., 2005; Cuitiño et al., 2012, 2015), as well as the development of prograding clastic wedges into the Atlantic Ocean (Biddle et al., 1986; Gallardo, 2014).

3. Materials and methods 3.1. Field and petrographic study

ACCEPTED MANUSCRIPT The field study was conducted in the Río Guanaco area, which was mapped in detail (Fig. 4A, 5). Complementary field observations were made in Bahía de la

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Lancha, located north of Lago Viedma (49ºS, Fig. 4B). Volcanic and sedimentary

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rocks were described in outcrop. Special attention was payed to the nature of the

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contacts between units and possible thickness variations.

The structural study was conducted firstly by the evaluation of the main structures,

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such as faults and folds, and the form they affect the stratigraphic units. For this approach, field observation was supplemented by high resolution satellital images and Azimuthal data of the brittle and ductile structures were

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outcrop photo mosaics.

collected. Attention was paid to the kinematic indicators of the extensional faults to

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evaluate whether there was overprinting of features produced during the compressional phase. The fracture pattern of the syn-rift El Quemado Complex was studied in detail

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to evaluate whether the tectonic fabric at the outcrop scale remains extensional or records only the last compressional phase. The statistical treatment of the azimuthal

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data was expressed using stereograms. Ten thin-sections of the El Quemado Complex and the herein proposed Río Guanaco dacite informal unit were described using an optical microscope. Qualitative petrographic analysis was performed to complement the volcanic rock descriptions and to recognize alteration patterns. 3.2. Seismic interpretation Two 2D seismic reflection sections from Petrobras Argentina S.A., acquired in the 1990s (AA´ and BB´ in Fig. 4A), were studied in order to compare the inversion styles at different scales. The sections have strikes of east-northeast and east-west and

ACCEPTED MANUSCRIPT represent dip sections orthogonal to the Andean structure. The stratigraphic horizons were interpreted by transposing them from another 2D seismic reflection section

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located east of AA´, where the horizons are controlled by well data (Fig. 4A). In

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addition, the syn-extensional master faults and the inversion-related conjugate features

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were also interpreted. 3.3. U-Pb geochronology

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One sample was collected from the Río Guanaco dacite (sample 27C) and was dated by zircon U-Pb geochronological method using a laser ablation multi-collector coupled

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spectrometer

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inductively

(LA-MC-ICP-MS)

at

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Geochronology Laboratory of the University of Brasília. Standard mineral separation

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methods were applied to concentrate the zircon grains, including vibrating Gemini table, Frantz magnetic separator and heavy liquid procedures. Final grain selection was

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obtained by hand-picking under a stereomicroscope. The selected grains were mounted in epoxy blocks and then polished to expose the grain interiors. Backscattered electron

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(BSE) images were obtained using a scanning electron microscope (SEM JEOL JSM 5800) to evaluate the internal structures of the zircon crystals and to select the target areas to be analyzed. The sample coordinates and U-Pb analytical methods and data are available in the Supplementary material.

4. Results 4.1. Stratigraphic units description 4.1.1. El Quemado Complex (Jeq)

ACCEPTED MANUSCRIPT In the Bahía de la Lancha region (Fig. 4B), it is possible to observe the stratigraphic relationships of this unit. The locally sub-horizontal strata rest

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unconformably on the folded Bahía de la Lancha Formation (Fig. 6A). The latter unit

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was thrusted and folded with northward vergence in the Late Paleozoic during the

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Gondwanides orogeny (Riccardi, 1971; Giacosa et al., 2012). A non-conformity delineates the upper contact of the volcanic El Quemado Complex and the sedimentary

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rocks of Río Mayer Formation (Fig. 6A) in both the Bahía de la Lancha and Río Guanaco areas. In the first area, the El Quemado Complex is up to 100 m thick.

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In the Río Guanaco section, only the upper contact with the Springhill Formation is observed. The exposed thickness is approximately 200 m. The volcanic

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rocks are gray colored or are greenish when altered. They are stratified, tilted about approximately 20º to the east (Fig. 5) and are massive or laminated due to the flow of

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highly viscous lava (flow banding). The lamination is concordant to highly discordant (sub-vertical) in relation to the bedding due to folding and internal shear of the silicic

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lava; delta-shaped structures suggest internal shear and rotation of semi-consolidated cores within the lava flow (Fig. 6B). The rocks exhibit a porphyritic texture, containing 30 to 50% phenocrysts of plagioclase and quartz (dominant) and biotite (subordinately); rare K-feldspar crystals are present in some samples. The phenocrysts are euhedral or subhedral, and the quartz and feldspar crystals present irregular and rounded borders, as well as embayments, due to chemical reactions with the groundmass (Fig. 6C). The main alteration mineral is sericite, which is present within feldspar grains and in the groundmass as well; subordinately, chlorite, epidote, carbonate and zeolite are observed. The groundmass is

ACCEPTED MANUSCRIPT microcrystalline and is significantly altered. Sericite and quartz-feldspar lenses resemble fiamme microstructures and are interpreted as patchy, two-phase alteration

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(cf. Bull and McPhie, 2007) (Fig. 6D). The rocks are interpreted as the crystallization

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product of viscous, dacitic lava flows.

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4.1.2. Springhill Formation (Jsh)

This unit is approximately 10 m thick in the Río Guanaco area. It is made of

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trough-cross and planar laminated coarse to fine sandstones forming a tabular bed (Fig. 7A). The upper contact with Río Mayer Formation is concordant and abrupt, as

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there are no recurrent sandstones within the overlying shaly unit. 4.1.3. Río Mayer Formation (Krm)

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In the Río Guanaco area, the Río Mayer Formation presents a thickness of approximately 370 m. Marine molluscs (belemnites, ammonites, bivalves) and plant

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remains are found in this unit, suggesting a Lower Cretaceous relative age (Kraemer and Riccardi, 1997; Ricchiano, 2014). Studies by Richiano et al. (2012) and Richiano

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(2014) subdivided the unit into three levels, which are clearly identified in the field (Figs. 7B, 10) and presented an updated facies analysis of the unit in the same study area. The lower Río Mayer Formation (Krml, Fig. 5) is composed of black shales rich in organic matter. Its mean thickness is approximately 80 m, although laterally it can be highly variable due to syn-depositional activity of normal faults. The middle level (Krmm, Fig. 5) is approximately 30 m thick and is composed of marls. The upper level (Krmu, Fig. 5) presents a thickness of approximately 260 m and is also composed of black shales. According to Ricchiano (2014), the upper shales are distinct from the

ACCEPTED MANUSCRIPT lower ones by the origin of the organic matter, which contains more continental detritus. The Río Mayer Formation is interpreted as deposited in the shelf

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paleoenvironment, based on the lithologies and fossil content.

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The upper boundary of the unit is concordant with the Cerro Toro Formation

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and is marked by a lithological change. The latter formation is composed of dark-gray siltstones. Marine bivalves (Innoceramus) are found within this unit. The outcropping

4.1.4 Río Guanaco Dacite (Krg)

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area of the Cerro Toro Formation extends beyond the eastern limit of the mapped area.

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This informal unit is described here for the first time and comprises a 20-mthick shallow intrusion or lava flow within the lower level of the Río Mayer Formation

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in the Río Guanaco area. The lower contact of this igneous body is strongly reactive with the lower Río Mayer Formation, whereas the upper contact with the same unit is

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not observed. In the basal part of Río Guanaco Dacite there is a network of fractures, which initiates at the basal contact with the Río Mayer Formation shale and propagates

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throughout the dacite (figs. 8A, B). It is remarkable that the fractures within the dacite appear to be filled with material from underlying bed, as if the black shale was reworked as a fluid. The contact between the dacite and the shale is a rough surface, rich in small scale embayments and irregularities (figs. 8B, C). This fact suggests that the dacitic lava was in contact with non-consolidated sediment. The dacite is gray in color, with a porphyritic texture and an equal proportion of phenocrysts and groundmass. The dominant phenocrysts are plagioclase (up to 3 mm long) and quartz (up to 2.2 mm long); biotite also occurs as up to 1.8-mm-long crystals. Quartz and plagioclase grains display irregular borders and embayments,

ACCEPTED MANUSCRIPT interpreted as chemical corrosion of original euhedral and subhedral crystal forms. The groundmass is microcrystalline, quartz-feldspathic, and altered to sericite, epidote and

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zeolite (Fig. 8D).

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The textural aspects of this rock suggest that it is a submarine lava flow or a

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shallow sill within the Río Mayer Formation. The mineralogical composition is very similar to that of the El Quemado Complex dacites in terms of both primary and

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alteration minerals, suggesting a common tectonic setting for the magma and a similar diagenetic history.

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A total of 29 zircon grains from sample 27C were dated but 8 analyses were rejected due to high discordance. The spectra of the 21 concordance ages (% of

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concordance 6/8 vs. 7/5 are between 98 and 102) are characterized by several group of ages between ca. 141 Ma and 357 Ma. The crystallization age of the Río Guanaco

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Dacite is given by the concordia age calculated with the youngest population of zircons (N=4) at 141.42 ± 0.95 Ma (Fig. 9). Other two older groups of zircons with

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concordance ages at 144.74 ± 0.68 Ma (N= 9) and 148.2 ± 1.2 Ma (N=3), separated from each other only by statistical parameters, were calculated and interpreted as inherited ages. An isolate Middle Jurassic age (166 ± 2 Ma) and a population of Early Carboniferous zircons (N=3) were recorded and also interpreted as inherited zircons. Early Carboniferous ages were recorded in several detrital zircon analyses from the Bahía La Lancha Formation which it is considered part of the stratigraphic basement of the Austral basin (Augustsson et al., 2006; Malkowski et al., 2015). The three zircon populations are considered as igneous in origin, and the central region of each grain was analyzed (Fig. 9B). Based on these premises, the younger age

ACCEPTED MANUSCRIPT is considered to be the crystallization age of the Río Guanaco Dacite, as discussed below.

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4.2. Structure

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In the Río Guanaco area, a major east-west lineament is recognized. It is

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partially covered by Quaternary sediments and is interpreted as an east-west strike and northward-dipping normal fault with a minimum throw of 90m, because it puts in

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lateral contact the El Quemado Complex and the lower Río Mayer Formation together with the Río Guanaco Dacite (Fig. 5). In this glacial landscape, the hanging wall is

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topographically high, exposing an east-west scarp facing south, where different types of structures were described. The footwall of the east-west normal fault presents a plan

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view of the El Quemado Complex rocks, where a study of the fracture pattern was performed. A description of the inversion structures in the seismic reflection sections

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in adjacent areas (Fig. 4) is also presented in the following. East-dipping backthrusts dominate the mapped area. The bedding (S0) dips

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approximately 30º to the east (Fig. 5). The backthrusts are genetically related to the main west-dipping thrusts of the Andes farther west and were described by Kraemer (1998) and Kraemer et al. (2002). 4.2.1. Structure in the outcropping east-west section The section shown in Figure 10 includes an anticline in its eastern part, with rocks of the El Quemado Complex in the core, capped with the Springhill Formation and the three levels of the Río Mayer Formation. Brittle structures striking approximately N-S are observed in the core of the anticline. An east-dipping backthrust is observed farther west. The footwall of the backthrust is composed of

ACCEPTED MANUSCRIPT rocks associated with the lower Río Mayer Formation and Río Guanaco dacite, whereas the hanging wall exhibits the succession of the El Quemado Complex,

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Springhill and Río Mayer formations. The westernmost structure shown in Figure 10 is

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a thrust dipping west, with rocks of the El Quemado Complex in the hanging wall.

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The different categories of recognized structures are described in the following and highlighted in Figure 10:

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(1) Normal faults. In a tectonic context dominated by compression and inversion, normal faults represent brittle structures in which the stratigraphic markers and

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kinematic indicators suggest downward displacement of the hanging wall. The two observed normal faults present orientation of N021E/72NW and N003E/65NW and

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throws of about 15m. As seen in Figure 10, the relative displacements of the El Quemado, Springhill and lower Río Mayer units have evidence of normal

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displacement. In addition, the lower Río Mayer beds in the vicinity of the fault zone present a widening-toward-the-fault wedge geometry, indicating syn-extensional

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deposition, and drag folds, which confirm the kinematics. Anti-Riedel shears were found intersecting the plane of one of these faults (exhibiting a north-south strike and a gentle eastward dip, see Figure 10D) and are additional evidence of normal displacement. The stereogram also displays the population of planes of the main fault zone, striking north-northeast and dipping westwards at steeper angles (approximately 65º). Additionally, there is an east-west striking and south-dipping population of fractures with grain comminution, which can be related to the previously mentioned east-west lineament seen at Figure 5.

ACCEPTED MANUSCRIPT (2) Normal faults with inversion features. This type of structure features a main fault plane with initially normal movement, inferred from the relative displacement of the

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stratigraphic units. The two faults present an orientation of N27E/75NW and

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N60E/40NW, and residual normal throws of 30-40m. Their distinctive characteristic is

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that there are features in their vicinities that suggest a second stage of inversion with reverse displacement during their evolution. For example, the drag folds in the beds of

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the El Quemado and Springhill units observed in the footwall of the easternmost fault of this type (in Figure 10) indicate reverse slip. In addition, part of the reverse

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displacement is transferred to conjugate faults that propagate throughout the hanging wall and affect at least the lower and middle levels of the Río Mayer Formation (Fig.

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10). Another normal fault with evidence of inversion is seen to the west of the previous one in Figure 10, and the features associated with the main fault plane can be

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seen in Figure 10E. A system of conjugate faults propagating throughout the hanging wall forms a push-up structure, which affects the lower and middle parts of the Río

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Mayer Formation. Contractional conjugate faults are also observed in the footwall of this fault (Fig. 10E). The term “push-up” is used for similar inversion features described by Pace and Calamita (2013) in seismic sections of the Apennines fold and thrust belt. In summary, the two inverted normal structures recognized show a distinct behaviour during contraction. The eastern fault concentrated the positive slip on the original main normal fault plane. Although the inversion was partial, as inferred from the stratigraphic markers, the reverse movement on the fault plane effectively produced an inverse drag fold in the footwall. In addition, another reverse fault created during the inversion process accommodated part of the displacement. In contrast, the

ACCEPTED MANUSCRIPT western fault does not show any evidence of reverse movement on the original fault plane, as inferred from the displacement of the stratigraphic markers and the drag fold

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in the hanging wall, which denote normal displacement. In this case, the

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compressional stress was transferred to conjugate faults in the hanging wall and

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footwall, which can be interpreted as push-up and shortcut faults, respectively. The only fault that present effective reverse movement on the main fault plane have a

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gentler angle in comparison with the other normal and inverted normal faults (approximately 40º).

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(3) Backthrust. This gently eastward-dipping fault (orientation of N004W/42NE) is seen in the western part of Figure 10. There are no confident elements to determine its

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throw, but it is estimated in 20m (Fig. 10). This compressional structure puts the El Quemado/Springhill units (east) into lateral contact with the lower Río Mayer/Río

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Guanaco dacite units (west).

(4) Thrust. One of the recognized thrusts is the westernmost structure shown in Figure

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10. It has a gentle dip to the west and vergence to the east (orientation of NS/08W). There are no markers in outcrop to evaluate its real throw; its minimum displacement is of about 20m (Fig. 10). This fault has carried the western block of the El Quemado Complex up over the block of lower Río Mayer/Río Guanaco dacite. Another thrust is observed close to the centre of the section shown in Figure 10 and exhibits an orientation of N62E/13NW and throw of about 10m. The middle Río Mayer Formation, which was previously folded, is locally broken, forming a blind, gentle west-dipping thrust fault.

ACCEPTED MANUSCRIPT (5) Accommodation folds. This type of structure is observed in the middle level of the Río Mayer Formation (Fig. 10). It is a product of the rheological contrast between the

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marls of this level and the shales of the underlying and overlying beds during the

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contraction that gave rise to the anticline.

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4.2.2. Brittle deformation pattern of the El Quemado Complex

The fracture analysis performed in the volcanic rocks is shown in the

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stereogram of Figure 11. The contour distribution exhibits three populations: (i) northwest-striking, northeast- and southwest-dipping, high-angle fractures; northeast-

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striking, southeast-dipping, lower-angle fractures; and (iii) north-south-striking, subvertical fractures.

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Some fractures are in fact faults, as their planes form true slickensides. The associated kinematic indicators (e.g., R and R´ shears intersections, polished and rough

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facets) are evidence that these fractures are reverse faults, which are displayed as great circles in Figure 11. These fractures are spatially related to the fracture populations (i)

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and (ii).

4.2.3 Structure in the 2D seismic sections Section AA´, located to the south of Lago Argentino (Fig. 4), exhibits an overall aspect of an anticline to the WSW and a syncline to the ENE, involving the Jurassic and Cretaceous units (Fig. 12). In terms of stratigraphy, the Jurassic rift section (El Quemado Complex) is interpreted as a series of east-dipping half-grabens. The Upper Jurassic Springhill Formation varies in thickness, although it is not controlled by the rift depocenters. The same situation is observed in the lower and middle/upper levels of the Río Mayer Formation.

ACCEPTED MANUSCRIPT As seen in Figure 12, the master faults of the half-grabens dip to the west (apparent fault dips ranging from 50o to 72o) and present a gently curved geometry and

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feature a spacing of 5 km to 10 km. A distinctive feature is the network of hanging

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wall push-up faults (sensu Pace and Calamita, 2013) that propagated away from the

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planes of the master faults. No substantial reverse displacement is observed on the master faults, and the compressive stresses seem to be accommodated by slip

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throughout the push-up structures. These features form structural highs in the hanging wall; this is particularly clear in the case of the anticline in the western side of the

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section (Fig. 12).

The BB´ section, located on the northern shore of Lago Argentino (Fig. 4),

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shows that the overall structure is a gentle anticline. The geometry of the rift section is more complex, featuring two grabens, as well as a half-graben on the eastern side,

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which are filled by rocks of the El Quemado Complex (Fig. 13). The dips of the master faults range from 62o to 70o. A gentle increase in the thickness of the Springhill

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Formation is also observed in the hanging wall of some extensional faults (indicated by the ellipses in Fig. 13), suggesting syn-extensional activity of the faults in the Tithonian-Berriasian. The deposition of the entire Río Mayer Formation was apparently not influenced by the normal faults. The spacing between the master faults ranges from 5 - 9 km. The geometry of the faults is planar to gently curved, and one fault also presents bends. The push-up hanging wall faults are the prominent feature and constitute the main structural highs (Fig. 13). Little reverse displacement, if any, is seen on the master fault planes. In addition, a shortcut fault is associated with the westernmost extensional master fault

ACCEPTED MANUSCRIPT (Fig. 13). One of the master faults of the westernmost graben presents bending (Fig. 13), and this may have influenced the development of push-up faults, according to the

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model presented by Mitra (1993).

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5. Discussion

5.1. Implication of the U-Pb geochronology data to the timing of the rift phase

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The volcanic event represented by the El Quemado Complex is related to the rift phase in the Austral Basin (Thomas, 1949; Dott et al., 1982; Wilson, 1991;

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Kraemer, 1998; Kraemer et al., 2002; Rodríguez et al., 2008; Barbeau et al., 2009; Giacosa et al., 2012). Pankhurst et al. (2000) presented U-Pb zircon ages for the El

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Quemado Complex and Tobífera Formation (the equivalent unit in Chile) between 156 and 153 Ma. Calderón et al. (2007) presented an U-Pb zircon age of 149 Ma for the

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Tobífera Formation. Coeval plutonic rocks in the Patagonian Batholith present U-Pb zircon ages ranging from 157 to 144 Ma (Hervé et al., 2007). Malkowski et al. (2015)

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reported an U-Pb zircon age of 152.0±2.0 Ma for the basal part of this unit, as well as ages of ca. 148.5 Ma for its upper part. In addition, Malkowski et al. (2015) pointed out to detrital zircons from the lower part of the Río Mayer Formation with ages between 152 and 146 Ma. The available data suggest that the volcanism related to the main rift phase occurred in a time span from 157 and 144 Ma. Our two older concordia ages (148.2±1.2 Ma and 144.74±0.68 Ma) are interpreted as inherited zircons, probably from previous igneous rocks of the El Quemado Complex and the Patagonian Batholith; i.e., ascending lava gave rise to the

ACCEPTED MANUSCRIPT Río Guanaco Dacite volcanic event and assimilated zircons from the El Quemado strata and associated plutonic rocks.

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The younger concordia age (141.42 ± 0.95 Ma) is considered to be the

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crystallization age of zircons within the Río Guanaco dacitic magma. According to the

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updated chronostratigraphic data (Cohen et al., 2013), this age is Berriasian, which is coeval with the lower level of the Río Mayer Formation based on the fossiliferous

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content of the latter (Kraemer and Riccardi, 1997; Arbe, 2002; Ricchiano et al., 2012; Ricchiano, 2014). Thus, it is here postulated that the Río Guanaco dacite is a late

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product of rift-related magmatic activity coeval with the deposition of Río Mayer Formation. The similar texture and mineralogical composition of the studied samples

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from both El Quemado Complex and Río Guanaco Dacite corroborate the idea of a

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common tectonic setting to the lavas from the two units.

5.2. Structural interpretation of the Río Guanaco area and the sequence of

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deformational events

5.2.1. Synthesis of the structure in the study area The dominant tectonic features of the area are the east-dipping backthrusts that produced the overall east-dipping strata configuration (Fig. 5). The north-south strike of these structures is considered to be inherited from the Jurassic half-grabens (Kraemer, 1998; Ghiglione et al., 2009; Giacosa et al., 2012). The analysis of the fracture pattern of the El Quemado Complex rocks in outcrop confirms the dominance of contractional structures (reverse faults). In this case, the reverse faults present northeast and northwest strikes, and this orientation is distinct from that of the main

ACCEPTED MANUSCRIPT backthrusts (Fig. 11). It is thought that this pattern reflects the meso-scale fabric of the volcanic rocks and that these fractures were reactivated as reverse faults even though

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they were oblique to the shortening. For instance, the Río Guanaco area presents a

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dominant contractional character, which is related to the last deformation phase, which

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occurred in the Neogene.

Other features are the two east-west-striking lineaments, displayed as inferred

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fractures at Figure 5. The northern one intersects the fault zone described in the stereogram in Figure 10D. The presence of west-northwest-striking fractures with

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grain comminution breccia (population 3 in Fig. 10D) is evidence that this lineament represents a fault zone. Transfer zones with east-west strikes were developed during

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Andean compression and are thought to be a reactivation of ancient transfer faults related to the Jurassic half-grabens (Ghiglione et al., 2009; Likerman et al., 2013). A

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latter reactivation of the northern east-west-striking lineament as a normal fault is interpreted, taking into account the stratigraphic relationships (Fig. 5). This final

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normal displacement could be related to a late extensional collapse of the anticline produced during the compressional phase. 5.2.2. Extensional phase The sequence of events interpreted here is summarized in Figure 14. The El Quemado Complex is represented in the area by acidic lava flows (Fig. 14A). These rocks are related to the extensional phase that occurred in the Early to Late Jurassic (Bruhn et al., 1978; Kraemer, 1998; Pankhurst et al., 2000; Kraemer et al., 2002; Rodríguez et al., 2008; Barbeau et al., 2009; Ghiglione et al., 2009; Zahid and Barbeau, 2010; Giacosa et al., 2012). The deposition of this unit was strongly

ACCEPTED MANUSCRIPT controlled by normal faulting, as evidenced by the seismic reflection data and some outcrops (e.g., Bahía de la Lancha, Fig. 4B). However, in the Río Guanaco area, the

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relationship between the El Quemado Complex and its basement is not visible, making

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it difficult to determine whether the exposed extensional faults (inverted or not) are

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half-graben master faults. This is likely not the case because the spacing between the master faults observed in the seismic reflection data (Figs. 12, 13) is larger (at least 5

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km) than that of the normal faults observed in the outcrop (ca. 300 m; Fig. 10). Instead, the smaller-scale faults at Rio Guanaco are interpreted as conjugate faults to a

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half-graben master fault.

The syn-rift volcanic activity related to the El Quemado Complex ceased in the

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Late Jurassic (Kraemer and Riccardi, 1997; Pankhurst et al., 2000; Malkowski et al., 2015). The El Quemado Complex is covered in the Río Guanaco area by the Springhill

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Formation. The latter unit is made of transgressive coastal sandstones of TithonianValanginian age (Kraemer and Riccardi, 1997; Schwarz et al., 2001; Arbe, 2002) (Fig.

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14A). In the Río Guanaco area, there is no significant thickness variation in the Springhill Formation due to fault activity, suggesting a period of tectonic quiescence. Despite this observation, thickening of the Springhill Formation is observed farther south, in the BB´ seismic reflection section (Fig. 13), suggesting that this interval was deposited during active faulting in some areas. The deposition of the Springhill Formation is coeval with the extensional tectonic activity which gave rise to the backarc Rocas Verdes basin (Calderón et al., 2012; Malkowski et al., 2015). If the normal faults observed in outcrop are not considered to be the master faults of the half-grabens, they probably played an important role in the late

ACCEPTED MANUSCRIPT extensional phase that developed in the Early Cretaceous. In the Berriasian, some of the observed faults were active, as evidenced by the growth strata in the lower level of

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the Río Mayer Formation (Fig. 14B). The field observations suggest that slip did not

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occur synchronously on the normal faults. A late growth strata sequence of the lower

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Río Mayer Formation is observed to unconformably overlie an older growth strata sequence (Fig. 14C).

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The dacitic lava flow or shallow intrusion of the Río Guanaco dacite is intimately related to the lower Río Mayer Formation, as suggested by field

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relationships and geochronology (Fig. 14B). The mineralogical and compositional similarities between this unit and the rocks of the El Quemado Complex influence the

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interpretation of the Rio Guanaco dacite as a late volcanic manifestation related to the extensional tectonics in this area, and it has not been described before. The late

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volcanic activity related to rifting in the Berriasian is in accordance with the growth strata in the lower section of the Río Mayer Formation in the hanging wall of the

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normal faults, as observed in outcrop (Fig. 10C). 5.2.3. The onset of the Andean compression During the rest of the Early Cretaceous, the area experienced a phase of tectonic quiescence during which thermal subsidence was dominant and the middle and upper levels of the Río Mayer Formation were deposited. In the Late Cretaceous, the onset of the first Andean compressional phase produced deformation only in the basement domain of the fold and thrust belt (Fildani et al., 2008; Ghiglione et al., 2009; 2014; Romans et al., 2010). In the Río Guanaco region, the Upper Cretaceous units were deposited in a foreland setting and the record of the tectonic activity is represented by

ACCEPTED MANUSCRIPT some unconformities (within the Cerro Toro Formation and between the Alta Vista and Anita formations, according to Ghiglione et al., 2014). The scenario at the end of

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the Cretaceous, following deposition of the entire Río Guanaco section, is shown in

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Figure 14D.

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The Eocene tectonic activity appears to have produced no deformation in the study region. Kraemer (1998) postulated that the Eocene deformation was restricted to

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the Masters Range, to the west of the Río Guanaco area, and produced the homonymous triangle zone (Fig. 4). Following the author, the compressional stresses

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would propagate farther east only in the Miocene, with the development of a basal décollement. In addition, there is no significant foreland terrigenous sedimentation

5.2.4. Inversion phase

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related to this phase between 44 and 51oS (Thomson et al., 2001).

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The deformation associated with the migrating thrust front reached the area under analysis during Miocene times. This is based on regional thermochronological,

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topographical and geochronological data showing patterns of strong Neogene exhumation, erosion and tilting during tectonic uplift (Thomson et al., 2001; Hashke et al., 2006; Guillaume et al., 2009; Fosdick et al., 2013, 2014), and the basal age of resulting foreland deposits (Blisniuk et al., 2005; Fosdick et al., 2011; Cuitiño et al., 2012; Cuitiño and Scasso, 2013). Those foreland deposits rest unconformably on top of the marine Maastrichtian Calafate/Dorotea Formation, bracketing a Paleocene-lower Eocene hiatus (Camacho et al., 2000; Malumián et al., 2000) related to the Eocene deformational event (Marenssi et al., 2002).

ACCEPTED MANUSCRIPT Zircon and apatite fission-track data presented by Thomson et al. (2001) pointed out to an eastward migration of the cooling (and uplift) between 44 and 51 oS in the

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Patagonian Andes from 30-23Ma to 12-8Ma. Between the 50 and 52°S parallels, just

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south of the study zone, zircon and apatite fission-track cooling ages show active

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denudation of the basement front and the fold and thrust belt between 24 and 5 Ma (Fosdick et al. 2013). On the same region, zircon (U–Th)/He indicates an approximate

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~6 km exhumation depth since ~22 Ma, while apatite (U–Th)/He records young ages centered between 6 and 4 Ma, indicating significant late Miocene and Pliocene

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denudation (Fosdick et al., 2013). A little bit to the north of the Río Guanaco area, at the northern mesetas of Lago Viedma, this latest deformation is shown by gently tilted

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early Pliocene basalts (Mercer et al., 1975) that unconformably overlie more deformed Paleogene sediments (Coutand et al., 1999). In addition, volcanic ashes intercalated

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with foreland deposits of the Estancia 25 de Mayo and Santa Cruz formations, south of Lago Argentino, suggested Early Miocene ages to these deposits (Fosdick et al., 2011;

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Cuitiño et al., 2012; Cuitiño and Scasso, 2013). According to the abovementioned studies, the period of the main tectonic activity was in the Lower Miocene. It is here postulated that, in the Río Guanaco area, the compressional deformation occurred first with the folding of the ductile layers of both the Río Mayer and Cerro Toro formations, producing the anticline represented in Figure 14E. The folding triggered the rotation of the easternmost normal fault, located in the limb of the anticline, to a gentler angle, which facilitated its reactivation as a reverse fault. In addition, a hanging wall conjugate fault became active (Figs. 10, 14E). Other originally extensional faults did not experience effective rotation, as they are

ACCEPTED MANUSCRIPT positioned in the core of the anticline. These faults remained inactive and are described as fossil normal faults. As an exception, one of these faults presents hanging wall

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push-up faults, indicating that inversion acted on the fault system despite the inactivity

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of the main fault plane (Figs. 10E, 14). The progression of compressional stresses

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produced the thrust deformation and late backthrusts shown in Figure 14F, cutting through the anticline structure. The north-south-striking backthrust could originally

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have been a half-graben master fault. In contrast, the thrust faults shown in Figure 14F present dips with gentler angles and are interpreted as newly formed faults under

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compressional stresses.

5.3. Mechanics of extensional fault inversion

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The studied section of the Río Guanaco area (Fig. 10) exhibits both inverted and non-inverted normal faults. This situation introduces a discussion in terms of the

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mechanical criteria controlling fault inversion. The reactivation of faults under a different stress regime in place of the

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production of new faults occurs under special conditions, such as (i) favourable orientation in relation to the new stress field and (ii) low coefficient of friction on the fault surface (Sibson, 1985). A detailed discussion on whether a pre-existing extensional fault may be reactivated as a compressional fault is made by Mescua and Giambiagi (2012). In brief, the spatial orientation of the pre-existing fault (strike and dip versus shortening direction), the pore pressure and the coefficient of internal friction in the fault zone are considered to be the main parameters controlling inversion (Mescua and Giambiagi, 2012). In addition, physical modeling performed by Bonini et al. (2012) noted that the fault dip angle is an important factor in triggering

ACCEPTED MANUSCRIPT inversion. In cases in which the fault orientation is unfavorable relative to the stress field and the rheological parameters of the fault zone do not reach critical values, the

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fault will not be reactivated. Alternatively, conjugate faults could be inverted or a new

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thrust could be produced.

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The only fault in the Río Guanaco section with evidence of inversion along the main plane is the easternmost one in Figure 10, which is the fault with the most gentle

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dip angle (ca. 40º) in the area. As discussed above, this angle variation is the product of rotation during the development of the anticline. The field data confirm the

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importance of the spatial orientation of the extensional fault in relation to the shortening, principally the dip angle, if the shortening is considered to be

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perpendicular to the strike of the extensional fault population. Steeper faults remained motionless during the compressional phase. One

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exception is the fault shown in Figure 10E (fault dip of 75o), which developed a series of hanging wall push-up faults and footwall shortcut faults. This situation, which was

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observed in the field, is particularly significant in the seismic scale, especially in relation to the rift master faults (Figs. 12, 13). While little or no reverse slip occurred along the fault plane, inversion produced a complex network of hanging wall structures, and their significance is discussed below. First, it is important to consider the origin of these structures. Hanging wall conjugate faults are common features developed under extensional stresses (Koopman et al., 1987; McClay and Scott, 1991; Peacock and Sanderson, 1991; McClay, 1995; Withjack et al., 1995; Fossen and Gabrielsen, 1996; Willsey et al., 2002; Eisenstadt and Sims, 2005; Burliga et al., 2010; Bonini et al., 2012). With the onset of

ACCEPTED MANUSCRIPT compression, these structures, represent zones of weakness and may be reactivated as inverted faults whenever the master fault does not satisfy the mechanical conditions of

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reactivation.

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The observations made in the Austral Basin at the seismic scale (Figs. 12 and

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13) show little or no displacement along the half-graben and graben master faults, as well as the formation of prominent hanging wall highs formed by a network of push-

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up faults. In this case, the mechanical character of the master fault zones did not facilitate reverse displacement. To accommodate contraction, two possibilities should

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be considered: (i) the formation of new faults, such as thrusts and shortcut faults or (ii) the transfer of stress to the hanging wall conjugate faults. Thrust faults were not

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observed in the studied seismic reflection sections, and shortcut faults are rare. For instance, the main mechanism associated with inversion is the reactivation of the

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conjugate faults, as observed in several physical models (Koopman et al., 1987; McClay, 1995; Burliga et al., 2010; Bonini et al., 2012).

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Despite the occurrence of these conjugate inverted faults, the experiments carried out by the authors above involve a considerable amount of displacement on the master fault planes, producing, for example, harpoon features by folding and upwelling of the extensional growth strata section. This was not observed in the seismic reflection sections investigated here or in the case of the steeper faults observed in outcrop. A similar situation was described by Pace and Calamita (2013) in seismic sections of the Apennines fold and thrust belt, where, in the case of some faults, the contraction was concentrated more in a push-up complex than on the main fault. Pace and Calamita (2013) consider the rock blocks involved to be competent

ACCEPTED MANUSCRIPT multi-layers, with a carbonatic composition in the example, producing a rigid fault ramp. The system of push-up conjugate faults is then reactivated, distributing the

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shortening with little displacement on each individual plane. The metamorphic

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basement and the predominantly volcanic rift section of the Austral Basin are

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rheologically competent. Thus, as the master fault planes are not capable of absorbing the shortening, a complex network of hanging wall faults and fractures develops to

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accommodate the shortening, forming the push-up features (Fig. 15). In the case of individual faults, as presented in Figure 13, fault bending can

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facilitate the formation of hanging wall antithetic faults that can be reactivated as push-up structures, according to the fault-bend folding model of inversion presented

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by Mitra (1993).

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6. Final remarks and conclusions The normal faults generated during continental extension, inverted or not,

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observed in the Río Guanaco area show a spacing of approximately 300 m (Fig. 10). At the seismic scale, the master faults of the grabens and half-grabens show a spacing of 5-10 km. Thus, the scale of these features differs by an order of magnitude. As the seismic sections are relatively near the field area, it is postulated here that the structural framework is the same and that the structures are seen at different scales. It is probable that the faults observed in the field are antithetic, west-dipping faults subordinate to the regional east-dipping master faults, as seen in the seismic reflection sections (Figs. 12, 13). For instance, the faults observed at the outcrop scale may be the same push-up structures in the seismic survey.

ACCEPTED MANUSCRIPT Push-up features are important in the generation of structural highs in the hanging walls of the inverted extensional sections and have potential as hydrocarbon

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traps. Fine examples at the seismic scale have been identified in the Apennines fold

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and thrust belt (Pace and Calamita, 2013) and the Caunton Oilfield, England (McClay,

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1995). However, physical modeling does not reproduce these features properly: even when antithetic hanging wall extensional faults are produced and then inverted, the

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main displacement occurs on the main faults planes. The situation in which the pushup highs are the main product of contraction has yet to be reproduced in experiments.

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Boundary conditions, such as the competence of the basement and the syn-extensional section, seem to be the key to solving this problem. Knowledge of the conditions

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needed to produce this type of feature is important to predicting the real geological conditions in which the process may take place.

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The main conclusions of the present work are summarized below: (i) In the Río Guanaco area (50ºS), the Jurassic syn-rift section is composed of dacitic

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lava flows related to the El Quemado Complex. (ii) An age of ca. 141 Ma has been determined for the Rio Guanaco dacite, which is ca. 15 myr younger than the volcanic rocks of the El Quemado Complex in the same area. The age is compatible with paleontological record of the Río Mayer Formation. (iii) The structure of the Río Guanaco is of an anticline capped by Cretaceous rocks of the Río Mayer and Cerro Toro formations. The rocks of the El Quemado Complex, Springhill Formation (Upper Jurassic), and lower Río Mayer Formation are cut by north-striking, west-dipping, fossil and inverted normal faults, as well as thrust and backthrusts. The normal faults were active in the final phase of extension (Early

ACCEPTED MANUSCRIPT Cretaceous), as attested by the growth strata in the lower Río Mayer Formation. In the Early Miocene, compressional stresses produced shortening, and an anticline

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developed. In this process, a normal fault that was rotated to a gentler dip was

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reactivated as a reverse fault. The other normal faults were not rotated and do not

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reveal substantial slip on the main plane. They remained as fossil normal faults or, in one example, developed hanging wall push-up and footwall conjugate faults in order

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to accommodate shortening. West-verging backthrusts are interpreted as regional structures on the eastern branch of the Masters triangle zone, and constitute the main

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structure accommodating shortening in the studied area. The thrusts are subordinate to the backthrusts and, in one case, are the result of the breaking of an accommodation

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fold limb.

(iv) The seismic reflection sections show the master faults of the half-grabens and

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grabens of the Jurassic extensional phase. The faults are approximately north-striking, west- and east-dipping, spaced by 5-10 km. The extensional growth strata section is

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mainly composed of the El Quemado Complex. A gentle increase in the thickness of the Springhill Formation in the hanging wall in the vicinities of the extensional faults is also observed in one seismic reflection section (Fig. 13). (v) Contraction produced during Neogene Andean compression was accommodated by a complex network of hanging wall push-up faults. The original structures are probably hanging wall antithetic faults that developed during the extensional phase. The rigid rheological characteristics of the basement and syn-rift section (metamorphic and volcanic rocks, respectively) are thought to be the cause of such mechanical behavior. The coefficient of internal friction of these rocks is too high to allow the

ACCEPTED MANUSCRIPT reactivation of steeper-angle (approximately 65º) normal faults. In this case, the contractional mechanisms work on the complex network of antithetic hanging wall

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faults with little displacement along each individual plane, producing the push-up

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structures.

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(vi) Push-up hanging wall features should be taken into account in exploratory activities. The presence of push-up highs means that the structural traps may not be

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directly above the inverted master faults but may instead be located some kilometers

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away in the hanging wall.

Acknowledgements

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This article is a result of the post-doctoral project of H.Z., which was funded by Petróleo Brasileiro S.A. (PETROBRAS) and performed at the Instituto de Estudios

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Andinos Don Pablo Groeber (IDEAN - Universidad de Buenos Aires, CONICET). Petrobras Argentina (PESA) is acknowledged for the logistic support and the

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permission for studying and publishing the seismic sections in Figures 12 and 13. Petrobras Research & Development Centre (CENPES) supported the U-Pb analyses at the University of Brasília. The authors thank to the Department of National Parks of the Republica Argentina for permission to perform field work in Los Glaciares National Park. The authors also wish to thank their colleagues M. Agüera, M. Cagnolatti, G. Conforto, G. Köhler, E. Nigro and J. Rodríguez for the fruitful discussions and M. Pimentel for the critical revision of the manuscript. The reviewers M. Calderón, M. Malkowski, Z. Sickmann and an anonymous reviewer are acknowledged for the useful suggestions that improved the final version.

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paleoambiental de la Formación Río Mayer, Cretácico Inferior, Cuenca Austral, Provincia de Santa Cruz, Argentina. Latin American Journal of Sedimentology

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and Basin Analysis, 19(1): 3-26.

Ricchiano, S., 2014. Lower Cretaceous anoxic conditions in the Austral basin,

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southwestern Gondwana, Patagonia Argentina. Journal of South American Earth

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Sciences, 54: 37-46.

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Rodríguez, J. F., Miller, M., Cagnolatti, M., 2008. Sistemas petroleros de Cuenca

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Austral, Argentina e Chile. VII Congreso de Exploración y Desarrollo de Hidrocarburos, Instituto Argentino del Petróleo y del Gas.

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Romans, B.W., Fildani, A., Graham, S. A., Hubbard, S. M., Covault, J.A., 2010. Importance of predecesor basin history on sedimentary fill of a retroarc foreland basin: provenance analysis of the Cretaceous Magallanes basin, Chile (50-52º S). Basin Reserach, 22(5): 640-658. Russo, A., Flores, M. A., Di Benedetto, H., 1980. Patagonia Austral Extraandina. II Simposio de Geología Regional Argentina, Actas 2: 1431-1462. Schwarz, E., Veiga, G. D., Spalletti, L. A., Massaferro, J. L., 2011. The transgressive infill of an inherited-valley system: The Springhill Formation (lower Cretaceous)

ACCEPTED MANUSCRIPT in southern Austral Basin, Argentina. Marine and Petroleum Geology, 28: 12181241.

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Scott, K. M., 1966. Sedimentology and dispersal pattern of a Cretaceous Flysch

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sequence, Patagonian Andes, southern Chile. AAPG Bulletin, 50: 72-107. Sdrolias, M., Müller, R. D., 2006. Controls on back‐arc basin formation.

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Geochemistry, Geophysics, Geosystems, 7(4): 10.1029/2005GC001090. Sibson, R. H., 1985. A note on fault reactivation. Journal of Structural Geology, 7(6):

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751-754.

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Somoza, R., Ghidella, M. E., 2005. Convergencia en el margen occidental de América

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del Sur durante el Cenozoico: Subducción de Nazca, Farallon y Aluk. Revista de la Asociación Geologica Argentina, 60: 797-809.

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Somoza, R., Ghidella, M. E., 2012. Late Cretaceous to recent plate motions in westrn South America revisited. Earth Planetary Science Letters, 331: 152-163.

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Stacey, J.S., Kramers, J.D., 1975. Approximation of terrestrial lead isotope evolution by a two-stage model. Earth and Planetary Science Letters, 26: 207-221. Stern, C. R., Mohseni, P. P., Fuenzalida, R., 1991.

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significance of Lower Cretaceous Barros Arana Formation basalts, southernmost Chilean Andes. Journal of South American Earth Sciences, 4: 331-342. Stern, C. R., De Wit, M. J., 2003. Rocas Verdes ophiolite, southernmost South America: Remnants of progressive stages of development of oceanic-type crust in a continental margin back-arc basin. In: Dilek, Y., Robinson, P. T. (eds.),

ACCEPTED MANUSCRIPT Ophiolites in Earth History. Geological Society Special Publication, 218: 665683.

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Thomas, C. R., 1949. Geology and Petroleum Exploration in Magallanes Province,

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Chile. AAPG Bulletin, 33: 1553-1578.

Thomson, S. N., Hervé, F., Stöckhert, B., 2001. Mesozoic-Cenozoic denidation history of the Patagonian Andes (southern Chile) and its correlation to different

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subduction processes. Tectonics, 20(5): 693-711.

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Thomson, S. N., Brandon, M. T., Reiners, P. W., Tomkin, J. H., Vásquez, C., Wilson, N. J., 2010. Glaciation as a destructive and constructive control on mountain

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building. Nature 467: 313-317.

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Uliana, M. A., Biddle, K. T., 1988. Mesozoic-Cenozoic paleogeographic and

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geodynamic evolution of Southern South America. Revista Brasileira de Geociências, 18(2): 172-190. Willsey, S. P., Umhoefer, P. J., Hilley, G. E., 2002. Early evolution of an extensional

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monocline by a propagating normal fault: 3D analysis from combined field study and numerical modelling. Journal of Structural Geology, 24: 651-669. Wilson, T.J., 1991. Transition from Back-Arc to Foreland Basin Development in the Southernmost Andes - Stratigraphic Record from the Ultima-Esperanza-District, Chile. Geological Society of America Bulletin, 103: 98-111. Withjack, M. O., Islam, Q. T., La Pointe, P. R., 1995. Normal faults and their hangingwall deformation: An experimental study. AAPG Bulletin, 79(1): 1-18. Winn, R. D., Dott, R. H., 1979. Deep-water fan-channel conglomerates of late Cretaceous age, southern Chile. Sedimentology, 26: 203-228.

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the Fueguian Andes Hinterland. Sedimentary Geology, 229: 64-74.

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America. The map shows the triple junction between South America, Nazca and

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Antarctic plates, the main geological provinces, and the location of the Austral and

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Malvinas foreland basins; compiled after Ramos (1982, 1999), Kraemer et al. (2002), Kraemer (2003), Ramos and Ghiglione (2008), Ghiglione et al. (2010, 2012).

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Figure 2 – Simplified palaeogeographic map of the Jurassic extension in Southern

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Patagonia, showing the intracontinental rifts and the backarc Rocas Verdes basin

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(weight = single column)

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(modified after Rodríguez et al., 2008).

Figure 3 – Simplified stratigraphic column of the Austral Basin in the Lago Argentino

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region (compiled and modified from Rodriguez et al., 2008, Ghiglione et al., 2014 and Ricchiano, 2014).

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Figure 4 – Regional geological maps of the studied areas. (A) Lago Argentino and Lago Viedma region showing the location of the area mapped in detail and the seismic lines (modified after Ghiglione et al., 2014) (B) Bahía de la Lancha region (compiled from Riccardi, 1971 and Giacosa et al., 2012). (weight = 2 columns) Figure 5 – Geological map of the Río Guanaco area. See explanation in the text. (weight = 2 columns)

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(unconformity) and upper Río Mayer (RM) (non-conformity) formations. The dashed

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lines point out axial planes of a synform (left) and an antiform (right) on Bahía de la

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Lancha Formation (Lago San Martin region). (B) Flow banding and rotation of a rigid core. (C) Photomicrograph of a dacite showing euhedral and subhedral phenocrysts of

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quartz (q) and plagioclase (p) exhibiting features of chemical corrosion, such as irregular borders and embayment; biotite phenocrysts (b) are also shown. (D)

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Photomicrograph of a dacite displaying sericite (s) and less altered quartz-feldspar lenses forming fiamme-like structures, interpreted as two-phase patchy alteration;

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quartz (q), plagioclase (p) and biotite (b) phenocrysts are indicated; B, C and D are

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from Río Guanaco area. (weight = 1.5 column)

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Figure 7 – Outcrop view of the sedimentary units and intercalated volcanic rock in Río Guanaco area. (A) Springhill Formation. Plane bed (Sp) and trough-cross bed (St) sandstone; one single plane of each kind of bedding is highlighted in yellow; paleocurrent to the left (SW). (B) Lower level of the Río Mayer Formation (RM); S0 is the sedimentary bedding and S1 the foliation. Above RM the Río Guanaco dacite (RG) is observed. (weight = 1.5 column) Figure 8 – Río Guanaco Dacite. (A) Basal part of the Río Guanaco Dacite, displaying a fracture network that propagates from the contact troughout the dacite, which are

ACCEPTED MANUSCRIPT filled with the black shale of Río Mayer Formation. (B) Detailed view of the contact zone between Río Mayer Formation (RM) and Río Guanaco Dacite (RG). The arrow

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indicates a zone with embayments and a crack fill with shaly material. (C) Detailed

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view of the contact zone displaying a rough surface, rich in embayments and small-

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scale irregularities. (D) Photomicrography displaying quartz (q) and plagioclase (p) phenocrysts and the microcrystalline mass made of quartz-feldspar (qf), partially

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altered to sericite (s); some phenocrysts exhibits irregular borders and embayments

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(Río Guanaco section). (weight = 2 columns)

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Figure 9 – Results of LA-ICP-MS U-Pb analyses of zircons from sample 27C of Río

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Guanaco dacite. (A) Scanning electron microprobe images of the younger zircons,

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showing the age and the spot location (circle). (B) Concordia plot displaying the three age populations. The open ellipses represent individual grain analysis and the filled

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ellipses represent the mean ellipse for each zircon population. (weight = 1.5 column) Figure 10 – Aspects of the Río Guanaco section. (A) Plan view of the section. (B) Structure of the section. The arrows indicate the corresponding structure in A. (1) Normal faults (fossil), (2) Inverted normal faults, (3) backthrust, (4) thrust, (5) accommodation folds in the middle Río Mayer Formation, (6) thrust fault by local breaking of the accommodation folds. (C) Sketch based on (B) displaying the structure and stratigraphic units. (D) Stereogram of one fossil extensional fault, representing the poles of planar structures in the vicinities of the main plane, projected at the lower

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probable are related to the E-W lineament at the foothill of the section (E) Detail of an

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inverted normal fault. The arrows indicate the main plane dipping to the west, and the

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original and inverted sense of displacement. Note the hanging wall push-up antithetic faults and the footwall pop-up-like feature, interpreted as a shortcut fault (see text for

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explanation). EQ = El Quemado Complex, Sh = Springhill Formation, LRM, MRM,

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URM = lower, middle and upper Río Mayer Formation, respectively. (weight = 2 columns)

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Figure 11 – Stereogram showing the overall pattern of brittle structures of El

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Quemado Complex. The contours represent the frequency of poles of fractures, and the

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great circles are true reverse faults, all projected in the lower hemisphere. Population 1 is made of fractures and reverse faults of northwest-strike; population 2 represent

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fractures and reverse faults of northeast-strike. Population 3 are north-south-striking fractures (see text for interpretation). (weight = single column) Figure 12 – Non-interpreted and interpreted 2D seismic section AA´ (see Fig. 4 for location). Thick solid lines are the interpreted master faults, fine solid lines are conjugate faults and dashed lines are stratigraphic contacts. (weight = 2 columns)

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Formation in the hangingwall of some faults. Keys for faults and stratigraphic surfaces

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are in the caption of Figure 12. (weight = 2 columns)

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Figure 14 – Structural evolution model for the Jurassic-Cretaceous section of the Río Guanaco area. The arrows indicate the direction of stretching (in A, B and C) and

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shortening (in E and F). See text for explanation.

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Figure 15 – Sketch representing the rheological conditions of the basement, syn-rift

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and post-rift section of the Austral Basin, and the development of a hangingwall pushup fault complex.

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Figure 3 – Simplified stratigraphic column of the Austral Basin in the Lago Argentino region (compiled and modified from Rodriguez et al., 2008, Ghiglione et al., 2014 and Ricchiano, 2014).

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A. (1) Normal faults (fossil), (2) Inverted normal faults, (3) backthrust, (4)

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thrust, (5) accommodation folds in the middle Río Mayer Formation, (6) thrust

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fault by local breaking of the accommodation folds. (C) Sketch based on (B) displaying the structure and stratigraphic units. (D) Stereogram of one fossil

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extensional fault, representing the poles of planar structures in the vicinities of the main plane, projected at the lower hemisphere. Population 1 is related to

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the main fault, population 2 the anti-riedel shears of the main fault; population 3 represents fractures with grain comminution probable are related to the E-W

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lineament at the foothill of the section (E) Detail of an inverted normal fault. The arrows indicate the main plane dipping to the west, and the original and

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inverted sense of displacement. Note the hanging wall push-up antithetic faults and the footwall pop-uplike feature, interpreted as a shortcut fault (see text for

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explanation). EQ = El Quemado Complex, Sh = Springhill Formation, LRM, MRM, URM = lower, middle and upper Río Mayer Formation, respectively. (weight = 2 columns)

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Graphical Abstract

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Extension in the Austral Basin is Jurassic to Lower Cretaceous in age

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The dominant inversion structures are push-ups

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Rift structures were inverted in the Miocene during the Andean orogeny