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Morphotectonic analysis of two axial tributary basins of the San Juan river controlled by the Precordillera fold and thrust belt, Central Andes of Argentina
Journal Pre-proof Morphotectonic analysis of two axial tributary basins of the San Juan river controlled by the Precordillera fold and thrust belt, Central Andes of Argentina Pablo A. Blanc, Flavia Tejada, Laura P. Perucca, Karen Espejo, Gabriela Lara, Nicolás Vargas PII:
S0895-9811(19)30281-0
DOI:
https://doi.org/10.1016/j.jsames.2019.102441
Reference:
SAMES 102441
To appear in:
Journal of South American Earth Sciences
Received Date: 7 June 2019 Revised Date:
23 November 2019
Accepted Date: 24 November 2019
Please cite this article as: Blanc, P.A., Tejada, F., Perucca, L.P., Espejo, K., Lara, G., Vargas, Nicolá., Morphotectonic analysis of two axial tributary basins of the San Juan river controlled by the Precordillera fold and thrust belt, Central Andes of Argentina, Journal of South American Earth Sciences (2019), doi: https://doi.org/10.1016/j.jsames.2019.102441. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
CRediT author statement Pablo Blanc: Conceptualization, methodology, software, validation, formal analysis, investigation, writing, review, editing Flavia Tejada: methodology, software, investigation Laura Perucca: Conceptualization, methodology, validation, formal analysis, investigation, writing, review, resources Karen Espejo: methodology, software Gabriela Lara: methodology, software Nicolas Vargas: Resources. Other analysis tools
1
Morphotectonic analysis of two axial tributary basins of the San Juan river
2
controlled by the Precordillera fold and thrust belt, Central Andes of Argentina
3
Pablo A. Blanc1, 2, Flavia Tejada1, 2, Laura P. Perucca1, 2, Karen Espejo3, Gabriela Lara1, Nicolás
4
Vargas3
5
1
6
Ignacio de la Roza 590, CP 5400 San Juan, Argentina.
7
2
8
Argentina.
9
3
Gabinete de Neotectónica y Geomorfología, INGEO-FCEFN, Universidad Nacional de San Juan, Av.
Grupo Geología del Cuaternario, CIGEOBIO-CONICET, Av. Ignacio de La Roza 590, CP 5400 San Juan,
Departamento Geología, FCEFN, Universidad Nacional de San Juan, Av. Ignacio de La Roza 590, CP 5400
10
San Juan, Argentina.
11
[email protected],
12
[email protected],
13
[email protected],
14
[email protected],
15
[email protected]
16
*Correspondence ([email protected])
17
Abstract
18
Analysis of several morphotectonics tools (longitudinal river profiles, knickpoints, swath profiles,
19
and geomorphic indices) together with geomorphological, lithological, and structural data provided
20
objective and quantifiable elements to improve the knowledge of the Quaternary tectonic activity in
21
the region. In this study, we analyzed and compared two large intermountain basins located in the
22
Precordillera of the Central Andes of Argentina. Local distortions in longitudinal river profiles and
23
river patterns may reflect resistant lithology, an increase in shear stress, or differential surface uplift.
24
Both basins have similar shapes, although one mirrors the other producing a large-scale broom-
25
shaped drainage pattern. The values of the Asymmetry Factor and Transverse Topographic
26
Symmetry Factor show opposite signs and almost identical values, suggesting that both basins have
27
mirror symmetry independently of order and scale. The asymmetry in both basins would be a product
28
of the progressive migration of the Andean orogenic front towards the east. Knickpoints were mainly
29
associated with resistant rocks, faults with the scarp facing upstream, or both. In the reaches where
30
the rivers flow across areas with presumed higher rates of uplift, we observed a reduction in the
31
width of the active channel belt by lateral confinement and river pattern changes. The results
32
presented in this paper constitute one of the first attempts to apply geomorphic indices to large
33
drainage basins in the Precordillera fold-and-thrust belt.
34
Keywords: morphotectonics, geomorphic index, fluvial basin, Precordillera.
35 36
1.1 Introduction
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The development of river networks can often provide clues about the extent and characteristics of
38
Quaternary tectonic deformation in an area (e.g., Jackson and Leeder 1994; Burbank and Anderson
39
2001; Delcaillau et al. 2006). Active tectonics have a significant influence on the evolution of fluvial
40
systems by controlling drainage patterns, watersheds erosion, and the morphology of valleys (e.g.,
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Keller and Pinter, 2002). Rivers are sensitive to these changes and, therefore, adjust their
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morphology over different periods depending on rock strength, climate, and tectonic activity (Hack,
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1973). Local distortions in the longitudinal river profile may reflect the presence of knickpoints
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caused by more resistant lithology, an increase in shear stress, or surface uplift. These anomalies in
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river profiles could reflect a stream in disequilibrium, where the headward erosion transmit base-
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level changes to the upstream valley (Bishop et al. 2005).
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A longitudinal river profile is a curve that arises from the relationship between elevation and
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upstream/downstream distance along a river channel and sometimes can thus provide insight into the
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tectonic evolution of a drainage basin.
50
In
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geomorphological or climatic processes can be quite difficult. The methods and tools commonly
52
used in the morphotectonic analysis (longitudinal profiles, knickpoints recognition, indices like basin
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elongation ratio, drainage basin asymmetry factor, transverse topographic symmetry factor, and
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stream sinuosity, among others) are highly sensitive to multiple variables such as lithology, tectonics,
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climate, and fluvial dynamics. However, fluvial anomalies—such as local development of meanders
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or a braided pattern, local widening or narrowing of channels, anomalous ponds, marshes or alluvial
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fills, variation in the width and continuity of the levees, and any abnormal curve or turn of the fluvial
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network—can also be indicators for active tectonics (Schumm et al., 2002; Struth et al., 2015;
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Perucca et al. 2018, among others).
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In this study, we analyzed and compared (from a morphotectonic point of view) two intermountain
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basins located between the orogenic fronts of Central and Eastern Precordillera (Fig. 1a, b). These
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two river basins, known as the De la Travesía and De la Ciénaga, are considerably extensive and
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stand out for their remarkable geological and tectonic complexity. Paleoseismological and
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morphotectonic studies (Bastías, 1986; Paredes and Perucca, 2000; Perucca et al., 2012, 2013,
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among others) have constrained long-term Quaternary deformation and active crustal faults in this
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region. These basins, drain to the Ullum-Zonda valley from the north and south respectively along a
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tectonic corridor called Matagusanos-Maradona-Acequión (Perucca, 1990) where two structural
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systems of opposite vergence converge and collide (Central Precordillera with eastern vergence and
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Eastern Precordillera with western vergence) (Fig. 1c). Both basins have a similar shape, and one
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appears to mirror the other (Fig. 1a), producing a large-scale broom-shaped drainage pattern
71
according to the concept of Audemard (1999). As stated by Suriano et al. (2015), during the
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mountain building of Precordillera, active folds and thrusts deviated several rivers from the original
geologically
complex
basins,
attributing
morphotectonic
indicators
resulting
from
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W-E regional slope. These N-S axial fluvial systems flow to a large W-E transference river (San
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Juan River) that connects these basins draining the fold and thrust belt to the foreland region. In this
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way, the San Juan River and its tributaries converge in a single channel that crosses the Eastern
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Precordillera, developing a mega broom-shaped river pattern (Audemard, 1999) (Fig. 1a). Most of
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the rivers that cross W-E the N-S trending mountain ranges are controlled by NW, SW, and W-E
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lineaments (Baldis and Chebli, 1969; Oriolo et al., 2015; Pantano, 2015; among others).
79
To recognize tectonic signals, we collected field data on the lithology and active faults features.
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Also, we constructed topographic swath profiles, longitudinal profiles, and analyzed the channel
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morphology of main rivers with the aid of river network geomorphic indices. The objective of this
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paper is to assess Quaternary and current deformation patterns and transient geomorphological
83
processes with the aid of suitable morphometric and morphotectonic analysis, remote sensing and,
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field data. This assessment would allow us to gain a better understanding of the evolution and
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behavior of the foreland area during the last stage of the Andean orogeny.
86 87 88 89 90 91 92 93 94
Fig. 1. (a) Location of the De la Travesía and De la Ciénaga basins in Precordillera within San Juan province and Argentina, (b) Map of the western border of South America showing the main geological provinces and the flat-slab subduction segment of the Nazca plate, between 28° and 33°S based on Cahill & Isacks (1992) (compared with the Benioff geometry proposed by Pardo et al. 2002 and Alvarado et al. 2005a,b) main basement uplifts of Sierras Pampeanas (Jordan et al. 1989) (colored in pink), and location of the Precordillera fold and thrust belt (dark grey) (Ramos et al. 2002), (Modified from Ramos and Folguera, 2009). The red line represents the cross-section location, (c) Crustal-scale cross-section of the Precordillera, and Sierras Pampeanas. Scale 1:1 (Modified from Vergés et al., 2007; Siame et al., 2015).
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1.2 Geological Setting
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The geological province of Precordillera is a fold and thrust belt located to the east of the Andes
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Cordillera, between 28° 37' S and 33° 05' S (Fig. 1a, b, c). This province is constituted mainly of
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marine and continental Paleozoic sedimentary rocks and Cenozoic synorogenic deposits. In this
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region, the structural evolution of the Andes is the result of the oblique convergence between the
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Nazca and South America plates. The sub-horizontal geometry (flat slab) of the Nazca Plate has been
101
held responsible for the migration of the orogenic front towards the east, the absence of volcanism,
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the intense intra-plate seismicity, the foreland uplift of the Sierras Pampeanas (Jordan et al., 1983)
103
and, the tectonic activity occurred during the Quaternary, localized mainly on the Eastern
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Precordillera (Ramos et al. 1997; Costa et al., 2000). This flat-slab configuration has been attributed
105
to the subduction of the Juan Fernández Ridge beneath the South American Plate (Pilger, 1981,
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Anderson et al., 2007) (Fig. 1b). At these latitudes, three main structural domains are defined in the
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eastern flank of the Andean orogen (Jordan et al., 1983; Ramos, 1988; Mpodozis and Ramos, 1989).
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From west to east, they are the Western and Central Precordillera, the Eastern Precordillera, and the
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Sierras Pampeanas (Fielding and Jordan, 1988; Ramos, 1988).
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The Central Precordillera is a thin-skinned fold and thrust belt with eastern vergence dipping
111
between 20º and 30º to the west (Zapata and Allmendinger, 1996). Marine Ordovician limestone and
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marls, Devonian shales, Carboniferous fluvial deposits with marine intercalations, and synorogenic
113
foreland sequences with the intrusion of Neogene acidic sub-volcanic bodies characterize this
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mountain range. The Quaternary is represented by heterogeneous Pleistocene-Holocene alluvial and
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lacustrine deposits (Fig. 1c, 2). The predominant lithology varies significantly, both texturally and
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compositionally, depending on the area of provenance. The alluvium exhibits a clear predominance
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of clasts of igneous rocks, sandstones, greywackes, and andesitic-dacitic rocks coming from the
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mountain ranges located to the west and within the study area.
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The Eastern Precordillera is characterized by west verging structures involving the basement, with a
120
detachment down to 30 km deep (Comínguez and Ramos, 1991; Zapata and Allmendinger, 1996).
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These structures cut but do not expose the basement, showing a thick-skinned deformation
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mechanism similar to that of Sierras Pampeanas (Rolleri, 1969; Ortiz and Zambrano, 1981; Ramos,
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1988; Allmendinger et al., 1990). A core of Cambro-Ordovician calcareous rocks and, to a lesser
124
extent, Silurian, Devonian and, Carboniferous marine siliciclastic rocks forms the Eastern
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Precordillera. A belt of Neogene synorogenic foreland sequences, constituted by alluvial deposits
126
(Fig. 1c, 2), surrounds this Paleozoic core.
127 128
1.3 Physical Geography
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The valley of Matagusanos (Fig. 1a) constitutes an elongated depression trending N-S with a
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maximum width of approximately 20 km. The Sierra de Villicum to the east and the Sierras de
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Talacasto and La Dehesa to the west delimit this depression, located at an average height of 900 m
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a.s.l. The ephemeral De la Travesía River is the primary collector of the basin. This river runs from
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N to S and flows into the San Juan River (Ullum Reservoir). The exoreic drainage allows us to
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define this depression as a "semibolson"—as opposed to closed depressions with centripetal
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drainage. Fine-grained deposits of an ancient playa lake (Fig. 2) occupy the central portion of the
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depression, with an approximate length of 20 km and a maximum width of 2 km. This playa lake,
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ephemerally flooded during the summer season, shows an almost flat surface with a scarce
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vegetation cover and incised gullies formed by headward erosion.
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140 141 142
Fig. 2. Lithological and structural map of the De la Travesía and De la Ciénaga basins (Modified from Paredes and Perucca, 2000; Ramos and Vujovich, 2000; Perucca et al., 2013, Lara et al., 2018; and Blanc, 2019).
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The valley of the De la Ciénaga River locates east of the Cerro Zonda, Cordón de las Osamentas and
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lower hills with an average N-S trend. To the east of the valley is the Sierra Chica de Zonda and
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Cerro Bayo (Figs. 1a and 2). This intermittent river drains from the southwest to the northeast,
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pouring its scarce flow into the valley of Ullum-Zonda. The maximum width of this depression
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(about 25 km) occurs in the De la Travesía river valley. At its narrower section, its width is less than
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5 km. The Barrancas playa lake locates in the southern sector of the depression and the upper section
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of the basin (Fig. 2). This ancient playa lake served as a depocenter for the rivers flowing from the
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eastern flank of the Cordón de Las Osamentas.
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In the study area, mountain ranges have a predominant N-S trend with an asymmetric transverse
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profile with its steeper flanks along the fault fronts. They show an extensive piedmont with elongated
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alluvial plains and ancient playa lakes with evidence of present fluvial incision. The Piedmont of the
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Central Precordillera has a remarkable width (~ 16 to 18 km) compared to the western piedmont of
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the Eastern Precordillera (~ 0.8 to 7 km). The depocenters along both basins are displaced towards
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the east, resulting in markedly asymmetric basins (Fig. 2). The piedmonts show alluvial fans,
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colluvial fans, debris cones, and some forms of erosion such as pediments (Fig. 2) with an alluvial
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cover of variable thickness. Fluvial plains and terrace levels associated with distributary fluvial
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systems of the collector-conoid river type (Suriano and Limarino, 2009) and playa lake deposits
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dominate the lower areas. The mouths of the De la Ciénaga and De la Travesía rivers are deeply
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affected by land use (i.e., crops, mines, alluvial control systems), all of which are exerting an
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influence on the current fluvial dynamics.
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The dominant climate is of the BWwka type according to the Köppen classification (Poblete and
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Minetti, 1989), where (BW) corresponds to dry desert climate with xerophyte vegetation or no
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vegetation with a summer concentration of rainfall (w), an annual average temperature of less than
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18ºC (k), and the temperature of the warmest month above 22ºC (a). Other features are intense solar
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radiation and high atmospheric transparency, as well as a marked annual and diurnal/nocturnal
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thermal amplitude.
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As mentioned before, streams that flow across the De la Travesía basin are ephemeral due to the
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scarce rainfall (approximately 80 mm/year). Instead, in the De la Ciénaga basin, rainfall is slightly
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higher (close to 100 mm/year on average). The latter presents an intermittent discharge. Some of its
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tributaries carry water throughout the year since they are partly fed by springs located along
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Quaternary faults such as the Papagayos River when it crosses the Maradona and Papagayos faults.
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(Lara et al., 2017, 2018) (Figs. 2 and 3). In both basins, most of the runoff comes from torrential
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summer precipitations. During short lapses, large flood peaks mobilize the thick detrital
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accumulations (debris and mudflows) on mountain slopes. However, snowmelt in the highest
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mountain ranges (mainly in the western part of the De la Ciénaga basin) also contributes to the basin
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runoff.
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Vegetation is predominantly shrubby, although there are also some low arboreal species and cacti.
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The distribution of vegetation is closely associated with the different landforms, soil type, and water
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availability (i.e., springs and water table position).
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Landforms in both basins relate to the fluvial dynamics of the San Juan River, which represents the
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local base level (Fig. 1a). This river presents a glacial-nival regime with an average annual flow of
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60 m3/sec and maximum runoff peaks occurring between December and January. Millenary floods
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might exceed 1,000 m3/sec (Perucca and Esper, 2009). Currently, numerous hydraulic works built for
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hydroelectric generation and irrigation purposes regulates the streamflow.
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1.4 Quaternary Tectonics
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Perucca (1990) characterized the succession of elongated N-S valleys in the boundary between
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Central and Eastern Precordillera as a tectonic corridor called Matagusanos-Maradona-Acequión
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(Fig. 1a). In the study area, its width varies between 5 km in its central portion (Zonda Valley) and
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25 km in the Matagusanos and the De la Ciénaga River valleys (Fig. 1a). To the east, this
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longitudinal tectonic valley is bordered by the sierras de Villicum, Marquesado, and Chica de Zonda
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(Eastern Precordillera). To the west, the Sierra de La Dehesa, Cerro Zonda, Sierra Alta de Zonda,
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Cordón de Las Osamentas, and low hills (Central Precordillera) borders this valley (Fig. 1a)
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(Perucca, 1990; Perucca and Onorato, 2011; Perucca et al., 2013).
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According to Zapata and Allmendinger (1996), the interaction between thrusts with opposite
197
vergences during the Cenozoic favored the development of a thick-skinned triangle zone (Fig. 1c).
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The formation of this triangular zone would have begun after 2.7 Ma when the thick-skinned
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deformation system of the Eastern Precordillera became active (Zapata and Allmendinger, 1996).
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The evidence of Quaternary faulting recognized throughout the Matagusanos-Maradona-Acequión
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tectonic corridor is consistent with this structural style (Perucca et al., 2013; Audemard et al., 2016).
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Smalley et al. (1993) mentioned the Matagusanos valley as one of the areas with the highest
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seismicity in the province of San Juan. This valley may have originated as a depression bordered by
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reverse faults (Rolleri, 1969). Its subsequent evolution, however, could be closely related to thrusts
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with western vergence, which would justify its marked asymmetry and the depocenter located
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towards the east (Paredes and Perucca, 2000).
207
On its eastern edge, the Matagusanos-Maradona-Acequión tectonic corridor is bordered by the
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Villicum-Zonda fault, which belongs to the Eastern Precordillera Fault System (Fig. 3) (Bastías,
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1986). This regional fault manifests itself along 145 km as a series of traces and segments (e1 to e9 in
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Fig. 3) with a mainly NNE trend, affecting the western flank of the mountain ranges of the Eastern
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Precordillera, between 31° S and 32° 20'S. These traces and segments are high-angle reverse faults
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with a western vergence that place Cambrian limestone and Neogene rocks over Quaternary alluvial
213
deposits. These Quaternary deposits frequently develop anomalous slopes opposite to the original
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slope of the piedmont, progressive unconformities, and fault propagation folds (Siame et al. 2002,
215
2005; Perucca et al., 2012; Blanc and Perucca, 2017; among others).
216
In the western piedmont of the Sierra de Villicum in the De la Travesía basin, the poorly
217
consolidated alluvial deposits make difficult the preservation of scarps and other morphological
218
evidence of Quaternary faulting. However, a few scarps can be observed affecting older terraces in
219
Quaternary alluvial fans, next to the mountain front of the Sierra de Villicum. Also, to the west and
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slightly away from this mountain front, Pliocene-Pleistocene deposits show discontinuous N-S
221
trending scarps facing west along ~ 20 km (structures e3 and e4, Fig. 3). Another evidence of
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Quaternary tectonic activity is the presence of alluvial deposits from the Sierra de Villicum overlying
223
the Pliocene-Pleistocene sequence with an anomalous slope (contrary to the original) possibly due to
224
the activity of the e4 structure located to the west (Paredes and Perucca, 2000).
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South of the San Juan River, in the De la Ciénaga basin, the western front of the Sierra Chica de
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Zonda shows a sinusoidal shape bounded to the west by several parallel traces of the Villicum-Zonda
227
fault (e1, e6, e7, e8 in Fig. 3). Horsetail-type faults branch from the main fault (e1) towards the
228
northeast (Fig. 3). These subsidiary faults show activity at least until the upper Neogene. In the
229
piedmont, between the CE-3 and CE-4 rivers, Perucca et al. (2012) pointed the presence of several
230
"hanging" Quaternary alluvial levels, progressive unconformities, and anomalous slopes in the fill
231
terraces dipping to the east with an average angle of 15 ° (Fig. 5a). Between the rivers CE-4 and CE-
232
6 (Fig. 3), a series of foothills (made of old Quaternary alluvial deposits and calcareous rocks of
233
Cambrian age) have been uplifted, together with their younger Quaternary alluvial cover, by a
234
reverse fault subparallel to the main trace of the Villicum-Zonda fault (fault e6, Figs. 3 and 5b).
235
Scarps related to this fault trend NW with the free face to the west (Perucca et al. 2012).
236
The Central Precordillera orogenic front, characterized by its eastern vergence, constitutes the
237
western edge of the Matagusanos-Maradona-Acequión tectonic corridor. In the piedmont of the
238
Sierra de la Dehesa (De la Travesía basin), Bastías (1986) identified the east-verging Talacasto-La
239
Dehesa fault (b1 and b2, Fig. 3). This fault has a length of 20 km, a ~N-S trend, and a dip of 60 ° to
240
the east, affecting the alluvial fan of the Tambolar River (TW-3). Where this river crosses the fault
241
trace, Neogene rocks can be seen overriding alluvial deposits assigned to the Quaternary, showing an
242
east-facing scarp up to 18 m high (Paredes and Perucca, 2000).
243
244 245 246
Fig. 3. Main Quaternary structural features from the De la Ciénaga and De la Travesía basins (Modified from Paredes and Perucca, 2000; Ramos and Vujovich, 2000; Perucca et al., 2013; Lara et al., 2018; and Blanc, 2019).
247
248 249 250 251 252 253 254
Fig. 4. (a) View to the south of the Matagusanos Playa Lake showing headward erosion of the De la Travesía River, (b) View to the east of the Tambolar River valley in Lomitas de Matagusanos area. Note the flat bottom of the riverbed (FBR). The Sierra de Villicum can be seen far to the east, (c) View to the south of the anticline valley of the Lomitas de Matagusanos. A couple of west-verging reverse faults (d1, Lomitas de Matagusanos fault system) produced progressive tilting to the east of Pleistocene strath terraces. NeS: Neogene sandstones; NeC: Neogene conglomerates; Qad: Quaternary alluvial deposits.
255
To the east and very close to the mountain front of the Sierra de la Dehesa, there are a series of low
256
hills called Lomas del Salto. These hills have an approximate length of 10 km and a maximum width
257
of 1.2 km and are affected on both flanks by high-angle opposite reverse faults. (C, Fig. 3). These
258
faults place Silurian marine rocks over fluvial sedimentary rocks of Neogene age. The Lomas del
259
Salto fault dips to the east between 40° and 70°. Conversely, the fault located on the eastern flank of
260
these hills dips towards the west from 70° to 75° (Paredes and Perucca, 2000). To the north, these
261
faults affect Quaternary alluvial deposits. Fine-grained clastic deposits (silts, light-colored clays),
262
and a bank of travertine in a fault scarp related sag pond yielded an age of 16,575 ± 300 14C ka BP
263
(Paredes and Perucca, 2000).
264
To the east of the Lomas del Salto, lies the Blanquitos fault area (d2, Fig. 3) where several alluvial
265
fans are affected by N-S trending subparallel faults. These faults dip to the east with a high angle at
266
the surface and show counter-slope scarps with the free face facing west. Paredes et al. (1997) dated
267
a bank of travertine along the Blanquitos fault plane at 28,422 ± 1,335 14C ka BP. Further east in the
268
Lomitas de Matagusanos area, Paredes and Perucca (2000) identified high-angle reverse faults with
269
western vergence (d1 in figs. 3, 4a-c). The Lomitas de Matagusanos hills are 35 km long, 3.5 to 4 km
270
wide, and trend NE. Neogene rocks that form a west-verging fault propagation fold (Baldis et al.,
271
1979) constitute these low hills. The entire sector shows clear evidence of Quaternary tectonic
272
activity in the form of tilted and uplifted relict alluvial fan surfaces (Fig. 4c).
273
To the south of the San Juan River, Perucca and Onorato (2011) described the Las Osamentas fault,
274
located to the west of the De la Ciénaga basin (a10, Fig. 3). This fault has an azimuth of 350 º, dips
275
30º to 45° to the west, and puts Carboniferous deposits over Pleistocene sediments. Towards the east,
276
in the piedmont of the Cordón del Espinacito and Cordón de las Osamentas, Bastías (1986) identified
277
the Maradona reverse fault (a12, Figs. 3, 5c, and 5e). Lara (2015) named this 30 km-long set of NNW
278
trending parallel structures Maradona fault system, which places Neogene rocks over Quaternary
279
alluvial deposits with an average 45° W dip and eastern vergence. These faults are easily
280
recognizable by a series of scarps and aligned springs with abundant vegetation. The alluvial cover
281
shows anomalous slopes and terraces (Perucca, 1990; Moreiras and Banchig, 2008; Perucca et al.,
282
2012; Lara, 2015; Lara et al., 2017, 2018). Based on the age of the affected alluvial levels, scarps
283
range from ~50 meters high to the north to small and eroded escarpments to the south where they
284
disappear beneath the alluvial cover. It is presumed that this fault continues further south as a blind
285
structure (Perucca and Vargas, 2014; Lara, 2015; Lara et al. 2017, 2018).
286
287 288 289 290 291 292 293 294 295 296
Fig. 5. (a) View to the northeast of the western flank of the Sierra Chica de Zonda in the reach 6 of the De la Ciénaga River. In the top of the foothills, piedmont related Quaternary deposits lie with an anomalous dip (opposite to the piedmont slope) on top of Cambrian limestone. The white arrow points to the location of the e8 structure. (b) View to the southeast. White arrows point Chica de Zonda fault scarp (e6). Piedmont related Quaternary alluvial deposits lie with anomalous dip on top of Cambrian limestone. (c) View to the west of the Maradona valley and the fault scarp (a12) pointed by the white arrows. In the background, the Cordón de las Osamentas. (d) View to the southwest of a natural exposure of the Papagallos fault (a17’) in the Papagallos River valley (CW-2), disrupting Pleistocene alluvial terraces and fine-grained deposits from fault-related springs. (e) Maradona thrust exposure, showing Neogene rocks overriding Quaternary alluvial deposits. Є: Cambrian; Ne: Neogene; Q: Quaternary.
297
Further east, the Cerro Tres Mogotes fault (a13, Fig. 3) constitutes a west-verging backthrust
298
associated with pre-Quaternary structures (Baldis and Chebli, 1969). On the eastern flank of the
299
Cerro Tres Mogotes, Perucca et al. (2012) recognized reverse faults with eastern vergence and N-S
300
trend in a natural exposure located on the left margin of the Papagallos River (CW-2, Figs. 3 and 5d).
301
These faults constitute a branch of the Papagayos fault (a17’, Fig. 3) (Perucca et al. 2012). They have
302
an almost N-S trend (8°) and dip 15° to 20° to the west affecting 20 m-thick Pleistocene alluvial
303
deposits and fine-grained deposits related to the upflow of groundwater along the fault. A second
304
fault, parallel to the previous one, shows an azimuth of 215° and dips 25° to the west. Fault-plane
305
displays striae suggesting a right-oblique movement (Perucca et al., 2012). In the fine-grained
306
deposits affected by these faults, these authors dated a carbonate level at 22,420 ± 390 years BP. The
307
last reactivation is considered to have occurred before this age.
308 309
2.1 Methodology
310
The analysis of morphometric parameters allows the identification of characteristics of a drainage
311
basin that could respond to active tectonic deformation (Keller and Pinter, 1996, 2002). In this work,
312
we analyze topographic, geological, and neotectonic data together with morphometric indexes to
313
describe and compare two genetically related basins in the Precordillera. We used GPS mapping,
314
topographic maps, Digital Elevation Models with a 45 and 5 m resolution from the National
315
Geographic Institute of Argentina (IGN), based on ASTER (Advanced Spaceborne Thermal
316
Emission and Reflection Radiometer) DEM (30 m), and satellite imagery interpretation to determine
317
the main geomorphic and morphotectonic features of the basins, due to the regional scope of the
318
study. Maps, profiles, and figures were plotted with QGIS (Quantum GIS) and Origin Software. For
319
greater clarity in the description, each one of the analyzed tributaries has been identified with a code
320
(T = De la Travesía, C = De la Ciénaga, E = east margin, W = west margin) and numbered in
321
upstream order.
322
The De la Travesía and De la Ciénaga basins were delineated based on the watershed divide concept
323
(an area in which all water flowing through it, goes to a common outlet). The stream order of a basin
324
assigns a numeric order to rivers based on the number of tributaries (Strahler, 1964). Being equal in
325
relation to the area, climate, and substrate, a larger order of a river basin implies a more developed
326
river (Horton, 1945).
327
To analyze the shape, geometry, and slope changes and to detect the presence of knickpoints, we
328
constructed longitudinal profiles of the main river and selected tributaries together with riverbed
329
lithology and main structures. Our regional morphometric analysis also relies on four basin-wide
330
topographical W-E swath profiles (3 km wide), obtained with SAGA GIS, with the mean, maximum,
331
and minimum elevations projected into cross-section planes. Such a window width ensures that the
332
minimum and the maximum elevations of the river valleys are measured. These profiles were
333
constructed W-E (i.e., transverse to the trends of river valleys: two across De La Travesía basin A-
334
A’, B-B’; and two across the De la Ciénaga Basin, C-C’ and D-D’) to identify several topographic
335
features, relief anomalies, and valley shape. To assess the behavior of the two primary collectors in
336
the study area (De la Travesía and De la Ciénaga rivers), we constructed two along-river swath
337
profiles, using their paths as baselines.
338
For river pattern analysis, we identified three types of channels: straight, bar-braided, and island-
339
braided, based on sinuosity, number of channels, and island stability (Schumm, 1985; Nanson and
340
Knighton, 1996). These patterns result from sediment size, load, stream power, and flow velocity
341
(Schumm, 1986), which in turn may respond to external forcing by tectonics or climate change
342
(Kirby and Whipple, 2012).
343
To determine the way in which Quaternary tectonic activity influences both basins, we used several
344
morphotectonic and morphometric indices together with the available geological, geomorphological,
345
and topographic data. Morphotectonic analysis using geomorphic indices can be used as a
346
preliminary reconnaissance tool to identify areas experiencing rapid tectonic deformation (e.g., Bull
347
and McFadden 1977; Keller and Pinter 1996, 2002). Drainage networks are highly sensitive to
348
tectonics, providing information about the tectonic behavior of a region. These indices have already
349
been used for basin characterization in other areas of the Central Andes of Argentina by Esper
350
Angillieri (2008, 2012); Perucca and Esper Angillieri (2011); Perucca et al. (2018), among others.
351
The morphometric and morphotectonic indices of both basins were quantitatively calculated using
352
GIS software. We divided these parameters into basic and derived parameters. Basic parameters are
353
area (A), perimeter (P), length (L), width (W), stream order (Nn), and main channel length (Mcl).
354
We calculated the main channel length (Mcl) and length (L), according to Schumm (1956). Derived
355
parameters are Basin Elongation Ratio (Er), Drainage Basin Asymmetry Factor (AF), Transverse
356
Topographic Symmetry Factor (T), and Stream Sinuosity (S) (Table 1). The Basin Elongation Ratio
357
(Er) is the ratio between the diameter of a circle, having the same area as that of the drainage basin,
358
and the maximum length of the basin (Schumm, 1956). When the value of Er deviates from one (0.6
359
to 0.8), the area is expected to have a sharp relief and a steep ground slope. The Asymmetry Factor
360
(AF) represents the drainage basin asymmetry and helps in determining the general tilt of the basin
361
landscape regardless of whether the tilt occurred due to local or regional tectonic deformation (Hare
362
and Gardner, 1984; Gardner et al., 1987; Keller and Pinter, 2002). It is determined by the formula:
363
AF = 100 (Ar /At) where Ar is the area of the basin to the right of the trunk stream while facing
364
downstream and At is the total area of the drainage basin. For a stream network that develops and
365
continues to flow in a stable setting and uniform lithology, the asymmetry factor should be equal to
366
50, whereas an unstable setting would give a variation from normal of either < 50 or > 50 (Keller and
367
Pinter, 1996).
368
The Transverse Topographic Symmetry Factor (T) is another way to establish the tilt direction. Cox
369
(1994) defined it as T = Da / Dd, where Da represents the distance from the midline of the drainage
370
basin to the midline of the active meander belt, and Dd corresponds to the distance from the basin
371
midline to the watershed divide. Perfectly symmetric basins have values of transverse topographic
372
symmetry (T) equal to zero. As the asymmetry increases, T increases and approaches the value of
373
one.
374
Stream Sinuosity (S) is defined as the ratio between the main channel length (Mcl) and valley length
375
(L). Schumm (1963, 1977) pointed out that stream sinuosity reflects the variability of the valley
376
slope and that changes in the valley floor slope produce downstream changes of sinuosity. Tectonic
377
activity is one of the conditions that control the changes in the valley floor slope (e.g., Schumm,
378
1986; Schumm et al., 2002).
379
3.1 Results
380
3.1.1 Longitudinal river profiles
381
The western tributaries of the De la Travesía River (TW-1 to TW-5), flow through several mountain
382
ranges and hills, and their paths lead them across a wide variety of active structures and rock types.
383
In these river profiles, most of the observed knickpoints are coincident with structures, contrasting
384
lithological contacts, or both (Figs. 6a, 2, and 3).
385
To the southwest, rivers TW-1 and TW-2 show very similar profiles and are affected by the same
386
structures. The most prominent knickpoints are related to the Talacasto-La Dehesa fault (b1),
387
especially where it exposes Ordovician limestone, and to the Lomitas de Matagusanos fault (d1)
388
which exposes Neogene sandstones and tuffs. However, in the Corral Viejo River (TW-2) between
389
structures a5 and B stands out a knickpoint that does not coincide with any known structure or
390
lithological contrast (Figs. 3 and 6a).
391
To the west, in the upper reaches of the Tambolar River (TW-3), subtle knickpoints coincide with the
392
Sierra de la Cantera fault traces (a1). Eastward, where TW-3 flows through the Cerro Tambolar fault
393
(a2), there is a marked knickpoint and a gradient reduction. Downstream of the structure a2, this river
394
shows entrenched meanders and a steepened valley floor in the reach coincident with the Cerro
395
Tambolar mountain range. At the end of the meandering reach, we observed a second knickpoint and
396
a gradient increase that does not coincide with any previously known structure or lithological
397
contrast. Downstream, coincident with the Los Gauchos fault (a4), there is an almost undetectable
398
convexity in the profile. The easternmost trace of the Talacasto-La Dehesa fault (b2, Figs. 3 and 9) is
399
one of the best examples of an active fault in the study area with at least 13 m of vertical uplift
400
during Late Pleistocene to Holocene. However, it was not possible to detect a knickpoint where TW-
401
3 crosses the fault trace. Contrarily, the older and presumed inactive main trace of the Talacasto-La
402
Dehesa fault (b1) that exposes Ordovician limestone produces a clearly visible knickpoint. We
403
observed the same situation in the river TW-5. The Lomas del Salto (c2) and Blanquitos (d2) faults,
404
despite showing in the field clear evidence of Quaternary tectonic activity, only seem to delimit river
405
reaches with slightly different slopes in the river profile. To the east of the Lomitas de Matagusanos
406
fault (d1), the TW-1, TW-2, and TW-3 rivers show a marked concavity. In the Lomitas de
407
Matagusanos, the Neogene strata outcrops forming an anticline valley or combe. As these sandstones
408
turn out to be easier to erode —even than the unconsolidated alluvial deposits— the profile of rivers
409
shows a strong concavity in this sector. Further east, after crossing the Lomitas Oriental fault (d3),
410
the profiles show a smooth convexity where the recent alluvial deposits follow the Neogene
411
conglomerates.
412
The tributary rivers that converge to the De la Travesía River from the east (TE-1 to TE-6) flow
413
across the western piedmont of the Sierra de Villicum and the Loma de Ullum. They are of a lesser
414
order than their western counterparts (TW-1 to TW-5). The TE-1 and TE-2 rivers flow across the
415
western piedmont of the Loma de Ullum, showing a slight concave to straight profile without
416
detectable knickpoints. Their beds are made entirely of alluvial deposits. The TE-4, TE-5, and TE-6
417
rivers flow across the western piedmont of the Sierra de Villicum. Their longitudinal profiles show a
418
significant gradient reduction where they leave the mountain front and enter into the valley (Fig. 6b).
419
This knickpoint coincides with the main trace of the Villicum-Zonda fault (e1, Fig. 3) that places
420
hard Cambrian limestone on top of poorly consolidated Neogene siltstones, sandstones, and
421
conglomerates (Fig. 2) (Ragona et al., 1995; Paredes et al. 1996; Paredes and Perucca, 2000; Ramos
422
and Vujovich, 2000; Meigs et al., 2006; among others). In their mid-reaches, these rivers show a
423
slightly convex profile and subtle knickpoints coinciding with structural lineaments and faults
424
(structures e3 and e4, Fig. 3).
425
Unlike the De la Travesía River —that flows along a depocenter unaffected directly by surficial
426
structures— the De la Ciénaga River seems to be controlled by faults (Fig. 3). It flows across or
427
parallel to the Papagallos (a17), Cerro Bayo occidental (e9), Chica de Zonda trace (e6), Cerro
428
Divisadero trace (e7), and the Río de la Ciénaga lineament (e8), taking advantage of the weakened
429
rocks or forced by the topography produced by these structures.
430
The western margin of the De la Ciénaga River (CW-3) and the Papagallos River (CW-2) show a
431
more graded profile. The Ancho River (CW-1) stands out as an exception showing several
432
anomalies. At first sight, we observed two marked gradient reductions located upstream of the
433
Osamentas fault (a10), and in the core of the Carboniferous rocks syncline (Fig. 6c, marked with an
434
asterisk). Contrarily, in the reach between the Sierra Alta de Zonda and the Cerro Zonda anticline,
435
CW-1 flows across the Alta de Zonda fault (a15, Fig. 3) developing a convex profile downstream and
436
exposing the Neogene sandstones and conglomerates bedrock.
437
In the eastern margin of the De la Ciénaga River, the tributary basins show a simpler geological
438
setting and are of a lesser order and size (<6 km2) than their western counterparts (e.g., the 140 km2
439
of the Papagallos River basin). Tributaries CE-1 to CE-6 flow across the steep western flank of the
440
Sierra Chica de Zonda constituted almost exclusively by Cambro-Ordovician limestone and
441
dolostones (Fig. 2). The faults affecting this piedmont seem to produce no knickpoints on the
442
longitudinal profile of rivers (e.g., the fault e1 in CE-4, Figs. 3 and 6d). Some of these rivers have
443
their headwaters on the flat summit of the Sierra Chica de Zonda, therefore showing a low gradient
444
followed by a sudden increase in slope on their upper reaches (CE-2 and CE-4 in Fig. 6d).
445
446 447 448 449 450
Fig. 6. Longitudinal profiles (upstream distance vs. altitude) of the main rivers and selected tributaries, showing lithology, primary structures, and knickpoints in the valley floor. The lithology and structure representation is schematic. (a) Western De la Travesía River tributaries; (b) eastern De la Travesía River tributaries; (c) western De la Ciénaga tributaries and (d) eastern De la Ciénaga River tributaries (see Fig. 3 for location).
451
3.1.2 Topographic swath profiles
452
Topographic swath profiles AA’, BB’, CC’, and DD’ exhibit a significant transverse asymmetry for
453
the De la Travesía and De la Ciénaga basins, where the axis of the Matagusanos and De la Ciénaga
454
valleys are displaced to the east (Figs. 7 and 8).
455
The analysis of these profiles shows that the mountain ranges of the Central Precordillera have a
456
highly incised and prominent relief. The maximum curve shows pointy peaks that usually coincide
457
with cohesive rocks in the hanging walls of thrusts, and deep valleys coincident with relatively
458
weaker rocks. The minimum curve also shows small peaks or steps that are related to cohesive rock
459
outcrops, fault scarps, or both. The separation between the maximum and minimum curves in the
460
piedmonts indicates the presence of ancient piedmont surfaces that show varying degrees of incision.
461
This configuration often occurs near to the mountain fronts where these surfaces are affected by
462
reverse faulting or up warping. On the other hand, when these curves converge indicate low to no
463
incision.
464
The Talacasto-La Dehesa fault (b1) (Figs. 3 and 6a) seems to coincide with a rapid decrease in the
465
max., min., and mean curves of the swath profile producing a knickpoint (Fig. 8, A-A’ and B-B’).
466
Also, the active trace (b2) (Fig. 9), together with the Lomas del Salto fault (c2) and its antithetic,
467
produces a gentle up-warping in the minimum curve (Fig. 8, A-A’ and B-B’). The Blanquitos fault
468
(d2) (Fig. 10a) generates an increase in the mean and maximum values, but its effect on the minimum
469
curve is not evident. On the other hand, the Lomitas de Matagusanos fault (d1) produces a clear peak
470
in the three curves. Finally, the Lomitas Oriental fault (d3) yields a strong knickpoint in the minimum
471
curve and a gentler counterslope in the mean and maximum curves (Fig. 8, B-B’). The Matagusanos
472
valley shows no marked incision (Fig. 10b) as the profile curves show similar and close maximum,
473
mean, and minimum values (Fig. 8, A-A’ and B-B’).
474
Conversely to the Matagusanos valley, the De la Ciénaga river valley shows several orographic units
475
related to surficial structures directly controlling the shape and position of the depocenter (e.g., the
476
Cerro Divisadero [related to fault e7] and the Cerro Bayo [related to fault e8]) (Fig. 8, C-C’). The
477
Osamentas fault (a10) seems to be coincident with a knickpoint in the max., min., and mean curves of
478
the swath profile (Fig. 8, D-D’). To the east, in the Maradona valley, it is noticeable that the active
479
Maradona fault (a12) shows little to no effect on the swath profile curves. Further east, the Tres
480
Mogotes fault (a13) coincides with a small peak in the maximum curve, indicating a topographic
481
high, yet it does not affect the minimum curve of the profile. Next, to the west of the Papagayos fault
482
(a17), the three profile curves show a gentle up-warping.
483
The mountain ranges of the Eastern Precordillera have steep mountain front slopes developed along
484
the high-angle and west-verging Villicum-Zonda reverse fault (Fig. 10c) with a steep piedmont. The
485
small relative distance between the maximum and minimum curves of the profile indicates a lower
486
degree of incision in the western flank of the Eastern Precordillera than those observed in the Central
487
Precordillera. In the western piedmont of the Sierra de Villicum, the three curves show a knickpoint
488
coincident with the Villicum occidental fault (e4). In the western piedmont of the Sierra Chica de
489
Zonda, there are several fault scarps not clearly visible in the swath profiles (Fig. 8, C-C’, and D-D’).
490 491 492
Fig. 7. Baselines of the swath profiles of the De la Travesía, Tambolar, and De la Ciénaga rivers, and the De la Travesía (AA’ – BB’), and De la Ciénaga (CC’ – DD’) basins.
493 494 495
Fig. 8. Transverse swath profiles of the De la Travesía (AA’ – BB’) and De la Ciénaga (CC’ – DD’) basins (see location in Fig. 7). The geometry of the main structures at depth is schematic.
496 497 498
Fig. 9. Southwest view of the easternmost scarp of the east-verging Talacasto-La Dehesa fault (b2) that crosses the Tambolar River (TW-3) (see Fig. 3). In the background, the Sierra de la Dehesa.
499 500 501 502 503 504 505
Fig. 10. (a) Aerial oblique view to the south of the west-verging Blanquitos reverse fault scarp (d2, Fig. 3). The quaternary fine-grained deposits shown in the picture, occupy an area with 1.6 km long and 400 m wide and are related to ancient fault aligned springs, (b) Aerial view to the southeast of the Matagusanos playa lake. In the background, the Loma de Ullum (right) and Sierra de Villicum (left), (c) View to the northeast of the western piedmont of the Sierra de Villicum. The white arrows point the location of the Villicum Occidental fault (e4) and the main Villicum-Zonda fault trace (e1).
506
3.1.3 Along-river swath profiles
507
To assess the behavior of the two primary collectors in the study area (De la Travesía and De la
508
Ciénaga rivers), we constructed two swath profiles along these rivers, using their paths as baselines
509
(Figs. 7 and 11). The along-river swath profiles show that the minimum curve coincides with the
510
riverbed while the mean and maximum curves describe the topography of the valley, low areas, and
511
river banks. The maximum curve indicates the steepness, shape, and incision of the mountain slopes
512
and the piedmont that confine the stream. The mean curve, whether approaches the maximum or
513
minimum curves, can be interpreted as indicative of the cross-section shape of these piedmonts or
514
mountain slopes. If the mean curve is similar in shape and close to the minimum curve, it indicates
515
that more or less graded piedmonts constitute the river banks. If the mean curve approaches the
516
maximum curve, it indicates incised river banks. Where the mean and maximum curves are irregular,
517
they are indicating deeply incised piedmonts or mountain slopes
518
The swath profile of the De la Travesía River shows a relatively small deviation between the
519
maximum, mean, and minimum curves, hence a high vertical exaggeration was required. Reaches 1
520
and 3 show higher maximum and mean values indicating steeper river banks. The reaches 2 and 4
521
show very close curves indicating a flat topography. Reach 2 coincides with the Matagusanos playa
522
lake. The high-frequency ruggedness of the mean and maximum curves, especially in reach 2, occurs
523
as a result of the presence of noise in the flat areas of the IGN DEM (45m).
524
In the De la Ciénaga River, reaches 1, 2, and 3 show a relatively high deviation between the
525
maximum, mean, and minimum curves suggesting steeper river banks and high confinement of the
526
stream related to the nearby presence of the Cordón del Infiernillo and the Sierra de la Cuchilla. In
527
the upper reach 4, the profile reflects the presence of the Cerro Bayo western flank on the right
528
margin of the river. The lower reach 4 and reach 5 show very close curves indicating a flat and
529
mostly unconfined channel belt. Finally, reaches 6 and 7 show a steep river bank controlled by the
530
western piedmont of the Sierra Chica de Zonda. The first two peaks in the maximum and mean
531
curves in the reach 6 coincide with the Cerro Divisadero, and the third and fourth peaks represent a
532
segment of the river lying down to the Sierra Chica de Zonda mountain slope.
533 534 535
Fig. 11. Along-river swath profiles of the De la Travesía and De la Ciénaga rivers showing the reach number used for river channel pattern analysis.
536 537
3.1.4 River channel patterns
538
The De la Travesía and De la Ciénaga main rivers were divided into reaches that show different
539
channel patterns (Figs. 11 and 12). We divided the De la Travesía River into four reaches, all of them
540
with riverbeds constituted by alluvial deposits (Figs. 11 and 12). Reach 1 shows a low sinuosity bar-
541
braided pattern and a mean active channel belt width of 65 m. Reach 2 shows a sort of straight
542
pattern as a result of a very gentle slope, with low sinuosity, and dominance of fine-grained deposits.
543
In this reach, the De la Travesía River cuts into deposits from the ancient Matagusanos playa lake.
544
Local streams cut the river banks and incise the flat surface of the playa lake by headward erosion,
545
forming gullies. The active channel belt width is approximately 950 m and shows a relatively high
546
width/depth ratio. In reach 3, the channel shows a straight pattern with an active channel belt width
547
of ~30 m, although its gradient remains similar to reach 2. The appearance of the Loma de Ullum in
548
the east could be responsible for this pattern change as a result of an increase in lateral confinement.
549
At last, reach 4 shows an island-braided pattern with moderate sinuosity and an active channel belt
550
width of ~330 m. In the lowest sector of this reach next to its mouth, the river enters the Ullum-
551
Zonda valley and develops unpaired downstream-divergent fluvial terraces (Fig. 12).
552 553 554 555
Fig. 12. Google Earth image showing river patterns for the De la Travesía River reaches (1) Single channel pattern (2) Single channel pattern. Small local tributaries formed by headward erosion in the playa lake show a dendritic pattern, (3) Very low sinuosity straight channel pattern, (4) Island-braided pattern.
556
Similar to the De la Travesía River, the De la Ciénaga River also shows an alluvial bed in its entire
557
length. Taking into account its marked structural control and more complex river patterns, we divide
558
it into seven reaches (Fig. 13). The reach 1 shows a bar-braided pattern, moderate sinuosity, an
559
average width of the active channel belt of ~95 m, and a scarce vegetation cover. In the reach 2, bar
560
size and stability increase (island-braided pattern) as the channel belt widens, reaching an average
561
width of ~322 m.
562 563 564 565
Fig. 13. Google Earth image showing river patterns for the De la Ciénaga River reaches: (1) Bar-braided channel pattern, (2) Island-braided pattern, (3) Bar-braided pattern, (4) Bar braided to island-braided pattern, (5) Island-braided pattern, (6) Single-channel to island-braided pattern, (7) Island-braided pattern.
566
The reach 3 shows a similar pattern to reach 1, more vegetation cover, and an average channel belt
567
width of ~170 m. In the reach 4, the valley is less confined, and the channel belt width increases.
568
After joining the Papagallos River (CW-2), the De la Ciénaga River develops a small and elongate
569
alluvial fan showing an island-braided pattern. The reach 5 varies from a low-sinuosity single-
570
channel pattern to an island-braided pattern, showing a variable active channel belt width. The reach
571
6 shows a single-channel pattern with moderate sinuosity and an active channel belt width of ~42 m.
572
In this reach, the river cuts through uplifted Quaternary alluvial deposits and Cambrian limestone of
573
the Cerro Divisadero and the western flank of the Sierra Chica de Zonda (Figs. 3 and 13). We
574
observed the presence of strath terraces carved on the right bank of the channel where the river
575
coincides with the trace of the De la Ciénaga lineament (e8) (Fig. 14a). Finally, in reach 7, the river
576
loses confinement and shows an island-braided pattern developing an elongated alluvial fan that
577
joins the Ullum-Zonda valley. In this reach, there are discontinuous fill terraces in the right bank of
578
the river (Fig. 13).
579
3.1.5 River basin morphometry
580
The characteristics and drainage pattern in young orogens constitute very good indicators of
581
Quaternary tectonic activity (Ouchi, 1985; Hallet and Molnar, 2001). Rivers are very sensitive
582
elements of fluvial landscape, and if a tectonic uplift occurs, it should be reflected in drainage basins.
583
The De la Travesía basin has an area (A) of 1120.52 km2, a perimeter (P) of 229.14 km, a length (L)
584
of 45.46 km, and a width of 24.65 km. The 6th order of the stream (Nn) indicates a moderate to high
585
discharge, channel width, and size of the basin (Table 1). The main river channel that crosses the
586
valley (De la Travesía River) is ephemeral in its entire course and shows a very low sinuosity
587
(S=1.06) (Table 1), with a main channel length (Mcl) of 48.44 km. The asymmetry factor (AF) of the
588
De la Travesía basin is 78.47, showing a deflection from the neutral value (Af =50). In this case, Af
589
>50 indicates that the main channel has shifted towards the downstream left side of the drainage
590
basin (to the east) (Keller and Pinter, 1996) (Table 1). Besides, the De la Travesía basin has a
591
Transverse Topographic Symmetry Factor (T) value of 0.55, suggesting an asymmetric river pattern.
592
On the other hand, the De la Ciénaga basin has an area (A) of 663.95 km2, a perimeter (P) of 174.34
593
km, a length (L) of 40.29 km and a width of 16.48 km. The 5th order of the stream (Nn) indicates a
594
moderate discharge, channel width, and size of the basin (Table 1). The main watercourse that
595
crosses the valley (De la Ciénaga River) is intermittent, showing a very low sinuosity (S=1.07)
596
(Table 1) with a main channel length (Mcl) of 43.04 km. The asymmetry factor (AF) of the De la
597
Ciénaga River basin is 22.6, indicating that the channel has shifted towards the downstream right
598
side of the drainage basin (Hare and Gardener, 1985), towards the east. Finally, the De la Ciénaga
599
River basin has a Transverse Topographic Symmetry Factor (T) value of 0.57, suggesting an
600
asymmetric river pattern.
601
The elongation ratio (Re) of the De la Travesía (Re= 0.83) and De la Ciénaga (Re= 0.72) basin,
602
indicates that the De la Ciénaga basin is barely more elongate. DE LA TRAVESÍA BASIN Area
0
A (km2)
1120,52 km2
Perimeter
0 0
P (km)
229,14 km
L (km)
45,46 km
W (km)
24,65 km
Mcl (km)
48,44 km
S
1,06
Nu
6
Re
0,83
AF
78,47
Length
= /
Width Main channel longitude
= /
Stream sinuosity Basin order (Strahler)
= ( 4/)
Elongation ratio
= /
Asymmetry factor
= 100(⁄ ) T
Transverse topographic symmetry factor
0,55
DE LA CIÉNAGA BASIN Area
0
A (km2)
663,95 km2
Perimeter
0
P (km)
174,34 km
Length
0
L (km)
40,29 km
W (km)
16,48 km
Mcl (km)
43,04 km
S
1,07
Nu
5
Re
0,72
AF
22,61
Width
= /
Main channel longitude Stream sinuosity
= /
Basin order (Strahler) Elongation ratio Asymmetry factor
= ( 4/) = / = 100(⁄ )
Transverse topographic symmetry factor
T
0,57
603
Table 1. Morphometric/morphotectonic indices for the De la Travesía and De la Ciénaga basins.
604
4.1 Discussion
605
4.1.1 Longitudinal river profiles
606
The analysis of the longitudinal profiles versus the riverbed geology proved to be a useful tool to
607
check the evidence of Quaternary tectonic activity in known structures and to detect possible new
608
structures, previously unknown or poorly documented.
609
Some knickpoints that did not coincide with any known structure or lithological contrast could be
610
possibly revealing their presence. For example, downstream of structure a2 in the Tambolar River
611
(TW-3), the profile shows a possible uplifted block (Fig. 6a). Morphotectonic evidence, like the
612
presence of an increased steepness of the valley floor and an anomalous river pattern forming incised
613
meanders, supports this hypothesis. Fairbridge (1968) defines as incised, intrenched, entrenched,
614
inclosed or ingrow meander, a meandering river valley that has cut down its bed into the bedrock,
615
because of uplift or lowered base level. The eastern border of the mentioned block, also marked by a
616
notable knickpoint, could be indicating the presence of a hypothetical second structure (a3, Figs. 3
617
and 6a) not mentioned in previous works. Similarly, the notable knickpoint located in the Corral
618
Viejo River (TW-2) between structures a5 and B could also be indicating the presence of a previously
619
unknown structure. However, more detailed studies and field data are required to confirm these
620
hypotheses.
621
Knickpoints related to faults with a marked lithological contrast between the hanging wall and the
622
footwall resulted not useful to assess tectonic activity. Lithology contrasts can have such a strong
623
influence on river morphology that completely masks tectonic signals. That is the case of the
624
knickpoints related to the Villicum-Zonda fault (e1) in the TE-4, TE-5, and TE-6 rivers, and the
625
knickpoint related to the Talacasto-La Dehesa fault (b1) in the Tambolar River (TW-3) (Fig. 6a).
626
Both faults expose Cambro-Ordovician limestone; however, the Villicum-Zonda fault shows plenty
627
of morphotectonic evidence of Quaternary activity, contrarily to the Talacasto-La Dehesa fault main
628
trace (b1) that shows no such evidence.
629
In poorly consolidated rocks or alluvial deposits, knickpoints related to active structures may not be
630
preserved, yet in many cases, the river profile appears up-warped in the hanging wall of the fault.
631
That is the case of the TE-4, TE-5, and TE-6 river profiles upstream of the Villicum Occidental fault
632
(e4) (Fig. 6b). The presence of Quaternary alluvial levels with an anomalous slope (contrary to the
633
normal slope of the landform) in the western piedmont of the Sierra de Villicum (Paredes et al. 1996;
634
Paredes and Perucca, 2000), provides morphotectonic evidence indicative of eastward tilting
635
produced by the Villicum Occidental fault (e4) during the Quaternary.
636
Conversely to the rivers that cut through the Lomitas de Matagusanos, the Ancho River (CW-1)
637
profile in the De la Ciénaga River basin shows a convexity where the river cuts through the same
638
Neogene sandstones (Figs. 3 and 6c). Within this reach, the river channel is incised in the bedrock
639
with a thin or without alluvial cover, suggesting a higher rate of relative tectonic uplift in this sector
640
during the Quaternary.
641
When crossing the Osamentas (a10) and Espinacito (a11) faults, CW-1, CW-2, and CW-3 show no
642
detectable knickpoints despite these faults are located in a place with favorable conditions for the
643
preservation of knickpoints, that is a hard rock substrate and low erosive power of the streams due to
644
low water flow and/or a high sediment load (debris flow). This situation could be indicative of a
645
relatively low Quaternary tectonic uplift rate for these faults when compared to nearby structures like
646
the Maradona fault (a12).
647
In the Papagallos River (CW-2), the most notable knickpoints coincide with the Papagallos (a17) and
648
Maradona (a12) faults. Both faults expose soft Neogene sandstones and mudstones (Fig. 6d). The
649
preservation of these knickpoints could be a result of the low erosive power of the streams, a high
650
tectonic uplift rate, or both. Perucca and Vargas (2014) estimated a 1 mm/yr rate of uplift for these
651
faults.
652
In the headwater of the De la Ciénaga River, the Barrancas fault (a12) exposes Carboniferous rocks
653
that dammed the local streams, forming during the late Pleistocene the Barrancas playa lake (Fig.
654
14b). This fault also deforms the fine-grained deposits of this ancient playa lake, suggesting that it
655
has remained active to at least the final stages of the Pleistocene epoch.
656
At the study area scale, we observed that knickpoints associated with active structures that do not
657
produce a lithological contrast were formed mainly on hard rocks, faults with the scarp facing
658
upstream, or both. On the other hand, structures with evidence of Quaternary tectonic activity did not
659
produce knickpoints in soft rocks or faults with the scarp facing downstream. For example, where the
660
Tambolar River (TW-3) crosses the easternmost trace of the Talacasto-La Dehesa fault (b2, Figs. 3
661
and 9). It seems that fault scarps formed facing downstream are less likely to be preserved than those
662
formed with the scarp facing upstream. In the first case, the scarp would produce a local increase in
663
the stream gradient, boosting stream power and, consequently, erosion. The scarp would be then
664
swiftly degraded by headward erosion. In the second case, the scarp would produce a local decrease
665
in the stream gradient, reducing stream power and therefore reducing the erosive power of the
666
torrent, favoring the preservation of the scarp and newly formed topography. This situation may
667
suggest that thrusts with vergence opposite to the flow direction of rivers would be more difficult to
668
cut through than those orographic units formed from thrusts with the same vergence than the river
669
flow direction —without taking into account other variables. In any case, the potential preservation
670
of any knickpoint, scarp, or topography also depends on the lithology and the erosive power of the
671
stream.
672
4.1.2 Topographic swath profiles
673
The topographic swath profiles show two asymmetrical basins, with the N-S river axis displaced
674
towards the east and the highest elevations to the west, in Central Precordillera (>3000 m a.s.l.).
675
Results suggest that the asymmetry of both basins would not be the result of a lateral tilting, but
676
rather a product of the different rock type, geometry, and vergence of thrusts between the Central and
677
Eastern Precordillera.
678
Quaternary deformation is not always evident in these swath profiles. However, an increase in the
679
deviation between maximum, mean, and minimum curves in the piedmonts, when showing a parallel
680
or up-warped geometry, usually coincides with areas with morphotectonic evidence of Quaternary
681
uplift.
682
Topographic swath profiles also indicate that Central Precordillera shows a higher degree of incision
683
than Eastern Precordillera, suggesting an older relief and a more developed river network.
684
4.1.3 Along-river swath profiles
685
The swath profile of the De la Travesía River shows a deviation between the max and min curves in
686
its headwaters as it crosses the main N-S mountain ranges, also suggesting greater slopes.
687
Downstream, the curves indicate a flat topography, when the river traverses the Matagusanos Playa
688
Lake. By contrast, the De la Ciénaga River is characterized by relatively high valley walls and
689
terraces that confine the channel forming a narrower floodplain. In this way, the along-river swath
690
profiles analysis shows that the De la Ciénaga River frequently interacts with steep piedmonts and
691
mountain slopes, suggesting a higher degree of structural control when compared to the De la
692
Travesía River.
693
4.1.4 River channel patterns
694
The change in the river pattern between reaches 1 and 2 of the De la Travesía River is due to a
695
widening of the Matagusanos valley and the reduction of the slope of the valley (Fig. 12). Between
696
reaches 2 and 3, the change in river pattern is a result of the narrowing of the valley due to the
697
appearance of the Loma de Ullum nearby to the east, which partially blocks the outlet of the valley to
698
the south. There is no significant variation in slope. The change in the river pattern between reaches
699
3 and 4 is coincident with a significant increase in the valley slope. The straight channel of reach 3
700
increases its sinuosity markedly in reach 4 transitioning into a bar-braided system. As the width of
701
the active channel belt grows progressively downstream, the pattern changes to an island-braided
702
system. This phenomenon is coincident with a remarkable morphological change in the Loma de
703
Ullum that reaches its maximum height at this point and considerably reduces its width and height
704
towards the south, controlled by an NW trending lineament (Fig. 3). This marked increase in the
705
slope and the presence of downstream-divergent terraces next to the river mouth would indicate that
706
the De la Travesía River is still adjusting to the local base level represented by the Ullum-Zonda
707
Valley. The tectonic uplift of the Loma de Ullum continuously interferes with this process, which
708
probably gives the Matagusanos valley its status of semi-bolson. However, the presence of streams
709
and gullies in the reach 2 within the ancient Matagusanos playa lake (Figs. 4a and 12) suggests that
710
fluvial incision rates prevail over the uplift rates of the Loma de Ullum. The downstream-divergent
711
terrace system possibly responds to successive base level drops associated with the Holocene capture
712
of the San Juan River by the Ullum gorge (Blanc and Perucca, 2017).
713
In the De la Ciénaga River, the channel pattern is braided along its entire length (Fig. 13). However,
714
we observed variations in the width of the active channel belt and the type of braiding: island-braided
715
vs. bar-braided. These variations could be related to the effects of Quaternary tectonic deformation.
716
In the reaches where the river flows across areas where Quaternary uplift is presumed, there is a
717
tendency to reduce the width of the active channel belt by lateral confinement and to develop a bar-
718
braided pattern. The narrowing of the valley causes this confinement by the presence of steep
719
piedmonts or the appearance of rocky outcrops out in the river bank. In the remaining reaches, the
720
width of the active channel belt increases, and the river develops an island-braided pattern with less
721
interference from nearby piedmonts.
722
The presence of Quaternary terraces on the right bank of the De la Ciénaga River in reaches 5, 6, and
723
7 (Fig. 14a), some of which are strath terraces carved in limestone suggests the presence of a reverse
724
fault along the e8 lineament. This proposed fault would have been active during the Late Pleistocene.
725
4.1.5 River basin morphometry
726
Even though the obtained river basin morphometric parameters are too general (due to the extension
727
and complexity of the basins), it is possible to make the following observations (Table 1).
728
The basin of the De la Ciénaga River is 41% smaller than the basin of the De la Travesía River. The
729
elongation ratio indicates that both are slightly elongated basins, with the De la Ciénaga being 13.6%
730
more elongated than the De la Travesía. In tectonically active regions, basin elongation is indicative
731
of regional stress direction. Siame et al. (2005, 2006) suggested that the predominant N-S fault
732
population in the Precordillera area shows approximately horizontal east–oriented maximum
733
principal stress (σ1).
734 735 736 737
Fig. 14. (a) View to the east of a strath terrace (Quaternary alluvial deposits [Q] on top of Cambrian limestone [CL]) in the right margin of De la Ciénaga River in the reach number 6. (b) View to the southwest of the Barrancas playa lake showing headward incision of the De la Ciénaga River.
738
The almost identical opposite values of the asymmetry factor and the transverse topographic
739
symmetry factor, suggest that both basins have mirror symmetry, independently of their order and
740
scale. As mentioned before, the asymmetry of both basins would not be the result of a lateral tilting,
741
but rather a product of the geometry and different vergences of thrusts between the Central and
742
Eastern Precordillera. Eastern Precordillera is a back-thrust system that involves basement rocks and
743
has western vergence. As previously stated, mountain fronts formed from structures with vergence
744
opposed to the flow of rivers, are a more difficult obstacle to cut through. This situation forced the
745
rivers to join and divert to take a new north-south trend, giving rise to a large-scale broom-shaped
746
pattern according to the concept of Audemard (1999).
747
5.1 Conclusions
748
The analysis of several morphotectonics tools (longitudinal river profiles, knickpoints, swath
749
profiles, and geomorphic indices) together with geomorphological, lithological, and structural data
750
provided objective and quantifiable elements to improve the knowledge of the Quaternary tectonic
751
activity in the region. This analysis also allowed us to test the effectiveness of these methods in a
752
geologically complex area with a well-known Quaternary tectonic setting.
753
The longitudinal river profiles and knickpoints yielded data that suggest the existence of previously
754
unknown structures or areas affected by Quaternary deformation. The knickpoints related to faults
755
with a marked lithological contrast between the hanging wall and the footwall resulted not useful to
756
assess tectonic activity. Lithology contrasts have a strong influence on river morphology that
757
completely masks tectonic signals. In poorly consolidated rocks or alluvial deposits, knickpoints
758
related to active structures were not preserved, yet in many cases, the river profile appeared up-
759
warped in the hanging wall of the fault.
760
At the study area scale, we observed that faults with evidence of Quaternary tectonic activity did not
761
produce knickpoints if soft rocks constitute the hanging wall and the footwall. The same situation
762
occurs if the faults generate a downstream facing scarp. These results suggested that thrusts with
763
vergence opposite to the direction of flow of rivers would be more difficult to cut through than those
764
with vergence in the same direction of the river flow, depending on whether the fault scarp increases
765
or decreases the local gradient (and the stream erosive power) of the river.
766
Quaternary deformation is not always evident in swath profiles. However, swath profiles curves that
767
show a parallel or up-warped geometry in the piedmonts, usually coincide with areas with
768
morphotectonic evidence of Quaternary uplift. The along-river swath profiles showed that the De la
769
Ciénaga River frequently interacts with steep piedmonts and mountain slopes, suggesting a higher
770
degree of structural control when compared to the De la Travesía River.
771
River pattern analysis showed variations in the width of the active channel belt and the type of
772
braiding related to the effects of Quaternary tectonic deformation. In the reaches where rivers flow
773
across areas where Quaternary uplift is presumed, we observed a tendency to reduce the width of the
774
active channel belt by lateral confinement and to develop a bar-braided pattern.
775
The W-E tributary rivers from the mountainous and piedmont areas flow to the main N-S collectors
776
(De La Travesía and La Ciénaga rivers). Both rivers drain to the W-E San Juan river, forming a large
777
scale broom-shaped pattern. This type of pattern is a common morpho-structural expression of thrust
778
faults on river networks.
779 780
Acknowledgments
781
We thank the editor and the reviewers for providing comments that helped to improve our
782
manuscript significantly. The authors acknowledge funding from PIP 92.CO (CONICET), PICTO
783
AGENCIA 2016-0995, and CICITCA 21/E1052. Pablo A. Blanc and Flavia Tejada acknowledge a
784
doctoral fellowship provided by the Consejo Nacional de Investigaciones Científicas y Técnicas
785
(CONICET).
786 787
788
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789
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Highlights
• • • •
Morphotectonic analysis of two specular river basins Active tectonics have a significant influence on the evolution of fluvial systems Results could be useful to understand Quaternary geomorphic and tectonic evolution Analysis of transient processes, tectonic-lithological knickpoints and topography
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0895-9811(19)30281-0
DOI:
https://doi.org/10.1016/j.jsames.2019.102441
Reference:
SAMES 102441
To appear in:
Journal of South American Earth Sciences
Received Date: 7 June 2019 Revised Date:
23 November 2019
Accepted Date: 24 November 2019
Please cite this article as: Blanc, P.A., Tejada, F., Perucca, L.P., Espejo, K., Lara, G., Vargas, Nicolá., Morphotectonic analysis of two axial tributary basins of the San Juan river controlled by the Precordillera fold and thrust belt, Central Andes of Argentina, Journal of South American Earth Sciences (2019), doi: https://doi.org/10.1016/j.jsames.2019.102441. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
CRediT author statement Pablo Blanc: Conceptualization, methodology, software, validation, formal analysis, investigation, writing, review, editing Flavia Tejada: methodology, software, investigation Laura Perucca: Conceptualization, methodology, validation, formal analysis, investigation, writing, review, resources Karen Espejo: methodology, software Gabriela Lara: methodology, software Nicolas Vargas: Resources. Other analysis tools
1
Morphotectonic analysis of two axial tributary basins of the San Juan river
2
controlled by the Precordillera fold and thrust belt, Central Andes of Argentina
3
Pablo A. Blanc1, 2, Flavia Tejada1, 2, Laura P. Perucca1, 2, Karen Espejo3, Gabriela Lara1, Nicolás
4
Vargas3
5
1
6
Ignacio de la Roza 590, CP 5400 San Juan, Argentina.
7
2
8
Argentina.
9
3
Gabinete de Neotectónica y Geomorfología, INGEO-FCEFN, Universidad Nacional de San Juan, Av.
Grupo Geología del Cuaternario, CIGEOBIO-CONICET, Av. Ignacio de La Roza 590, CP 5400 San Juan,
Departamento Geología, FCEFN, Universidad Nacional de San Juan, Av. Ignacio de La Roza 590, CP 5400
10
San Juan, Argentina.
11
[email protected],
12
[email protected],
13
[email protected],
14
[email protected],
15
[email protected]
16
*Correspondence ([email protected])
17
Abstract
18
Analysis of several morphotectonics tools (longitudinal river profiles, knickpoints, swath profiles,
19
and geomorphic indices) together with geomorphological, lithological, and structural data provided
20
objective and quantifiable elements to improve the knowledge of the Quaternary tectonic activity in
21
the region. In this study, we analyzed and compared two large intermountain basins located in the
22
Precordillera of the Central Andes of Argentina. Local distortions in longitudinal river profiles and
23
river patterns may reflect resistant lithology, an increase in shear stress, or differential surface uplift.
24
Both basins have similar shapes, although one mirrors the other producing a large-scale broom-
25
shaped drainage pattern. The values of the Asymmetry Factor and Transverse Topographic
26
Symmetry Factor show opposite signs and almost identical values, suggesting that both basins have
27
mirror symmetry independently of order and scale. The asymmetry in both basins would be a product
28
of the progressive migration of the Andean orogenic front towards the east. Knickpoints were mainly
29
associated with resistant rocks, faults with the scarp facing upstream, or both. In the reaches where
30
the rivers flow across areas with presumed higher rates of uplift, we observed a reduction in the
31
width of the active channel belt by lateral confinement and river pattern changes. The results
32
presented in this paper constitute one of the first attempts to apply geomorphic indices to large
33
drainage basins in the Precordillera fold-and-thrust belt.
34
Keywords: morphotectonics, geomorphic index, fluvial basin, Precordillera.
35 36
1.1 Introduction
37
The development of river networks can often provide clues about the extent and characteristics of
38
Quaternary tectonic deformation in an area (e.g., Jackson and Leeder 1994; Burbank and Anderson
39
2001; Delcaillau et al. 2006). Active tectonics have a significant influence on the evolution of fluvial
40
systems by controlling drainage patterns, watersheds erosion, and the morphology of valleys (e.g.,
41
Keller and Pinter, 2002). Rivers are sensitive to these changes and, therefore, adjust their
42
morphology over different periods depending on rock strength, climate, and tectonic activity (Hack,
43
1973). Local distortions in the longitudinal river profile may reflect the presence of knickpoints
44
caused by more resistant lithology, an increase in shear stress, or surface uplift. These anomalies in
45
river profiles could reflect a stream in disequilibrium, where the headward erosion transmit base-
46
level changes to the upstream valley (Bishop et al. 2005).
47
A longitudinal river profile is a curve that arises from the relationship between elevation and
48
upstream/downstream distance along a river channel and sometimes can thus provide insight into the
49
tectonic evolution of a drainage basin.
50
In
51
geomorphological or climatic processes can be quite difficult. The methods and tools commonly
52
used in the morphotectonic analysis (longitudinal profiles, knickpoints recognition, indices like basin
53
elongation ratio, drainage basin asymmetry factor, transverse topographic symmetry factor, and
54
stream sinuosity, among others) are highly sensitive to multiple variables such as lithology, tectonics,
55
climate, and fluvial dynamics. However, fluvial anomalies—such as local development of meanders
56
or a braided pattern, local widening or narrowing of channels, anomalous ponds, marshes or alluvial
57
fills, variation in the width and continuity of the levees, and any abnormal curve or turn of the fluvial
58
network—can also be indicators for active tectonics (Schumm et al., 2002; Struth et al., 2015;
59
Perucca et al. 2018, among others).
60
In this study, we analyzed and compared (from a morphotectonic point of view) two intermountain
61
basins located between the orogenic fronts of Central and Eastern Precordillera (Fig. 1a, b). These
62
two river basins, known as the De la Travesía and De la Ciénaga, are considerably extensive and
63
stand out for their remarkable geological and tectonic complexity. Paleoseismological and
64
morphotectonic studies (Bastías, 1986; Paredes and Perucca, 2000; Perucca et al., 2012, 2013,
65
among others) have constrained long-term Quaternary deformation and active crustal faults in this
66
region. These basins, drain to the Ullum-Zonda valley from the north and south respectively along a
67
tectonic corridor called Matagusanos-Maradona-Acequión (Perucca, 1990) where two structural
68
systems of opposite vergence converge and collide (Central Precordillera with eastern vergence and
69
Eastern Precordillera with western vergence) (Fig. 1c). Both basins have a similar shape, and one
70
appears to mirror the other (Fig. 1a), producing a large-scale broom-shaped drainage pattern
71
according to the concept of Audemard (1999). As stated by Suriano et al. (2015), during the
72
mountain building of Precordillera, active folds and thrusts deviated several rivers from the original
geologically
complex
basins,
attributing
morphotectonic
indicators
resulting
from
73
W-E regional slope. These N-S axial fluvial systems flow to a large W-E transference river (San
74
Juan River) that connects these basins draining the fold and thrust belt to the foreland region. In this
75
way, the San Juan River and its tributaries converge in a single channel that crosses the Eastern
76
Precordillera, developing a mega broom-shaped river pattern (Audemard, 1999) (Fig. 1a). Most of
77
the rivers that cross W-E the N-S trending mountain ranges are controlled by NW, SW, and W-E
78
lineaments (Baldis and Chebli, 1969; Oriolo et al., 2015; Pantano, 2015; among others).
79
To recognize tectonic signals, we collected field data on the lithology and active faults features.
80
Also, we constructed topographic swath profiles, longitudinal profiles, and analyzed the channel
81
morphology of main rivers with the aid of river network geomorphic indices. The objective of this
82
paper is to assess Quaternary and current deformation patterns and transient geomorphological
83
processes with the aid of suitable morphometric and morphotectonic analysis, remote sensing and,
84
field data. This assessment would allow us to gain a better understanding of the evolution and
85
behavior of the foreland area during the last stage of the Andean orogeny.
86 87 88 89 90 91 92 93 94
Fig. 1. (a) Location of the De la Travesía and De la Ciénaga basins in Precordillera within San Juan province and Argentina, (b) Map of the western border of South America showing the main geological provinces and the flat-slab subduction segment of the Nazca plate, between 28° and 33°S based on Cahill & Isacks (1992) (compared with the Benioff geometry proposed by Pardo et al. 2002 and Alvarado et al. 2005a,b) main basement uplifts of Sierras Pampeanas (Jordan et al. 1989) (colored in pink), and location of the Precordillera fold and thrust belt (dark grey) (Ramos et al. 2002), (Modified from Ramos and Folguera, 2009). The red line represents the cross-section location, (c) Crustal-scale cross-section of the Precordillera, and Sierras Pampeanas. Scale 1:1 (Modified from Vergés et al., 2007; Siame et al., 2015).
95
1.2 Geological Setting
96
The geological province of Precordillera is a fold and thrust belt located to the east of the Andes
97
Cordillera, between 28° 37' S and 33° 05' S (Fig. 1a, b, c). This province is constituted mainly of
98
marine and continental Paleozoic sedimentary rocks and Cenozoic synorogenic deposits. In this
99
region, the structural evolution of the Andes is the result of the oblique convergence between the
100
Nazca and South America plates. The sub-horizontal geometry (flat slab) of the Nazca Plate has been
101
held responsible for the migration of the orogenic front towards the east, the absence of volcanism,
102
the intense intra-plate seismicity, the foreland uplift of the Sierras Pampeanas (Jordan et al., 1983)
103
and, the tectonic activity occurred during the Quaternary, localized mainly on the Eastern
104
Precordillera (Ramos et al. 1997; Costa et al., 2000). This flat-slab configuration has been attributed
105
to the subduction of the Juan Fernández Ridge beneath the South American Plate (Pilger, 1981,
106
Anderson et al., 2007) (Fig. 1b). At these latitudes, three main structural domains are defined in the
107
eastern flank of the Andean orogen (Jordan et al., 1983; Ramos, 1988; Mpodozis and Ramos, 1989).
108
From west to east, they are the Western and Central Precordillera, the Eastern Precordillera, and the
109
Sierras Pampeanas (Fielding and Jordan, 1988; Ramos, 1988).
110
The Central Precordillera is a thin-skinned fold and thrust belt with eastern vergence dipping
111
between 20º and 30º to the west (Zapata and Allmendinger, 1996). Marine Ordovician limestone and
112
marls, Devonian shales, Carboniferous fluvial deposits with marine intercalations, and synorogenic
113
foreland sequences with the intrusion of Neogene acidic sub-volcanic bodies characterize this
114
mountain range. The Quaternary is represented by heterogeneous Pleistocene-Holocene alluvial and
115
lacustrine deposits (Fig. 1c, 2). The predominant lithology varies significantly, both texturally and
116
compositionally, depending on the area of provenance. The alluvium exhibits a clear predominance
117
of clasts of igneous rocks, sandstones, greywackes, and andesitic-dacitic rocks coming from the
118
mountain ranges located to the west and within the study area.
119
The Eastern Precordillera is characterized by west verging structures involving the basement, with a
120
detachment down to 30 km deep (Comínguez and Ramos, 1991; Zapata and Allmendinger, 1996).
121
These structures cut but do not expose the basement, showing a thick-skinned deformation
122
mechanism similar to that of Sierras Pampeanas (Rolleri, 1969; Ortiz and Zambrano, 1981; Ramos,
123
1988; Allmendinger et al., 1990). A core of Cambro-Ordovician calcareous rocks and, to a lesser
124
extent, Silurian, Devonian and, Carboniferous marine siliciclastic rocks forms the Eastern
125
Precordillera. A belt of Neogene synorogenic foreland sequences, constituted by alluvial deposits
126
(Fig. 1c, 2), surrounds this Paleozoic core.
127 128
1.3 Physical Geography
129
The valley of Matagusanos (Fig. 1a) constitutes an elongated depression trending N-S with a
130
maximum width of approximately 20 km. The Sierra de Villicum to the east and the Sierras de
131
Talacasto and La Dehesa to the west delimit this depression, located at an average height of 900 m
132
a.s.l. The ephemeral De la Travesía River is the primary collector of the basin. This river runs from
133
N to S and flows into the San Juan River (Ullum Reservoir). The exoreic drainage allows us to
134
define this depression as a "semibolson"—as opposed to closed depressions with centripetal
135
drainage. Fine-grained deposits of an ancient playa lake (Fig. 2) occupy the central portion of the
136
depression, with an approximate length of 20 km and a maximum width of 2 km. This playa lake,
137
ephemerally flooded during the summer season, shows an almost flat surface with a scarce
138
vegetation cover and incised gullies formed by headward erosion.
139
140 141 142
Fig. 2. Lithological and structural map of the De la Travesía and De la Ciénaga basins (Modified from Paredes and Perucca, 2000; Ramos and Vujovich, 2000; Perucca et al., 2013, Lara et al., 2018; and Blanc, 2019).
143
The valley of the De la Ciénaga River locates east of the Cerro Zonda, Cordón de las Osamentas and
144
lower hills with an average N-S trend. To the east of the valley is the Sierra Chica de Zonda and
145
Cerro Bayo (Figs. 1a and 2). This intermittent river drains from the southwest to the northeast,
146
pouring its scarce flow into the valley of Ullum-Zonda. The maximum width of this depression
147
(about 25 km) occurs in the De la Travesía river valley. At its narrower section, its width is less than
148
5 km. The Barrancas playa lake locates in the southern sector of the depression and the upper section
149
of the basin (Fig. 2). This ancient playa lake served as a depocenter for the rivers flowing from the
150
eastern flank of the Cordón de Las Osamentas.
151
In the study area, mountain ranges have a predominant N-S trend with an asymmetric transverse
152
profile with its steeper flanks along the fault fronts. They show an extensive piedmont with elongated
153
alluvial plains and ancient playa lakes with evidence of present fluvial incision. The Piedmont of the
154
Central Precordillera has a remarkable width (~ 16 to 18 km) compared to the western piedmont of
155
the Eastern Precordillera (~ 0.8 to 7 km). The depocenters along both basins are displaced towards
156
the east, resulting in markedly asymmetric basins (Fig. 2). The piedmonts show alluvial fans,
157
colluvial fans, debris cones, and some forms of erosion such as pediments (Fig. 2) with an alluvial
158
cover of variable thickness. Fluvial plains and terrace levels associated with distributary fluvial
159
systems of the collector-conoid river type (Suriano and Limarino, 2009) and playa lake deposits
160
dominate the lower areas. The mouths of the De la Ciénaga and De la Travesía rivers are deeply
161
affected by land use (i.e., crops, mines, alluvial control systems), all of which are exerting an
162
influence on the current fluvial dynamics.
163
The dominant climate is of the BWwka type according to the Köppen classification (Poblete and
164
Minetti, 1989), where (BW) corresponds to dry desert climate with xerophyte vegetation or no
165
vegetation with a summer concentration of rainfall (w), an annual average temperature of less than
166
18ºC (k), and the temperature of the warmest month above 22ºC (a). Other features are intense solar
167
radiation and high atmospheric transparency, as well as a marked annual and diurnal/nocturnal
168
thermal amplitude.
169
As mentioned before, streams that flow across the De la Travesía basin are ephemeral due to the
170
scarce rainfall (approximately 80 mm/year). Instead, in the De la Ciénaga basin, rainfall is slightly
171
higher (close to 100 mm/year on average). The latter presents an intermittent discharge. Some of its
172
tributaries carry water throughout the year since they are partly fed by springs located along
173
Quaternary faults such as the Papagayos River when it crosses the Maradona and Papagayos faults.
174
(Lara et al., 2017, 2018) (Figs. 2 and 3). In both basins, most of the runoff comes from torrential
175
summer precipitations. During short lapses, large flood peaks mobilize the thick detrital
176
accumulations (debris and mudflows) on mountain slopes. However, snowmelt in the highest
177
mountain ranges (mainly in the western part of the De la Ciénaga basin) also contributes to the basin
178
runoff.
179
Vegetation is predominantly shrubby, although there are also some low arboreal species and cacti.
180
The distribution of vegetation is closely associated with the different landforms, soil type, and water
181
availability (i.e., springs and water table position).
182
Landforms in both basins relate to the fluvial dynamics of the San Juan River, which represents the
183
local base level (Fig. 1a). This river presents a glacial-nival regime with an average annual flow of
184
60 m3/sec and maximum runoff peaks occurring between December and January. Millenary floods
185
might exceed 1,000 m3/sec (Perucca and Esper, 2009). Currently, numerous hydraulic works built for
186
hydroelectric generation and irrigation purposes regulates the streamflow.
187
1.4 Quaternary Tectonics
188
Perucca (1990) characterized the succession of elongated N-S valleys in the boundary between
189
Central and Eastern Precordillera as a tectonic corridor called Matagusanos-Maradona-Acequión
190
(Fig. 1a). In the study area, its width varies between 5 km in its central portion (Zonda Valley) and
191
25 km in the Matagusanos and the De la Ciénaga River valleys (Fig. 1a). To the east, this
192
longitudinal tectonic valley is bordered by the sierras de Villicum, Marquesado, and Chica de Zonda
193
(Eastern Precordillera). To the west, the Sierra de La Dehesa, Cerro Zonda, Sierra Alta de Zonda,
194
Cordón de Las Osamentas, and low hills (Central Precordillera) borders this valley (Fig. 1a)
195
(Perucca, 1990; Perucca and Onorato, 2011; Perucca et al., 2013).
196
According to Zapata and Allmendinger (1996), the interaction between thrusts with opposite
197
vergences during the Cenozoic favored the development of a thick-skinned triangle zone (Fig. 1c).
198
The formation of this triangular zone would have begun after 2.7 Ma when the thick-skinned
199
deformation system of the Eastern Precordillera became active (Zapata and Allmendinger, 1996).
200
The evidence of Quaternary faulting recognized throughout the Matagusanos-Maradona-Acequión
201
tectonic corridor is consistent with this structural style (Perucca et al., 2013; Audemard et al., 2016).
202
Smalley et al. (1993) mentioned the Matagusanos valley as one of the areas with the highest
203
seismicity in the province of San Juan. This valley may have originated as a depression bordered by
204
reverse faults (Rolleri, 1969). Its subsequent evolution, however, could be closely related to thrusts
205
with western vergence, which would justify its marked asymmetry and the depocenter located
206
towards the east (Paredes and Perucca, 2000).
207
On its eastern edge, the Matagusanos-Maradona-Acequión tectonic corridor is bordered by the
208
Villicum-Zonda fault, which belongs to the Eastern Precordillera Fault System (Fig. 3) (Bastías,
209
1986). This regional fault manifests itself along 145 km as a series of traces and segments (e1 to e9 in
210
Fig. 3) with a mainly NNE trend, affecting the western flank of the mountain ranges of the Eastern
211
Precordillera, between 31° S and 32° 20'S. These traces and segments are high-angle reverse faults
212
with a western vergence that place Cambrian limestone and Neogene rocks over Quaternary alluvial
213
deposits. These Quaternary deposits frequently develop anomalous slopes opposite to the original
214
slope of the piedmont, progressive unconformities, and fault propagation folds (Siame et al. 2002,
215
2005; Perucca et al., 2012; Blanc and Perucca, 2017; among others).
216
In the western piedmont of the Sierra de Villicum in the De la Travesía basin, the poorly
217
consolidated alluvial deposits make difficult the preservation of scarps and other morphological
218
evidence of Quaternary faulting. However, a few scarps can be observed affecting older terraces in
219
Quaternary alluvial fans, next to the mountain front of the Sierra de Villicum. Also, to the west and
220
slightly away from this mountain front, Pliocene-Pleistocene deposits show discontinuous N-S
221
trending scarps facing west along ~ 20 km (structures e3 and e4, Fig. 3). Another evidence of
222
Quaternary tectonic activity is the presence of alluvial deposits from the Sierra de Villicum overlying
223
the Pliocene-Pleistocene sequence with an anomalous slope (contrary to the original) possibly due to
224
the activity of the e4 structure located to the west (Paredes and Perucca, 2000).
225
South of the San Juan River, in the De la Ciénaga basin, the western front of the Sierra Chica de
226
Zonda shows a sinusoidal shape bounded to the west by several parallel traces of the Villicum-Zonda
227
fault (e1, e6, e7, e8 in Fig. 3). Horsetail-type faults branch from the main fault (e1) towards the
228
northeast (Fig. 3). These subsidiary faults show activity at least until the upper Neogene. In the
229
piedmont, between the CE-3 and CE-4 rivers, Perucca et al. (2012) pointed the presence of several
230
"hanging" Quaternary alluvial levels, progressive unconformities, and anomalous slopes in the fill
231
terraces dipping to the east with an average angle of 15 ° (Fig. 5a). Between the rivers CE-4 and CE-
232
6 (Fig. 3), a series of foothills (made of old Quaternary alluvial deposits and calcareous rocks of
233
Cambrian age) have been uplifted, together with their younger Quaternary alluvial cover, by a
234
reverse fault subparallel to the main trace of the Villicum-Zonda fault (fault e6, Figs. 3 and 5b).
235
Scarps related to this fault trend NW with the free face to the west (Perucca et al. 2012).
236
The Central Precordillera orogenic front, characterized by its eastern vergence, constitutes the
237
western edge of the Matagusanos-Maradona-Acequión tectonic corridor. In the piedmont of the
238
Sierra de la Dehesa (De la Travesía basin), Bastías (1986) identified the east-verging Talacasto-La
239
Dehesa fault (b1 and b2, Fig. 3). This fault has a length of 20 km, a ~N-S trend, and a dip of 60 ° to
240
the east, affecting the alluvial fan of the Tambolar River (TW-3). Where this river crosses the fault
241
trace, Neogene rocks can be seen overriding alluvial deposits assigned to the Quaternary, showing an
242
east-facing scarp up to 18 m high (Paredes and Perucca, 2000).
243
244 245 246
Fig. 3. Main Quaternary structural features from the De la Ciénaga and De la Travesía basins (Modified from Paredes and Perucca, 2000; Ramos and Vujovich, 2000; Perucca et al., 2013; Lara et al., 2018; and Blanc, 2019).
247
248 249 250 251 252 253 254
Fig. 4. (a) View to the south of the Matagusanos Playa Lake showing headward erosion of the De la Travesía River, (b) View to the east of the Tambolar River valley in Lomitas de Matagusanos area. Note the flat bottom of the riverbed (FBR). The Sierra de Villicum can be seen far to the east, (c) View to the south of the anticline valley of the Lomitas de Matagusanos. A couple of west-verging reverse faults (d1, Lomitas de Matagusanos fault system) produced progressive tilting to the east of Pleistocene strath terraces. NeS: Neogene sandstones; NeC: Neogene conglomerates; Qad: Quaternary alluvial deposits.
255
To the east and very close to the mountain front of the Sierra de la Dehesa, there are a series of low
256
hills called Lomas del Salto. These hills have an approximate length of 10 km and a maximum width
257
of 1.2 km and are affected on both flanks by high-angle opposite reverse faults. (C, Fig. 3). These
258
faults place Silurian marine rocks over fluvial sedimentary rocks of Neogene age. The Lomas del
259
Salto fault dips to the east between 40° and 70°. Conversely, the fault located on the eastern flank of
260
these hills dips towards the west from 70° to 75° (Paredes and Perucca, 2000). To the north, these
261
faults affect Quaternary alluvial deposits. Fine-grained clastic deposits (silts, light-colored clays),
262
and a bank of travertine in a fault scarp related sag pond yielded an age of 16,575 ± 300 14C ka BP
263
(Paredes and Perucca, 2000).
264
To the east of the Lomas del Salto, lies the Blanquitos fault area (d2, Fig. 3) where several alluvial
265
fans are affected by N-S trending subparallel faults. These faults dip to the east with a high angle at
266
the surface and show counter-slope scarps with the free face facing west. Paredes et al. (1997) dated
267
a bank of travertine along the Blanquitos fault plane at 28,422 ± 1,335 14C ka BP. Further east in the
268
Lomitas de Matagusanos area, Paredes and Perucca (2000) identified high-angle reverse faults with
269
western vergence (d1 in figs. 3, 4a-c). The Lomitas de Matagusanos hills are 35 km long, 3.5 to 4 km
270
wide, and trend NE. Neogene rocks that form a west-verging fault propagation fold (Baldis et al.,
271
1979) constitute these low hills. The entire sector shows clear evidence of Quaternary tectonic
272
activity in the form of tilted and uplifted relict alluvial fan surfaces (Fig. 4c).
273
To the south of the San Juan River, Perucca and Onorato (2011) described the Las Osamentas fault,
274
located to the west of the De la Ciénaga basin (a10, Fig. 3). This fault has an azimuth of 350 º, dips
275
30º to 45° to the west, and puts Carboniferous deposits over Pleistocene sediments. Towards the east,
276
in the piedmont of the Cordón del Espinacito and Cordón de las Osamentas, Bastías (1986) identified
277
the Maradona reverse fault (a12, Figs. 3, 5c, and 5e). Lara (2015) named this 30 km-long set of NNW
278
trending parallel structures Maradona fault system, which places Neogene rocks over Quaternary
279
alluvial deposits with an average 45° W dip and eastern vergence. These faults are easily
280
recognizable by a series of scarps and aligned springs with abundant vegetation. The alluvial cover
281
shows anomalous slopes and terraces (Perucca, 1990; Moreiras and Banchig, 2008; Perucca et al.,
282
2012; Lara, 2015; Lara et al., 2017, 2018). Based on the age of the affected alluvial levels, scarps
283
range from ~50 meters high to the north to small and eroded escarpments to the south where they
284
disappear beneath the alluvial cover. It is presumed that this fault continues further south as a blind
285
structure (Perucca and Vargas, 2014; Lara, 2015; Lara et al. 2017, 2018).
286
287 288 289 290 291 292 293 294 295 296
Fig. 5. (a) View to the northeast of the western flank of the Sierra Chica de Zonda in the reach 6 of the De la Ciénaga River. In the top of the foothills, piedmont related Quaternary deposits lie with an anomalous dip (opposite to the piedmont slope) on top of Cambrian limestone. The white arrow points to the location of the e8 structure. (b) View to the southeast. White arrows point Chica de Zonda fault scarp (e6). Piedmont related Quaternary alluvial deposits lie with anomalous dip on top of Cambrian limestone. (c) View to the west of the Maradona valley and the fault scarp (a12) pointed by the white arrows. In the background, the Cordón de las Osamentas. (d) View to the southwest of a natural exposure of the Papagallos fault (a17’) in the Papagallos River valley (CW-2), disrupting Pleistocene alluvial terraces and fine-grained deposits from fault-related springs. (e) Maradona thrust exposure, showing Neogene rocks overriding Quaternary alluvial deposits. Є: Cambrian; Ne: Neogene; Q: Quaternary.
297
Further east, the Cerro Tres Mogotes fault (a13, Fig. 3) constitutes a west-verging backthrust
298
associated with pre-Quaternary structures (Baldis and Chebli, 1969). On the eastern flank of the
299
Cerro Tres Mogotes, Perucca et al. (2012) recognized reverse faults with eastern vergence and N-S
300
trend in a natural exposure located on the left margin of the Papagallos River (CW-2, Figs. 3 and 5d).
301
These faults constitute a branch of the Papagayos fault (a17’, Fig. 3) (Perucca et al. 2012). They have
302
an almost N-S trend (8°) and dip 15° to 20° to the west affecting 20 m-thick Pleistocene alluvial
303
deposits and fine-grained deposits related to the upflow of groundwater along the fault. A second
304
fault, parallel to the previous one, shows an azimuth of 215° and dips 25° to the west. Fault-plane
305
displays striae suggesting a right-oblique movement (Perucca et al., 2012). In the fine-grained
306
deposits affected by these faults, these authors dated a carbonate level at 22,420 ± 390 years BP. The
307
last reactivation is considered to have occurred before this age.
308 309
2.1 Methodology
310
The analysis of morphometric parameters allows the identification of characteristics of a drainage
311
basin that could respond to active tectonic deformation (Keller and Pinter, 1996, 2002). In this work,
312
we analyze topographic, geological, and neotectonic data together with morphometric indexes to
313
describe and compare two genetically related basins in the Precordillera. We used GPS mapping,
314
topographic maps, Digital Elevation Models with a 45 and 5 m resolution from the National
315
Geographic Institute of Argentina (IGN), based on ASTER (Advanced Spaceborne Thermal
316
Emission and Reflection Radiometer) DEM (30 m), and satellite imagery interpretation to determine
317
the main geomorphic and morphotectonic features of the basins, due to the regional scope of the
318
study. Maps, profiles, and figures were plotted with QGIS (Quantum GIS) and Origin Software. For
319
greater clarity in the description, each one of the analyzed tributaries has been identified with a code
320
(T = De la Travesía, C = De la Ciénaga, E = east margin, W = west margin) and numbered in
321
upstream order.
322
The De la Travesía and De la Ciénaga basins were delineated based on the watershed divide concept
323
(an area in which all water flowing through it, goes to a common outlet). The stream order of a basin
324
assigns a numeric order to rivers based on the number of tributaries (Strahler, 1964). Being equal in
325
relation to the area, climate, and substrate, a larger order of a river basin implies a more developed
326
river (Horton, 1945).
327
To analyze the shape, geometry, and slope changes and to detect the presence of knickpoints, we
328
constructed longitudinal profiles of the main river and selected tributaries together with riverbed
329
lithology and main structures. Our regional morphometric analysis also relies on four basin-wide
330
topographical W-E swath profiles (3 km wide), obtained with SAGA GIS, with the mean, maximum,
331
and minimum elevations projected into cross-section planes. Such a window width ensures that the
332
minimum and the maximum elevations of the river valleys are measured. These profiles were
333
constructed W-E (i.e., transverse to the trends of river valleys: two across De La Travesía basin A-
334
A’, B-B’; and two across the De la Ciénaga Basin, C-C’ and D-D’) to identify several topographic
335
features, relief anomalies, and valley shape. To assess the behavior of the two primary collectors in
336
the study area (De la Travesía and De la Ciénaga rivers), we constructed two along-river swath
337
profiles, using their paths as baselines.
338
For river pattern analysis, we identified three types of channels: straight, bar-braided, and island-
339
braided, based on sinuosity, number of channels, and island stability (Schumm, 1985; Nanson and
340
Knighton, 1996). These patterns result from sediment size, load, stream power, and flow velocity
341
(Schumm, 1986), which in turn may respond to external forcing by tectonics or climate change
342
(Kirby and Whipple, 2012).
343
To determine the way in which Quaternary tectonic activity influences both basins, we used several
344
morphotectonic and morphometric indices together with the available geological, geomorphological,
345
and topographic data. Morphotectonic analysis using geomorphic indices can be used as a
346
preliminary reconnaissance tool to identify areas experiencing rapid tectonic deformation (e.g., Bull
347
and McFadden 1977; Keller and Pinter 1996, 2002). Drainage networks are highly sensitive to
348
tectonics, providing information about the tectonic behavior of a region. These indices have already
349
been used for basin characterization in other areas of the Central Andes of Argentina by Esper
350
Angillieri (2008, 2012); Perucca and Esper Angillieri (2011); Perucca et al. (2018), among others.
351
The morphometric and morphotectonic indices of both basins were quantitatively calculated using
352
GIS software. We divided these parameters into basic and derived parameters. Basic parameters are
353
area (A), perimeter (P), length (L), width (W), stream order (Nn), and main channel length (Mcl).
354
We calculated the main channel length (Mcl) and length (L), according to Schumm (1956). Derived
355
parameters are Basin Elongation Ratio (Er), Drainage Basin Asymmetry Factor (AF), Transverse
356
Topographic Symmetry Factor (T), and Stream Sinuosity (S) (Table 1). The Basin Elongation Ratio
357
(Er) is the ratio between the diameter of a circle, having the same area as that of the drainage basin,
358
and the maximum length of the basin (Schumm, 1956). When the value of Er deviates from one (0.6
359
to 0.8), the area is expected to have a sharp relief and a steep ground slope. The Asymmetry Factor
360
(AF) represents the drainage basin asymmetry and helps in determining the general tilt of the basin
361
landscape regardless of whether the tilt occurred due to local or regional tectonic deformation (Hare
362
and Gardner, 1984; Gardner et al., 1987; Keller and Pinter, 2002). It is determined by the formula:
363
AF = 100 (Ar /At) where Ar is the area of the basin to the right of the trunk stream while facing
364
downstream and At is the total area of the drainage basin. For a stream network that develops and
365
continues to flow in a stable setting and uniform lithology, the asymmetry factor should be equal to
366
50, whereas an unstable setting would give a variation from normal of either < 50 or > 50 (Keller and
367
Pinter, 1996).
368
The Transverse Topographic Symmetry Factor (T) is another way to establish the tilt direction. Cox
369
(1994) defined it as T = Da / Dd, where Da represents the distance from the midline of the drainage
370
basin to the midline of the active meander belt, and Dd corresponds to the distance from the basin
371
midline to the watershed divide. Perfectly symmetric basins have values of transverse topographic
372
symmetry (T) equal to zero. As the asymmetry increases, T increases and approaches the value of
373
one.
374
Stream Sinuosity (S) is defined as the ratio between the main channel length (Mcl) and valley length
375
(L). Schumm (1963, 1977) pointed out that stream sinuosity reflects the variability of the valley
376
slope and that changes in the valley floor slope produce downstream changes of sinuosity. Tectonic
377
activity is one of the conditions that control the changes in the valley floor slope (e.g., Schumm,
378
1986; Schumm et al., 2002).
379
3.1 Results
380
3.1.1 Longitudinal river profiles
381
The western tributaries of the De la Travesía River (TW-1 to TW-5), flow through several mountain
382
ranges and hills, and their paths lead them across a wide variety of active structures and rock types.
383
In these river profiles, most of the observed knickpoints are coincident with structures, contrasting
384
lithological contacts, or both (Figs. 6a, 2, and 3).
385
To the southwest, rivers TW-1 and TW-2 show very similar profiles and are affected by the same
386
structures. The most prominent knickpoints are related to the Talacasto-La Dehesa fault (b1),
387
especially where it exposes Ordovician limestone, and to the Lomitas de Matagusanos fault (d1)
388
which exposes Neogene sandstones and tuffs. However, in the Corral Viejo River (TW-2) between
389
structures a5 and B stands out a knickpoint that does not coincide with any known structure or
390
lithological contrast (Figs. 3 and 6a).
391
To the west, in the upper reaches of the Tambolar River (TW-3), subtle knickpoints coincide with the
392
Sierra de la Cantera fault traces (a1). Eastward, where TW-3 flows through the Cerro Tambolar fault
393
(a2), there is a marked knickpoint and a gradient reduction. Downstream of the structure a2, this river
394
shows entrenched meanders and a steepened valley floor in the reach coincident with the Cerro
395
Tambolar mountain range. At the end of the meandering reach, we observed a second knickpoint and
396
a gradient increase that does not coincide with any previously known structure or lithological
397
contrast. Downstream, coincident with the Los Gauchos fault (a4), there is an almost undetectable
398
convexity in the profile. The easternmost trace of the Talacasto-La Dehesa fault (b2, Figs. 3 and 9) is
399
one of the best examples of an active fault in the study area with at least 13 m of vertical uplift
400
during Late Pleistocene to Holocene. However, it was not possible to detect a knickpoint where TW-
401
3 crosses the fault trace. Contrarily, the older and presumed inactive main trace of the Talacasto-La
402
Dehesa fault (b1) that exposes Ordovician limestone produces a clearly visible knickpoint. We
403
observed the same situation in the river TW-5. The Lomas del Salto (c2) and Blanquitos (d2) faults,
404
despite showing in the field clear evidence of Quaternary tectonic activity, only seem to delimit river
405
reaches with slightly different slopes in the river profile. To the east of the Lomitas de Matagusanos
406
fault (d1), the TW-1, TW-2, and TW-3 rivers show a marked concavity. In the Lomitas de
407
Matagusanos, the Neogene strata outcrops forming an anticline valley or combe. As these sandstones
408
turn out to be easier to erode —even than the unconsolidated alluvial deposits— the profile of rivers
409
shows a strong concavity in this sector. Further east, after crossing the Lomitas Oriental fault (d3),
410
the profiles show a smooth convexity where the recent alluvial deposits follow the Neogene
411
conglomerates.
412
The tributary rivers that converge to the De la Travesía River from the east (TE-1 to TE-6) flow
413
across the western piedmont of the Sierra de Villicum and the Loma de Ullum. They are of a lesser
414
order than their western counterparts (TW-1 to TW-5). The TE-1 and TE-2 rivers flow across the
415
western piedmont of the Loma de Ullum, showing a slight concave to straight profile without
416
detectable knickpoints. Their beds are made entirely of alluvial deposits. The TE-4, TE-5, and TE-6
417
rivers flow across the western piedmont of the Sierra de Villicum. Their longitudinal profiles show a
418
significant gradient reduction where they leave the mountain front and enter into the valley (Fig. 6b).
419
This knickpoint coincides with the main trace of the Villicum-Zonda fault (e1, Fig. 3) that places
420
hard Cambrian limestone on top of poorly consolidated Neogene siltstones, sandstones, and
421
conglomerates (Fig. 2) (Ragona et al., 1995; Paredes et al. 1996; Paredes and Perucca, 2000; Ramos
422
and Vujovich, 2000; Meigs et al., 2006; among others). In their mid-reaches, these rivers show a
423
slightly convex profile and subtle knickpoints coinciding with structural lineaments and faults
424
(structures e3 and e4, Fig. 3).
425
Unlike the De la Travesía River —that flows along a depocenter unaffected directly by surficial
426
structures— the De la Ciénaga River seems to be controlled by faults (Fig. 3). It flows across or
427
parallel to the Papagallos (a17), Cerro Bayo occidental (e9), Chica de Zonda trace (e6), Cerro
428
Divisadero trace (e7), and the Río de la Ciénaga lineament (e8), taking advantage of the weakened
429
rocks or forced by the topography produced by these structures.
430
The western margin of the De la Ciénaga River (CW-3) and the Papagallos River (CW-2) show a
431
more graded profile. The Ancho River (CW-1) stands out as an exception showing several
432
anomalies. At first sight, we observed two marked gradient reductions located upstream of the
433
Osamentas fault (a10), and in the core of the Carboniferous rocks syncline (Fig. 6c, marked with an
434
asterisk). Contrarily, in the reach between the Sierra Alta de Zonda and the Cerro Zonda anticline,
435
CW-1 flows across the Alta de Zonda fault (a15, Fig. 3) developing a convex profile downstream and
436
exposing the Neogene sandstones and conglomerates bedrock.
437
In the eastern margin of the De la Ciénaga River, the tributary basins show a simpler geological
438
setting and are of a lesser order and size (<6 km2) than their western counterparts (e.g., the 140 km2
439
of the Papagallos River basin). Tributaries CE-1 to CE-6 flow across the steep western flank of the
440
Sierra Chica de Zonda constituted almost exclusively by Cambro-Ordovician limestone and
441
dolostones (Fig. 2). The faults affecting this piedmont seem to produce no knickpoints on the
442
longitudinal profile of rivers (e.g., the fault e1 in CE-4, Figs. 3 and 6d). Some of these rivers have
443
their headwaters on the flat summit of the Sierra Chica de Zonda, therefore showing a low gradient
444
followed by a sudden increase in slope on their upper reaches (CE-2 and CE-4 in Fig. 6d).
445
446 447 448 449 450
Fig. 6. Longitudinal profiles (upstream distance vs. altitude) of the main rivers and selected tributaries, showing lithology, primary structures, and knickpoints in the valley floor. The lithology and structure representation is schematic. (a) Western De la Travesía River tributaries; (b) eastern De la Travesía River tributaries; (c) western De la Ciénaga tributaries and (d) eastern De la Ciénaga River tributaries (see Fig. 3 for location).
451
3.1.2 Topographic swath profiles
452
Topographic swath profiles AA’, BB’, CC’, and DD’ exhibit a significant transverse asymmetry for
453
the De la Travesía and De la Ciénaga basins, where the axis of the Matagusanos and De la Ciénaga
454
valleys are displaced to the east (Figs. 7 and 8).
455
The analysis of these profiles shows that the mountain ranges of the Central Precordillera have a
456
highly incised and prominent relief. The maximum curve shows pointy peaks that usually coincide
457
with cohesive rocks in the hanging walls of thrusts, and deep valleys coincident with relatively
458
weaker rocks. The minimum curve also shows small peaks or steps that are related to cohesive rock
459
outcrops, fault scarps, or both. The separation between the maximum and minimum curves in the
460
piedmonts indicates the presence of ancient piedmont surfaces that show varying degrees of incision.
461
This configuration often occurs near to the mountain fronts where these surfaces are affected by
462
reverse faulting or up warping. On the other hand, when these curves converge indicate low to no
463
incision.
464
The Talacasto-La Dehesa fault (b1) (Figs. 3 and 6a) seems to coincide with a rapid decrease in the
465
max., min., and mean curves of the swath profile producing a knickpoint (Fig. 8, A-A’ and B-B’).
466
Also, the active trace (b2) (Fig. 9), together with the Lomas del Salto fault (c2) and its antithetic,
467
produces a gentle up-warping in the minimum curve (Fig. 8, A-A’ and B-B’). The Blanquitos fault
468
(d2) (Fig. 10a) generates an increase in the mean and maximum values, but its effect on the minimum
469
curve is not evident. On the other hand, the Lomitas de Matagusanos fault (d1) produces a clear peak
470
in the three curves. Finally, the Lomitas Oriental fault (d3) yields a strong knickpoint in the minimum
471
curve and a gentler counterslope in the mean and maximum curves (Fig. 8, B-B’). The Matagusanos
472
valley shows no marked incision (Fig. 10b) as the profile curves show similar and close maximum,
473
mean, and minimum values (Fig. 8, A-A’ and B-B’).
474
Conversely to the Matagusanos valley, the De la Ciénaga river valley shows several orographic units
475
related to surficial structures directly controlling the shape and position of the depocenter (e.g., the
476
Cerro Divisadero [related to fault e7] and the Cerro Bayo [related to fault e8]) (Fig. 8, C-C’). The
477
Osamentas fault (a10) seems to be coincident with a knickpoint in the max., min., and mean curves of
478
the swath profile (Fig. 8, D-D’). To the east, in the Maradona valley, it is noticeable that the active
479
Maradona fault (a12) shows little to no effect on the swath profile curves. Further east, the Tres
480
Mogotes fault (a13) coincides with a small peak in the maximum curve, indicating a topographic
481
high, yet it does not affect the minimum curve of the profile. Next, to the west of the Papagayos fault
482
(a17), the three profile curves show a gentle up-warping.
483
The mountain ranges of the Eastern Precordillera have steep mountain front slopes developed along
484
the high-angle and west-verging Villicum-Zonda reverse fault (Fig. 10c) with a steep piedmont. The
485
small relative distance between the maximum and minimum curves of the profile indicates a lower
486
degree of incision in the western flank of the Eastern Precordillera than those observed in the Central
487
Precordillera. In the western piedmont of the Sierra de Villicum, the three curves show a knickpoint
488
coincident with the Villicum occidental fault (e4). In the western piedmont of the Sierra Chica de
489
Zonda, there are several fault scarps not clearly visible in the swath profiles (Fig. 8, C-C’, and D-D’).
490 491 492
Fig. 7. Baselines of the swath profiles of the De la Travesía, Tambolar, and De la Ciénaga rivers, and the De la Travesía (AA’ – BB’), and De la Ciénaga (CC’ – DD’) basins.
493 494 495
Fig. 8. Transverse swath profiles of the De la Travesía (AA’ – BB’) and De la Ciénaga (CC’ – DD’) basins (see location in Fig. 7). The geometry of the main structures at depth is schematic.
496 497 498
Fig. 9. Southwest view of the easternmost scarp of the east-verging Talacasto-La Dehesa fault (b2) that crosses the Tambolar River (TW-3) (see Fig. 3). In the background, the Sierra de la Dehesa.
499 500 501 502 503 504 505
Fig. 10. (a) Aerial oblique view to the south of the west-verging Blanquitos reverse fault scarp (d2, Fig. 3). The quaternary fine-grained deposits shown in the picture, occupy an area with 1.6 km long and 400 m wide and are related to ancient fault aligned springs, (b) Aerial view to the southeast of the Matagusanos playa lake. In the background, the Loma de Ullum (right) and Sierra de Villicum (left), (c) View to the northeast of the western piedmont of the Sierra de Villicum. The white arrows point the location of the Villicum Occidental fault (e4) and the main Villicum-Zonda fault trace (e1).
506
3.1.3 Along-river swath profiles
507
To assess the behavior of the two primary collectors in the study area (De la Travesía and De la
508
Ciénaga rivers), we constructed two swath profiles along these rivers, using their paths as baselines
509
(Figs. 7 and 11). The along-river swath profiles show that the minimum curve coincides with the
510
riverbed while the mean and maximum curves describe the topography of the valley, low areas, and
511
river banks. The maximum curve indicates the steepness, shape, and incision of the mountain slopes
512
and the piedmont that confine the stream. The mean curve, whether approaches the maximum or
513
minimum curves, can be interpreted as indicative of the cross-section shape of these piedmonts or
514
mountain slopes. If the mean curve is similar in shape and close to the minimum curve, it indicates
515
that more or less graded piedmonts constitute the river banks. If the mean curve approaches the
516
maximum curve, it indicates incised river banks. Where the mean and maximum curves are irregular,
517
they are indicating deeply incised piedmonts or mountain slopes
518
The swath profile of the De la Travesía River shows a relatively small deviation between the
519
maximum, mean, and minimum curves, hence a high vertical exaggeration was required. Reaches 1
520
and 3 show higher maximum and mean values indicating steeper river banks. The reaches 2 and 4
521
show very close curves indicating a flat topography. Reach 2 coincides with the Matagusanos playa
522
lake. The high-frequency ruggedness of the mean and maximum curves, especially in reach 2, occurs
523
as a result of the presence of noise in the flat areas of the IGN DEM (45m).
524
In the De la Ciénaga River, reaches 1, 2, and 3 show a relatively high deviation between the
525
maximum, mean, and minimum curves suggesting steeper river banks and high confinement of the
526
stream related to the nearby presence of the Cordón del Infiernillo and the Sierra de la Cuchilla. In
527
the upper reach 4, the profile reflects the presence of the Cerro Bayo western flank on the right
528
margin of the river. The lower reach 4 and reach 5 show very close curves indicating a flat and
529
mostly unconfined channel belt. Finally, reaches 6 and 7 show a steep river bank controlled by the
530
western piedmont of the Sierra Chica de Zonda. The first two peaks in the maximum and mean
531
curves in the reach 6 coincide with the Cerro Divisadero, and the third and fourth peaks represent a
532
segment of the river lying down to the Sierra Chica de Zonda mountain slope.
533 534 535
Fig. 11. Along-river swath profiles of the De la Travesía and De la Ciénaga rivers showing the reach number used for river channel pattern analysis.
536 537
3.1.4 River channel patterns
538
The De la Travesía and De la Ciénaga main rivers were divided into reaches that show different
539
channel patterns (Figs. 11 and 12). We divided the De la Travesía River into four reaches, all of them
540
with riverbeds constituted by alluvial deposits (Figs. 11 and 12). Reach 1 shows a low sinuosity bar-
541
braided pattern and a mean active channel belt width of 65 m. Reach 2 shows a sort of straight
542
pattern as a result of a very gentle slope, with low sinuosity, and dominance of fine-grained deposits.
543
In this reach, the De la Travesía River cuts into deposits from the ancient Matagusanos playa lake.
544
Local streams cut the river banks and incise the flat surface of the playa lake by headward erosion,
545
forming gullies. The active channel belt width is approximately 950 m and shows a relatively high
546
width/depth ratio. In reach 3, the channel shows a straight pattern with an active channel belt width
547
of ~30 m, although its gradient remains similar to reach 2. The appearance of the Loma de Ullum in
548
the east could be responsible for this pattern change as a result of an increase in lateral confinement.
549
At last, reach 4 shows an island-braided pattern with moderate sinuosity and an active channel belt
550
width of ~330 m. In the lowest sector of this reach next to its mouth, the river enters the Ullum-
551
Zonda valley and develops unpaired downstream-divergent fluvial terraces (Fig. 12).
552 553 554 555
Fig. 12. Google Earth image showing river patterns for the De la Travesía River reaches (1) Single channel pattern (2) Single channel pattern. Small local tributaries formed by headward erosion in the playa lake show a dendritic pattern, (3) Very low sinuosity straight channel pattern, (4) Island-braided pattern.
556
Similar to the De la Travesía River, the De la Ciénaga River also shows an alluvial bed in its entire
557
length. Taking into account its marked structural control and more complex river patterns, we divide
558
it into seven reaches (Fig. 13). The reach 1 shows a bar-braided pattern, moderate sinuosity, an
559
average width of the active channel belt of ~95 m, and a scarce vegetation cover. In the reach 2, bar
560
size and stability increase (island-braided pattern) as the channel belt widens, reaching an average
561
width of ~322 m.
562 563 564 565
Fig. 13. Google Earth image showing river patterns for the De la Ciénaga River reaches: (1) Bar-braided channel pattern, (2) Island-braided pattern, (3) Bar-braided pattern, (4) Bar braided to island-braided pattern, (5) Island-braided pattern, (6) Single-channel to island-braided pattern, (7) Island-braided pattern.
566
The reach 3 shows a similar pattern to reach 1, more vegetation cover, and an average channel belt
567
width of ~170 m. In the reach 4, the valley is less confined, and the channel belt width increases.
568
After joining the Papagallos River (CW-2), the De la Ciénaga River develops a small and elongate
569
alluvial fan showing an island-braided pattern. The reach 5 varies from a low-sinuosity single-
570
channel pattern to an island-braided pattern, showing a variable active channel belt width. The reach
571
6 shows a single-channel pattern with moderate sinuosity and an active channel belt width of ~42 m.
572
In this reach, the river cuts through uplifted Quaternary alluvial deposits and Cambrian limestone of
573
the Cerro Divisadero and the western flank of the Sierra Chica de Zonda (Figs. 3 and 13). We
574
observed the presence of strath terraces carved on the right bank of the channel where the river
575
coincides with the trace of the De la Ciénaga lineament (e8) (Fig. 14a). Finally, in reach 7, the river
576
loses confinement and shows an island-braided pattern developing an elongated alluvial fan that
577
joins the Ullum-Zonda valley. In this reach, there are discontinuous fill terraces in the right bank of
578
the river (Fig. 13).
579
3.1.5 River basin morphometry
580
The characteristics and drainage pattern in young orogens constitute very good indicators of
581
Quaternary tectonic activity (Ouchi, 1985; Hallet and Molnar, 2001). Rivers are very sensitive
582
elements of fluvial landscape, and if a tectonic uplift occurs, it should be reflected in drainage basins.
583
The De la Travesía basin has an area (A) of 1120.52 km2, a perimeter (P) of 229.14 km, a length (L)
584
of 45.46 km, and a width of 24.65 km. The 6th order of the stream (Nn) indicates a moderate to high
585
discharge, channel width, and size of the basin (Table 1). The main river channel that crosses the
586
valley (De la Travesía River) is ephemeral in its entire course and shows a very low sinuosity
587
(S=1.06) (Table 1), with a main channel length (Mcl) of 48.44 km. The asymmetry factor (AF) of the
588
De la Travesía basin is 78.47, showing a deflection from the neutral value (Af =50). In this case, Af
589
>50 indicates that the main channel has shifted towards the downstream left side of the drainage
590
basin (to the east) (Keller and Pinter, 1996) (Table 1). Besides, the De la Travesía basin has a
591
Transverse Topographic Symmetry Factor (T) value of 0.55, suggesting an asymmetric river pattern.
592
On the other hand, the De la Ciénaga basin has an area (A) of 663.95 km2, a perimeter (P) of 174.34
593
km, a length (L) of 40.29 km and a width of 16.48 km. The 5th order of the stream (Nn) indicates a
594
moderate discharge, channel width, and size of the basin (Table 1). The main watercourse that
595
crosses the valley (De la Ciénaga River) is intermittent, showing a very low sinuosity (S=1.07)
596
(Table 1) with a main channel length (Mcl) of 43.04 km. The asymmetry factor (AF) of the De la
597
Ciénaga River basin is 22.6, indicating that the channel has shifted towards the downstream right
598
side of the drainage basin (Hare and Gardener, 1985), towards the east. Finally, the De la Ciénaga
599
River basin has a Transverse Topographic Symmetry Factor (T) value of 0.57, suggesting an
600
asymmetric river pattern.
601
The elongation ratio (Re) of the De la Travesía (Re= 0.83) and De la Ciénaga (Re= 0.72) basin,
602
indicates that the De la Ciénaga basin is barely more elongate. DE LA TRAVESÍA BASIN Area
0
A (km2)
1120,52 km2
Perimeter
0 0
P (km)
229,14 km
L (km)
45,46 km
W (km)
24,65 km
Mcl (km)
48,44 km
S
1,06
Nu
6
Re
0,83
AF
78,47
Length
= /
Width Main channel longitude
= /
Stream sinuosity Basin order (Strahler)
= ( 4/)
Elongation ratio
= /
Asymmetry factor
= 100(⁄ ) T
Transverse topographic symmetry factor
0,55
DE LA CIÉNAGA BASIN Area
0
A (km2)
663,95 km2
Perimeter
0
P (km)
174,34 km
Length
0
L (km)
40,29 km
W (km)
16,48 km
Mcl (km)
43,04 km
S
1,07
Nu
5
Re
0,72
AF
22,61
Width
= /
Main channel longitude Stream sinuosity
= /
Basin order (Strahler) Elongation ratio Asymmetry factor
= ( 4/) = / = 100(⁄ )
Transverse topographic symmetry factor
T
0,57
603
Table 1. Morphometric/morphotectonic indices for the De la Travesía and De la Ciénaga basins.
604
4.1 Discussion
605
4.1.1 Longitudinal river profiles
606
The analysis of the longitudinal profiles versus the riverbed geology proved to be a useful tool to
607
check the evidence of Quaternary tectonic activity in known structures and to detect possible new
608
structures, previously unknown or poorly documented.
609
Some knickpoints that did not coincide with any known structure or lithological contrast could be
610
possibly revealing their presence. For example, downstream of structure a2 in the Tambolar River
611
(TW-3), the profile shows a possible uplifted block (Fig. 6a). Morphotectonic evidence, like the
612
presence of an increased steepness of the valley floor and an anomalous river pattern forming incised
613
meanders, supports this hypothesis. Fairbridge (1968) defines as incised, intrenched, entrenched,
614
inclosed or ingrow meander, a meandering river valley that has cut down its bed into the bedrock,
615
because of uplift or lowered base level. The eastern border of the mentioned block, also marked by a
616
notable knickpoint, could be indicating the presence of a hypothetical second structure (a3, Figs. 3
617
and 6a) not mentioned in previous works. Similarly, the notable knickpoint located in the Corral
618
Viejo River (TW-2) between structures a5 and B could also be indicating the presence of a previously
619
unknown structure. However, more detailed studies and field data are required to confirm these
620
hypotheses.
621
Knickpoints related to faults with a marked lithological contrast between the hanging wall and the
622
footwall resulted not useful to assess tectonic activity. Lithology contrasts can have such a strong
623
influence on river morphology that completely masks tectonic signals. That is the case of the
624
knickpoints related to the Villicum-Zonda fault (e1) in the TE-4, TE-5, and TE-6 rivers, and the
625
knickpoint related to the Talacasto-La Dehesa fault (b1) in the Tambolar River (TW-3) (Fig. 6a).
626
Both faults expose Cambro-Ordovician limestone; however, the Villicum-Zonda fault shows plenty
627
of morphotectonic evidence of Quaternary activity, contrarily to the Talacasto-La Dehesa fault main
628
trace (b1) that shows no such evidence.
629
In poorly consolidated rocks or alluvial deposits, knickpoints related to active structures may not be
630
preserved, yet in many cases, the river profile appears up-warped in the hanging wall of the fault.
631
That is the case of the TE-4, TE-5, and TE-6 river profiles upstream of the Villicum Occidental fault
632
(e4) (Fig. 6b). The presence of Quaternary alluvial levels with an anomalous slope (contrary to the
633
normal slope of the landform) in the western piedmont of the Sierra de Villicum (Paredes et al. 1996;
634
Paredes and Perucca, 2000), provides morphotectonic evidence indicative of eastward tilting
635
produced by the Villicum Occidental fault (e4) during the Quaternary.
636
Conversely to the rivers that cut through the Lomitas de Matagusanos, the Ancho River (CW-1)
637
profile in the De la Ciénaga River basin shows a convexity where the river cuts through the same
638
Neogene sandstones (Figs. 3 and 6c). Within this reach, the river channel is incised in the bedrock
639
with a thin or without alluvial cover, suggesting a higher rate of relative tectonic uplift in this sector
640
during the Quaternary.
641
When crossing the Osamentas (a10) and Espinacito (a11) faults, CW-1, CW-2, and CW-3 show no
642
detectable knickpoints despite these faults are located in a place with favorable conditions for the
643
preservation of knickpoints, that is a hard rock substrate and low erosive power of the streams due to
644
low water flow and/or a high sediment load (debris flow). This situation could be indicative of a
645
relatively low Quaternary tectonic uplift rate for these faults when compared to nearby structures like
646
the Maradona fault (a12).
647
In the Papagallos River (CW-2), the most notable knickpoints coincide with the Papagallos (a17) and
648
Maradona (a12) faults. Both faults expose soft Neogene sandstones and mudstones (Fig. 6d). The
649
preservation of these knickpoints could be a result of the low erosive power of the streams, a high
650
tectonic uplift rate, or both. Perucca and Vargas (2014) estimated a 1 mm/yr rate of uplift for these
651
faults.
652
In the headwater of the De la Ciénaga River, the Barrancas fault (a12) exposes Carboniferous rocks
653
that dammed the local streams, forming during the late Pleistocene the Barrancas playa lake (Fig.
654
14b). This fault also deforms the fine-grained deposits of this ancient playa lake, suggesting that it
655
has remained active to at least the final stages of the Pleistocene epoch.
656
At the study area scale, we observed that knickpoints associated with active structures that do not
657
produce a lithological contrast were formed mainly on hard rocks, faults with the scarp facing
658
upstream, or both. On the other hand, structures with evidence of Quaternary tectonic activity did not
659
produce knickpoints in soft rocks or faults with the scarp facing downstream. For example, where the
660
Tambolar River (TW-3) crosses the easternmost trace of the Talacasto-La Dehesa fault (b2, Figs. 3
661
and 9). It seems that fault scarps formed facing downstream are less likely to be preserved than those
662
formed with the scarp facing upstream. In the first case, the scarp would produce a local increase in
663
the stream gradient, boosting stream power and, consequently, erosion. The scarp would be then
664
swiftly degraded by headward erosion. In the second case, the scarp would produce a local decrease
665
in the stream gradient, reducing stream power and therefore reducing the erosive power of the
666
torrent, favoring the preservation of the scarp and newly formed topography. This situation may
667
suggest that thrusts with vergence opposite to the flow direction of rivers would be more difficult to
668
cut through than those orographic units formed from thrusts with the same vergence than the river
669
flow direction —without taking into account other variables. In any case, the potential preservation
670
of any knickpoint, scarp, or topography also depends on the lithology and the erosive power of the
671
stream.
672
4.1.2 Topographic swath profiles
673
The topographic swath profiles show two asymmetrical basins, with the N-S river axis displaced
674
towards the east and the highest elevations to the west, in Central Precordillera (>3000 m a.s.l.).
675
Results suggest that the asymmetry of both basins would not be the result of a lateral tilting, but
676
rather a product of the different rock type, geometry, and vergence of thrusts between the Central and
677
Eastern Precordillera.
678
Quaternary deformation is not always evident in these swath profiles. However, an increase in the
679
deviation between maximum, mean, and minimum curves in the piedmonts, when showing a parallel
680
or up-warped geometry, usually coincides with areas with morphotectonic evidence of Quaternary
681
uplift.
682
Topographic swath profiles also indicate that Central Precordillera shows a higher degree of incision
683
than Eastern Precordillera, suggesting an older relief and a more developed river network.
684
4.1.3 Along-river swath profiles
685
The swath profile of the De la Travesía River shows a deviation between the max and min curves in
686
its headwaters as it crosses the main N-S mountain ranges, also suggesting greater slopes.
687
Downstream, the curves indicate a flat topography, when the river traverses the Matagusanos Playa
688
Lake. By contrast, the De la Ciénaga River is characterized by relatively high valley walls and
689
terraces that confine the channel forming a narrower floodplain. In this way, the along-river swath
690
profiles analysis shows that the De la Ciénaga River frequently interacts with steep piedmonts and
691
mountain slopes, suggesting a higher degree of structural control when compared to the De la
692
Travesía River.
693
4.1.4 River channel patterns
694
The change in the river pattern between reaches 1 and 2 of the De la Travesía River is due to a
695
widening of the Matagusanos valley and the reduction of the slope of the valley (Fig. 12). Between
696
reaches 2 and 3, the change in river pattern is a result of the narrowing of the valley due to the
697
appearance of the Loma de Ullum nearby to the east, which partially blocks the outlet of the valley to
698
the south. There is no significant variation in slope. The change in the river pattern between reaches
699
3 and 4 is coincident with a significant increase in the valley slope. The straight channel of reach 3
700
increases its sinuosity markedly in reach 4 transitioning into a bar-braided system. As the width of
701
the active channel belt grows progressively downstream, the pattern changes to an island-braided
702
system. This phenomenon is coincident with a remarkable morphological change in the Loma de
703
Ullum that reaches its maximum height at this point and considerably reduces its width and height
704
towards the south, controlled by an NW trending lineament (Fig. 3). This marked increase in the
705
slope and the presence of downstream-divergent terraces next to the river mouth would indicate that
706
the De la Travesía River is still adjusting to the local base level represented by the Ullum-Zonda
707
Valley. The tectonic uplift of the Loma de Ullum continuously interferes with this process, which
708
probably gives the Matagusanos valley its status of semi-bolson. However, the presence of streams
709
and gullies in the reach 2 within the ancient Matagusanos playa lake (Figs. 4a and 12) suggests that
710
fluvial incision rates prevail over the uplift rates of the Loma de Ullum. The downstream-divergent
711
terrace system possibly responds to successive base level drops associated with the Holocene capture
712
of the San Juan River by the Ullum gorge (Blanc and Perucca, 2017).
713
In the De la Ciénaga River, the channel pattern is braided along its entire length (Fig. 13). However,
714
we observed variations in the width of the active channel belt and the type of braiding: island-braided
715
vs. bar-braided. These variations could be related to the effects of Quaternary tectonic deformation.
716
In the reaches where the river flows across areas where Quaternary uplift is presumed, there is a
717
tendency to reduce the width of the active channel belt by lateral confinement and to develop a bar-
718
braided pattern. The narrowing of the valley causes this confinement by the presence of steep
719
piedmonts or the appearance of rocky outcrops out in the river bank. In the remaining reaches, the
720
width of the active channel belt increases, and the river develops an island-braided pattern with less
721
interference from nearby piedmonts.
722
The presence of Quaternary terraces on the right bank of the De la Ciénaga River in reaches 5, 6, and
723
7 (Fig. 14a), some of which are strath terraces carved in limestone suggests the presence of a reverse
724
fault along the e8 lineament. This proposed fault would have been active during the Late Pleistocene.
725
4.1.5 River basin morphometry
726
Even though the obtained river basin morphometric parameters are too general (due to the extension
727
and complexity of the basins), it is possible to make the following observations (Table 1).
728
The basin of the De la Ciénaga River is 41% smaller than the basin of the De la Travesía River. The
729
elongation ratio indicates that both are slightly elongated basins, with the De la Ciénaga being 13.6%
730
more elongated than the De la Travesía. In tectonically active regions, basin elongation is indicative
731
of regional stress direction. Siame et al. (2005, 2006) suggested that the predominant N-S fault
732
population in the Precordillera area shows approximately horizontal east–oriented maximum
733
principal stress (σ1).
734 735 736 737
Fig. 14. (a) View to the east of a strath terrace (Quaternary alluvial deposits [Q] on top of Cambrian limestone [CL]) in the right margin of De la Ciénaga River in the reach number 6. (b) View to the southwest of the Barrancas playa lake showing headward incision of the De la Ciénaga River.
738
The almost identical opposite values of the asymmetry factor and the transverse topographic
739
symmetry factor, suggest that both basins have mirror symmetry, independently of their order and
740
scale. As mentioned before, the asymmetry of both basins would not be the result of a lateral tilting,
741
but rather a product of the geometry and different vergences of thrusts between the Central and
742
Eastern Precordillera. Eastern Precordillera is a back-thrust system that involves basement rocks and
743
has western vergence. As previously stated, mountain fronts formed from structures with vergence
744
opposed to the flow of rivers, are a more difficult obstacle to cut through. This situation forced the
745
rivers to join and divert to take a new north-south trend, giving rise to a large-scale broom-shaped
746
pattern according to the concept of Audemard (1999).
747
5.1 Conclusions
748
The analysis of several morphotectonics tools (longitudinal river profiles, knickpoints, swath
749
profiles, and geomorphic indices) together with geomorphological, lithological, and structural data
750
provided objective and quantifiable elements to improve the knowledge of the Quaternary tectonic
751
activity in the region. This analysis also allowed us to test the effectiveness of these methods in a
752
geologically complex area with a well-known Quaternary tectonic setting.
753
The longitudinal river profiles and knickpoints yielded data that suggest the existence of previously
754
unknown structures or areas affected by Quaternary deformation. The knickpoints related to faults
755
with a marked lithological contrast between the hanging wall and the footwall resulted not useful to
756
assess tectonic activity. Lithology contrasts have a strong influence on river morphology that
757
completely masks tectonic signals. In poorly consolidated rocks or alluvial deposits, knickpoints
758
related to active structures were not preserved, yet in many cases, the river profile appeared up-
759
warped in the hanging wall of the fault.
760
At the study area scale, we observed that faults with evidence of Quaternary tectonic activity did not
761
produce knickpoints if soft rocks constitute the hanging wall and the footwall. The same situation
762
occurs if the faults generate a downstream facing scarp. These results suggested that thrusts with
763
vergence opposite to the direction of flow of rivers would be more difficult to cut through than those
764
with vergence in the same direction of the river flow, depending on whether the fault scarp increases
765
or decreases the local gradient (and the stream erosive power) of the river.
766
Quaternary deformation is not always evident in swath profiles. However, swath profiles curves that
767
show a parallel or up-warped geometry in the piedmonts, usually coincide with areas with
768
morphotectonic evidence of Quaternary uplift. The along-river swath profiles showed that the De la
769
Ciénaga River frequently interacts with steep piedmonts and mountain slopes, suggesting a higher
770
degree of structural control when compared to the De la Travesía River.
771
River pattern analysis showed variations in the width of the active channel belt and the type of
772
braiding related to the effects of Quaternary tectonic deformation. In the reaches where rivers flow
773
across areas where Quaternary uplift is presumed, we observed a tendency to reduce the width of the
774
active channel belt by lateral confinement and to develop a bar-braided pattern.
775
The W-E tributary rivers from the mountainous and piedmont areas flow to the main N-S collectors
776
(De La Travesía and La Ciénaga rivers). Both rivers drain to the W-E San Juan river, forming a large
777
scale broom-shaped pattern. This type of pattern is a common morpho-structural expression of thrust
778
faults on river networks.
779 780
Acknowledgments
781
We thank the editor and the reviewers for providing comments that helped to improve our
782
manuscript significantly. The authors acknowledge funding from PIP 92.CO (CONICET), PICTO
783
AGENCIA 2016-0995, and CICITCA 21/E1052. Pablo A. Blanc and Flavia Tejada acknowledge a
784
doctoral fellowship provided by the Consejo Nacional de Investigaciones Científicas y Técnicas
785
(CONICET).
786 787
788
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Highlights
• • • •
Morphotectonic analysis of two specular river basins Active tectonics have a significant influence on the evolution of fluvial systems Results could be useful to understand Quaternary geomorphic and tectonic evolution Analysis of transient processes, tectonic-lithological knickpoints and topography
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: