Tectonic evolution of the western “Pampean” flat segment (28°–30°S)

CHAPTER

Tectonic evolution of the western “Pampean” flat segment (28°–30°S)

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F. Martínez⁎, C. Arriagada†, C. López⁎, Mauricio, Parra‡

Department of Geological Sciences, Catholic University of the North, Antofagasta, Chile* Department of Geology, University of Chile, Santiago, Chile† Institute of Energy and Environment, University of São Paulo, São Paulo, Brazil‡

1 ­Introduction The flat-slab or “Pampean” region of Chile and Argentina in the Central Andes is located between 28° and 30°S (Fig.  1). Along this region, the oceanic Nazca plate is subducting horizontally under the overriding continental margin of South America, which is mainly characterized by a Quaternary volcanism gap (Barazangi and Isacks, 1976; Ramos et al., 2002; Martinod et al., 2010). Two main causes have been proposed for this geodynamic context; the first is related to the westward, fast motion of the continental margin toward the Nazca trench in the hot spot reference frame (Scholz and Campos, 1995; Silver et al., 1998), and the second is related to the subduction of buoyant anomalies, such as aseismic ridges and/or oceanic plateaus (Pilger, 1981; Gutscher et al., 2000). The tectonic configuration of the overriding continental margin along the “Pampean” region consists of large (more than 50 km long) basement-involved structures that lie well exposed in the Coastal and Frontal cordilleras in northern Chile and the Sierras Pampeanas in Argentina (Fig. 1), which have accumulated no more than 160 km of crustal shortening (Kley et al., 1999; Coutand et al., 2001; Kley and Monaldi, 2002; Cristallini et al., 2004; Martínez et al., 2016). This structure is usually compared with those present in other deformed belts characterized by basement-involved deformation and commonly associated with flat-slab subduction segments, such as the Rocky Mountains of North America, the Sierra Madres of Mexico, and the Western Cordillera of Colombia, and, also the western Peruvian Andes, among others (Jordan et al., 1983; Jordan and Allmendinger, 1986; Moscoso and Mpodozis, 1988; Ramos, 2010; Martínez et al., 2016). In northern Chile, the Coastal and Frontal cordilleras represent the first-order tectonic provinces that form the western “Pampean” flat-slab segment, being the Frontal Cordillera the site that records most deformation in the region. They are composed of a series of intercalated, NNE-oriented mountain belts of granitic rocks and Mesozoic and Cenozoic strata that reach around 5 km in elevation. Regional, structural, and geochronological studies (Moscoso and Mpodozis, 1988; Ramos, 1999; Arévalo, 2005; Salazar et al., 2013; Martínez et al., 2015a; Rossel et al., 2016) have interpreted that the structure of the region resulted from short contractional episodes (lasting between 10 and 15 Ma) that occurred diachronically from the Late Cretaceous onwards. These are related to the horizontal contraction experienced by the overriding continental margin after the shallowing of the Nazca oceanic plate beneath of it. Andean Tectonics. https://doi.org/10.1016/B978-0-12-816009-1.00019-8 © 2019 Elsevier Inc. All rights reserved.

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FIG. 1 DEM topography image of the Central Andes along the “Pampean” flat-slab subduction, segment indicating the main tectonic provinces located in Chile and Argentina, and the location of the study area.

In order to provide a general background of the tectonic evolution of this region, we have compiled a complete set of previous and new tectonic interpretations, which are mainly based on geological mapping, balanced cross-sections, geochronological (U-Pb, K-Ar, etc.) and thermochronological data (apatite fission track). These results are here used to discuss the Mesozoic and Cenozoic tectonic history of the western “Pampean” flat-slab segment of northern Chile between 28° and 29°S.

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2 ­Stratigraphy of the western “Pampean” flat-slab segment 2.1  ­Coastal Cordillera This province occupies the westernmost part of this segment (Figs. 1 and 2) and consists of Devonian to Carboniferous metasedimentary and granitic plutonic rocks that range between 190 and 110–90 Ma (Dallmeyer et al., 1996; Grocott and Taylor, 2002; Naranjo and Puig, 1984). Its eastern side is composed of Jurassic and Lower Cretaceous volcanic and sedimentary deposits that lie well exposed along narrow NNE-oriented stratigraphic belts (Figs. 2 and 4). The Jurassic deposits mainly consist of volcanic successions (andesitic lavas, basaltic andesites volcanic breccias, etc.), which unconformably overlie, Lower Mesozoic granitic rocks (Fig. 3). These have been assigned to the La Negra (Lower Jurassic) and Punta del Cobre Formations (Upper Jurassic-Lower Cretaceous), both interpreted as products of an ancient volcanic arc setting (Arévalo et al., 1999; Marschik and Fontboté, 2001; Oliveros et al., 2006; López et al., 2017). The Lower Cretaceous deposits comprise nearly 4000 m of marine and fossiliferous sedimentary successions (limestones, sandstones, and shales), which rest unconformably over Jurassic rocks (Figs. 3 and 4). These form a large stratigraphic wedge that thickens toward the easternmost part of the Coastal Cordillera, and has been defined as the Chañarcillo Group (Arévalo, 1999; Mourgues, 2004; Martínez et al., 2013). The Chañarcillo Group is followed by ~3000 m of continental volcanic and sedimentary successions composed of andesites, sedimentary breccias, conglomerates, sandstones, among others, known as the Cerrillos Formation (Segerstrom and Ruiz, 1962; Arévalo, 1999; Maksaev et al., 2009). This complete Mesozoic stratigraphic record is associated with the infill of the Lower Cretaceous Chañarcillo Basin; a back-arc extensional basin that was established along the western continental margin of central South America during the break-up of Gondwana (Mpodozis and Ramos, 1989, 2008; Aguirre-Urreta, 1993; Ramos, 2009).

2.2 ­Frontal Cordillera The main and oldest rocks of this province correspond to Permo-Triassic complexes made of granites, granodiorites, monzogranites, and similar igneous rocks that report crystallization ages (U-Pb) between c.245 and 250 Ma (Salazar et al., 2013; Martínez et al., 2014, 2015a; Del Rey et al., 2016; Coloma et al., 2017). They are grouped into the Chollay, Montosa, and Guanta Plutonic Complexes, among others (Mpodozis and Kay, 1990; Martínez et al., 2014; Ortiz and Merino, 2015), representing the prerift basement of the ancient Mesozoic back-arc extensional-related basins (Lautaro, Lagunillas, La Totora) (Fig. 2). These are usually unconformably overlain by Upper Triassic continental volcanic (lavas, andesites, basalts) and sedimentary (red conglomerates and sandstones) successions, which constitute the basal synrift infill of the back-arc basins, assigned to the La Ternera and/or La Totora formations (Suárez and Bell, 1992; Jensen, 1976; Charrier, 1979; Salazar et al., 2013; Fig. 3). This succession is unconformably overlain by thick stratigraphic wedges of Lower Jurassic marine and continental volcanic and clastic deposits, interpreted as the upper synrift successions of the Mesozoic extensional basin described earlier (Martínez et al., 2012), defined as the Lautaro and Lagunillas formations (Jensen, 1976; Arévalo, 2005; Oliveros et al., 2013). This formation is commonly covered by Upper Jurassic continental volcanic and sedimentary deposits (lavas, tuffs, sandstones, shales) that complete the postrift record of the basins, which have been described under different names (e.g., Picudo, Algarrobal, etc.).

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FIG. 2 Simplified geological map of the western “Pampean” flat-slab segment of northern Chile, showing the distribution of major geological units and structures. In the map, are also indicated the location of the different regions analyzed in this work (modified from Martínez et al., 2016).

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FIG. 3 Mesozoic and Cenozoic stratigraphy of the western “Pampean” flat-slab segment of northern Chile, based on geological observations.

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The Cretaceous to Cenozoic is mainly composed of Upper Cretaceous to Miocene successions. The oldest Upper Cretaceous-Paleocene deposits lie well exposed along the westernmost and easternmost parts of the Frontal Cordillera (Fig. 2). They correspond to thick continental synorogenic successions comprising volcanic (tuffs, lavas, andesites, volcanic breccias, and volcanic flows) and sedimentary deposits (conglomerates and sandstones) known as the Viñitas, Quebrada Seca, and/or Hornitos formations that usually display a marked angular unconformity with the Mesozoic synrift deposits (Figs. 3 and 4). They form asymmetrical wedges composed of growth strata with ages c.70–80 Ma (Peña et al., 2013; Salazar et al., 2013; Martínez et al., 2013; Rossel et al., 2016). In contrast to Mesozoic deposits, these have been interpreted to be accumulated in a contractional setting (Peña et al., 2013; Rossel et al., 2016; Martínez et al., 2016), and mark the oldest contractional deformation in the region. The rest of the Cenozoic rocks are mainly composed of kilometric-scale Eocene intrusive bodies, (granodiorites, granites, syenogranites, etc.), and thick Miocene to Pliocene deposits of unconsolidated sediments and interbedded volcanic complexes, which are mostly exposed in the eastern parts of the region (Fig. 3).

3 ­Structural styles present in the western “Pampean” flat-slab segment Two major structural styles have been interpreted along this segment: inverted structures and basementinvolved reverse faults and folds. The best examples are found in the eastern Coastal Cordillera, the westernmost part of the Frontal Cordillera, and along the central and easternmost parts of the Frontal Cordillera (Fig. 2). Inverted structures consist of large NNE-striking inversion anticlines (Figs. 4 and 5B), which are mostly exposed in the eastern Coastal Cordillera and the western Frontal Cordillera. In the Coastal Cordillera, the structure consists of an east-verging asymmetrical anticline called the Tierra Amarilla Anticline (Segerstrom, 1960; Fig. 4A). This fold comprises the Lower Cretaceous synrift and mid-Cretaceous postrift deposits of the Chañarcillo Group and Cerrillos Formation, respectively. The anticline commonly shows an arrowhead geometry with a large and semihorizontal back limb, and a short and steep frontal limb (Fig. 4B). The fold lies in the hanging wall of the NNE-striking and eastverging Elisa de Bordos Fault, located at the eastern edge of the Chañarcillo Basin (Fig. 4C). Field observations, along with gravimetry studies and balanced cross-sections (Martínez et al., 2015b), show that this anticline overlies a deeply subsided half-graben structure, and that the boundary of the fold lies above the shoulder of the graben. Consequently, this structure is interpreted as a “basin margin inversion fold” related to the tectonic inversion of the Lower Cretaceous Chañarcillo Basin. In the western Frontal Cordillera (Fig. 2), other large and small inversion folds are present in the Triassic-Jurassic Lautaro and La Totora basins (Fig. 2). They commonly consist of NNE-striking and west-verging anticlines that raise the Triassic-Lower Jurassic synrift and Upper Jurassic postrift deposits of the Lautaro, Picudo, and Algarrobal formations (Fig. 5B). Geometrically, these structures show two main shapes. (1) Along the western Lautaro Basin (Fig. 2), the anticlines have a long-wavelength arrowhead shape, similar to those observed in the Tierra Amarilla Anticline in the Coastal Cordillera. These folds have kilometric-scale Tertiary intrusive bodies trapped in their cores. The anticline lies in the hanging wall of the so-called Calquis Fault, which forms part of the western shoulder of the Lautaro Basin (Fig. 5B). (2) The top of this anticline lies above the thickest, main depocenter of the basin. Unlike the previous case, along the La Totora Basin (Fig. 2), the structure consists of a narrow and asymmetrical anticline located in the hanging wall of the Pinte Fault (Fig. 6), which forms part of the western shoulder of the La Totora Basin. In this area, the anticline is characterized by a frontal limb strongly shortened

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FIG. 4 (A) West to east regional view of the structural architecture of the inverted Chañarcillo Basin in the eastern Coastal Cordillera (GoogleEarth), (B) transversal view of the folded synrift deposits of the Lower Cretaceous Chañarcillo Group in the central part of the Chañarcillo Basin, (C) aspect of the relationship between the post-rift deposits of the Cerrillos Formation and the Upper Cretaceous synorogenic deposits along the Elisa de Bordos Fault.

against the fault, along which the Lower Jurassic synrift deposits of the Lautaro Basin are buttressed (Fig. 6). The Pinte Fault is a NNE-striking high-angle fault that shows some along-strike dip variations. Kinematically, both anticlines in the Lautaro and La Totora basins have been interpreted as the result of positive reactivation of their master faults (Martínez et al., 2012; Salazar et al., 2013). Some previous structural models (Martínez et al., 2012; Salazar et al., 2013, among others), based on geological mapping, tectonic restorations and analogue modeling, have argued that these structures can be explained by the closure and tectonic inversion of Lower Jurassic asymmetrical half-grabens. However, the original displacement of the faults has not fully inverted.

FIG. 5 (A) West to east regional view of the Frontal Cordillera in northern Chile at 27°S latitude (GoogleEarth), showing the distribution of the Lautaro and Lagunillas basins, as well as the relationship between the basement granitic rocks and the Meso-Cenozoic cover in both basins, (B) panoramic view of a west-verging inversion anticline located along the western shoulder of the Lautaro Basin.

FIG. 6 See figure caption on opposite page.

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The basement-involved reverse faults and folds are mostly located in the central and eastern parts of the Frontal Cordillera (Fig. 2). They consist of large NNE-striking and irregular structures that uplift the Permo-Triassic crystalline and granitic rocks that form the prerift of the Lautaro, Lagunillas, and La Totora basins (Figs. 2 and 3), thus separating large basement blocks by narrow belts of stratified rocks. Good examples of these structures are represented by the Iglesia Colorada, Vizcachas, El Potro, and Chacay, Valeriano faults, among others (see Salazar et al., 2013; Martínez et al., 2014, Fig. 7). Reverse faults usually alternate between west- and east-verging structures with moderate-to-high angle (60°–70°), which place the Permo-Triassic granitic rocks over the Meso-Cenozoic cover, where narrow footwall synclines are usually developed (Fig. 7A). Good examples of these structures are found at the eastern border of the Lautaro Basin (e.g., Montosa High, Figs. 5 and 7), the western and eastern borders of the Lagunillas Basin (Figs. 2 and 5), and along the La Totora Basin (Fig. 2). Folds commonly correspond to large NNE-striking, basement-cored asymmetrical anticlines that lie above the hanging wall of the east-verging reverse faults (Fig. 8). The best example of these structures is found along the Lagunillas Basin near the Chile-Argentina border, where it is known as the El Colorado Anticline (Fig. 8). Tectonic restorations (Martínez et al., 2016) have interpreted that these structures are associated with the propagation of large basement ramps, which could have been emplaced along ancient weakness zones, such as previous shear zones, normal faults, suture zones, among others. However, in the field, there is not enough geological evidence for it.

4 ­Mechanisms of deformation 4.1  ­Normal faults reactivation Based on the structural styles described, three main Andean deformation mechanisms are proposed to have acted in the western “Pampean” region, as illustrated in Fig. 9. The first is related to pure inversion, where previous Mesozoic (Triassic to Early Cretaceous) basement normal faults were partially reactivated. Structures related to basin inversion are usually concentrated in the western part of the region (e.g., Tierra Amarilla Anticline), thus indicating that the former Mesozoic normal faults in this area had the following conditions: initial moderate dip angles (between 45° and 50°), low coefficient of friction, low cohesion, and that the σ1 direction that resulted in fault reactivation was nearly perpendicular to the original orientation of the normal fault. Many of these inverted faults and inversion anticlines are commonly affected by magmatic intrusions and veins, which appear to have been contemporaneous to the growth of the inverted structures. This indicates that the emplacement of magma played an important role during basin inversion, acting as a lubricant to facilitate the reactivation of normal faults. Other basin inversion mechanism is buttressing (Fig.  9), which is preferably developed in those basement-involved, high-angle faults of the Frontal Cordillera. This mechanism is especially evidenced by the strong folding of the thin-bedded Jurassic limestones of the Lautaro Formation in the hanging wall of the inverted normal faults along the La Totora Basin. The inversion anticlines created from this mechanism are usually narrow, with the frontal limbs strongly shortened against the fault planes FIG. 6–CONT'D Panoramic oblique view of the inverted Pinte Fault along the westernmost part of the La Totora Basin. Aspect of the folding and buttressing of the Jurassic synrift deposits of the Lautaro Formation in the hanging wall fault. See location in Fig. 2.

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FIG. 7 (A) Aspect of the Iglesia Colorada Fault and a footwall syncline fold exposed at the eastern border of the Lautaro Basin, (B) relation observed between the pre-rift basement rocks and the marine Jurassic synrift deposits along the Vizcachas Fault in the western Lagunillas Basin. See location in Fig. 5.

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FIG. 8 West to east panoramic view showing the geometry of the El Colorado Anticline in the easternmost part of the Lagunillas Basin (Fig. 5), and the stratigraphic relation between the different units exposed in the region.

Normal fault reactivations

Pure inversion

Buttressing

Basement-involved reverse faulting

Trishear fault propagation folding

Decapitation of previous normal fault

FIG. 9 Styles of inverted structures and basement contractional structures along the “Pampean” region of the northern Chile.

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(Fig. 6). During basin inversion, the upper sections of the normal faults probably had dips over 60° and, consequently, they were not well oriented for reactivation. Therefore, these faults were locked, and the shortening was mostly concentrated within the weak layers of the synrift Lautaro Formation, where minor mesoscale thrusts and detachments were created.

4.2 ­Basement-involved reverse faulting Two main mechanisms of basement-involved contractional deformation are recognizable; the first is related to trishear fault propagation folding (Erslev, 1991; Fig. 9), which is mostly identified along the eastern border of the Lautaro Basin and other major basement faults located in the La Totora Basin (Salazar et al., 2013). According to previous tectonic restorations (Martínez et al., 2016), these structures mainly deform paleostructural highs that separate the Mesozoic basins. Usually, the movement along major faults deforms the basement in these sectors, and fault slip is consumed by folding within the thin volcanic and sedimentary layers of the synrift and postrift deposits of both basins (Martínez et al., 2012). Under this mechanism, deformation is especially concentrated within a triangular deformation zone that widens upward (Fig. 9), where rocks tend to be sheared. The rocks in this triangular zone commonly form narrow footwall synclines whose geometry will depend on the propagation and slip ratio of the fault (Narr and Suppe, 1994; Mitra and Mount, 1998; Allmendinger, 1998). This model is similar to those proposed by Erslev (1991) and Erslev and Rogers (1993). Kinematically, this mechanism depends on the competence between the basement and cover, the rheology of the basement rocks, the thickness of the cover and the physical conditions of deformation. Based on their current geometries, previous works (Martínez et al., 2012) have considered that the geometry of the hanging wall blocks of these structures can be modeled assuming a p/s (propagation/slip) relation = 0, and an average of upward displacements of 10–15 m of the hanging wall blocks along the fault planes. These structures usually deform the paleostructural highs that separated the Mesozoic basins during their previous extensional stages (Martínez et al., 2016). The other mechanism for basement-involved reverse faulting is related to the decapitation of previous basement normal faults (Fig. 9). Here, new basement thrust ramps cut some of the inherited structures (commonly Mesozoic normal faults) that were not fully reactivated. Evidence for this mechanism is found in the western border of the Lagunillas Basin (Fig.  7), along which a major east-verging basement thrust fault known as the Vizcachas Fault (Jensen, 1976; Martínez et al., 2014) overlies the synrift marine deposits of the Lagunillas Basin located in the main depocenter. This situation is usually governed by some conditions; for example, previous high-angle normal faults are especially prone to be decapitated during contractional deformation, especially when the contractional stress field is nearly perpendicular to the normal faults. Many of these structures have associated secondary back-thrusts, which form pop-ups, thus making the determination of Andean deformation vergence difficult.

5 ­Chronology of Andean deformation The chronology and sequence of the contractional deformation and exhumation experienced in this region were mainly constrained by U-Pb and thermochronological data. The ages of deformation have been usually determined from the U-Pb ages of detrital and volcanic zircons of the synorogenic deposits preferentially exposed in the frontal limbs of the inversion anticlines, and also in the footwall blocks of the basement-involved reverse faults. The ages of exhumation were obtained from apatite fission track analysis.

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The volcanic and sedimentary successions of the Hornitos Formation in the Coastal Cordillera (Fig. 3) show growth strata in the limbs of some synclines exposed in the easternmost part of this region, and over the frontal limbs of inversion anticlines, thus indicating that they were accumulated synchronically with crustal shortening and therefore are interpreted as synorogenic deposits. Samples of ignimbrites and tuffs of the basal section of the Hornitos Formation, which unconformably overlie the tectonic inversionrelated Tierra Amarilla Anticline in the eastern Coastal Cordillera, have reported U-Pb ages between 70 and 80 Ma (Peña et  al., 2013; Fig.  10B). Similar ages have also been recently determined for the basal conglomerates of the Quebrada Seca Formation in the easternmost part of the Frontal Cordillera (Martínez et al., 2016; Fig. 10A), and also in some volcanic and sedimentary deposits exposed to the south in the Vallenar area (Fig. 2) that unconformably overlie contractional structures. These Upper Cretaceous deposits usually cover unconformably synrift Mezosoic successions. Based on this stratigraphic relationship, we interpreted that the ages of these synorogenic deposits correspond to the oldest deformation

FIG. 10 (A) Aspect of the Upper Cretaceous synorogenic deposits accumulated in the footwall blocks of the basementinvolved reverse faults exposed in the eastern Frontal Cordillera, (B) east to west view of the folded Upper Cretaceous to Paleocene synorogenic deposits exposed on the eastern section of the Coastal Cordillera, (C) aspect of the overthrusted Paleocene synorogenic deposits exposed along the Frontal Cordillera and the angular unconformity between the Paleocene and Oligo-Miocene synorogenic deposits.

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ages determined for the “Pampean” region in northern Chile. As such, a first contractional pulse has been interpreted to occur during the Late Cretaceous. This pulse is mostly related to the tectonic inversion of Mesozoic basins previously established in the region as described earlier (see Section 2). This tectonic event is usually marked by a notorious angular unconformity between the Mesozoic (Jurassic-Lower Cretaceous) synrift and the Upper Cretaceous synorogenic deposits, which is well exposed along the eastern edge of the Chañarcillo Basin in the Coastal Cordillera. Some intrusive bodies located in the core of the inversion anticlines and along some inverted faults have reported similar ages as the synorogenic deposits, indicating that they were emplaced synchronically with tectonic inversion (Peña et al., 2013). A second contractional episode occurred during the Paleocene (Fig. 11), which is mostly recorded within the synorogenic deposits of the middle and upper sections of the Quebrada Seca Formation that lie in the footwall of the basement-involved reverse faults of the Frontal Cordillera. The rocks of these sections have reported U-Pb ages between c.60 and 65 Ma (Iriarte et al., 1999; Moscoso et al., 2010; Salazar et al., 2013; Martínez et al., 2014), indicating that they were accumulated during basementinvolved deformation of the Frontal Cordillera (Fig. 11). Good examples of this situation are found along the east of the Montosa High in the Lagunillas Basin (Fig. 10C). This Paleocene contractional event is mostly correlated with the so-called K-T Andean deformation (Cornejo et al., 2003), which is well recognized along the northern Chile. Recent thermochronological data (apatite fission track ages; Fig. 11) from prerift basement blocks have indicated that a rapid and important cooling episode related to crustal exhumation (Martinez et al., 2017; Lossada et al., 2017) occurred during the Eocene (c.55–30 Ma). This episode seems to have been accompanied by the progressive shortening and uplift of the basement blocks in the Frontal Cordillera along large doubly vergent reverse faults. The footwall blocks of these faults, frequently created narrow intermontane basins, whose depth and complete infill are unknown. Some of the Paleocene synorogenic deposits of the Quebrada Seca Formation are well preserved in narrow contractional minibasins located along unusual basement-involved triangle

FIG. 11 Geological cross-section along the “Pampean” region of the northern Chile showing the distribution of the firstorder structural styles, as well as the location of the chronological data.

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zones (Fig. 11). Eocene synorogenic deposits associated with the basement uplift reported by the thermochronological data are absent in our study area, but these crop out immediately to the northern and southern neighboring regions. This episode is correlated with the “Incaic” tectonic phase (Steinmann, 1929), which provoked an important crustal thickening and shortening of the continental margin in northern Chile, mainly due to the eastward propagation of basement-involved thrust systems (Coutand et al., 2001; Amilibia et al., 2008; Charrier et al., 2009; Martínez et al., 2015a; Lossada et al., 2017). A regional angular unconformity between the Paleocene synorogenic deposits of the Quebrada Seca Formation and the younger folded and faulted volcanic Miocene to Pliocene (e.g., Doña Ana) is observed along the easternmost part of the study area in the Lagunillas Basin (Fig. 2); however this stratigraphic relationship is not completely observed in other sectors of the western Pampean region. Commonly the Miocene volcanic rocks exposed in the sectors lie folded and faulted, thus indicating that they were affected by a younger Late Cenozoic contractional deformational episode. Nevertheless, the Miocene volcanic and sedimentary deposits related to the Atacama Gravels exposed on the eastern Coastal Cordillera are undeformed. This situation suggests that the Late Cenozoic Andean deformation was mostly concentered along the eastern Pampean region. This Late Cenozoic contractional episode is related to the eastward migration of deformation to the Sierras Pampeanas in Argentina. Previous works (Winocur et al., 2014) on the Argentinean side have related Oligocene to Miocene volcanic deposits with an extensional setting. Similarly, Mid-Cenozoic extensional deformation has been interpreted to some regions in central Chile (e.g., Abanico Basin; Charrier et al., 2009) and also to other regions in northern Chile (e.g., Salar de Atacama Basin; Jordan et al., 2007). However, structural and stratigraphic evidence as normal faults, synrift stratigraphic wedges, among others, have not been observed in our study region and therefore there is no sufficient evidence to interpret that extensional deformation occurred during the Cenozoic. On the contrary, the Miocene volcanic deposits are affected by thin-skinned folds and thrust faults (see earlier) (Fig. 12).

6 ­General discussions The present-day configuration of the western “Pampean” flat-slab subduction segment is dominated by a mix of structural styles that resulted from shortening and deformation of a previous extensional fault system. The interaction between ancient Mesozoic normal faults and the generalized contractional stress field overimposed along the continental margin caused the creation of large inversion anticlines, basementcored anticlines, and reverse faults, along which, Permo-Triassic prerift basement blocks and thick marine and continental synrift successions were shortened and uplifted. Usually, the tectonic inversion of previous normal faults occurs under transpressional deformation and, therefore, the inverted faults show strike and dip slips; however, it is very difficult determine a strike-slip component for the inverted and basement faults exposed in the region. In map view, some thin-skinned folds have an oblique orientation respect to the regional structures, thus indicating that these could have experienced transpression, but it is highly speculative. Other kinematics indicators for strike-slip motion are usually absent. Based on it, we interpreted that a major compression dominated the shortening and crustal uplift of the region. Further, reverse faulting is more efficient than strike-slip faults to elevate large basement blocks near of 4500 km a.s.l. The structure of this segment is frequently compared with the observed in the Sierras Pampeanas, which is dominated by intercalated basement blocks and intramontane basins, in a broken foreland style (Jordan and Allmendinger, 1986; Ramos et al., 2002; Dávila and Carter, 2013, among others). However, this comparison is only valid for the structure of the central and eastern parts of the Frontal Cordillera,

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FIG. 12 Semi-balanced cross-section and palinspastic restoration of the “Pampean” region of the northern Chile (modified from Martínez et al., 2016).

and not for the complete segment that includes the Coastal Cordillera. The persistence of inverted structures in the region and their along-strike variations are strongly controlled by three main factors: the initial dip angle of the Mesozoic normal faults, their orientation with respect to the contractional stress field, and the presence or absence of magmatic fluids during tectonic inversion. Based on this, the master faults of the Chañarcillo, Lautaro, and La Totora basins were the structures most favored for reactivation. The crustal structure of this segment has been usually related to vertical uplifts associated with the flat-slab subduction of the Nazca Plate under the continental margin, which seems to have occurred during the Late Cenozoic (Jordan et al., 1983; Ramos et al., 2002). However, based on the structural evidence described earlier, we hypothesize that the initial deformation episodes in this segment (tectonic inversion and basement fault propagation folding) were mostly controlled by the horizontal regional stress associated with the coupling between both tectonic plates. This interpretation considers that the

7 ­Conclusions

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region was affected by synorogenic magmatism during the Cenozoic, indicating thus that the flat-slab process occurred after its deformation. Major structural and stratigraphic evidences for crustal extension are absent in this region, unlike other regions of the southern Central Andes (e.g., Abanico and Cura Mallin basins) in Chile and Argentina, where an important Mid-Cenozoic extensional deformation is recognized (Charrier et al., 2009; Jordan et al., 2001; Burns et al., 2006; Horton, 2018, among others). In map view, this segment consists of a doubly verging contractional system, different to the eastverging thrust and fold belts observed in other Andean regions, such as the Sub-Andean, the Eastern Cordillera, and/or those located to the east of the Principal Cordillera in Argentina. Generally, the advance of contractional deformation in this region is marked by the occurrence of synorogenic deposits accumulated at the front of inverted structures and in the footwall of basement thrusts. These deposits are rejuvenating toward the Argentinean side, evidencing thus the eastward advance of the deformation front at these latitudes. Late Cretaceous deformation ages determined in the Coastal and Frontal Cordillera allow proposing that this episode affected completely the northern Chile at least to the latitude of the study region, thus suggesting that this had a long wavelength, because it affected a broad continental region. This type of contractional deformation commonly is interpreted as the result of pure shear uplift (Allmendinger and Gubbels, 1996), mechanically controlled by tectonic inversion. However, the Paleocene, Eocene, and Miocene tectonic episodes seem to be associated with simple shear, controlled by the eastward propagation of basement-involved thrusts. Locally, the major crustal shortening and uplift in the region is estimated to have occurred between the Paleocene and Eocene (Martínez et al., 2016; Lossada et al., 2017) and not during the Miocene, such as was previously proposed by some studies (Ramos et al., 2002, among others); even Oligocene and Early Miocene extensional deformation is not recognized in the region. Possibly a major crustal uplift occurred in the Late Miocene; however, this is mainly interpreted in the Argentinean side. We interpreted that the major relief in the Chilean side was created by the rapid propagation of broad and large prerift basement blocks as is indicated by field observations and the apatite fission track data. In Chile, usually, the contractional structures related to the Coastal Cordillera are covered by undeformed Miocene volcanic and sedimentary rocks, while that the basement-involved structures in the Frontal Cordillera are covered by softly folded Miocene volcanic and sedimentary deposits. This situation indicates that the major shortening occurred previous to the Miocene. Even, previous palinspastic restorations (Martínez et al., 2016) show that nearly 70% of the total shortening in the Chilean side was accumulated between the Paleocene and Eocene. Narrow contractional basins related to doubly verging basement reverse fault commonly contain Paleocene to Eocene synorogenic deposits. This suggests that at least the Eocene was a major episode of vertical crustal uplift. Previous works (Martínez et al., 2016) have reported nearly 41 km of crustal shortening in this region. Similar to what occurs to the south of the study area, the Neogene deformation is mostly related to the propagation of secondary structures and back-thrusts that accommodated major horizontal shortening (Lossada et al., 2017).

7 ­Conclusions The tectonic configuration of the western Pampean region of the Central Andes was largely controlled by the initial position of Mesozoic extensional systems established along the continental margin, dominated by intercalated half-grabens and structural highs. The generalized crustal shortening experienced by the region seems to have started after the opening of the Atlantic Ocean (110 Ma) considering the oldest ages (80–70 Ma) of the synorogenic deposits exposed in the region; however the major

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r­ eorganization of the continental plates (South America, Africa) provoked by the continental separation triggers an important deformation along the western margin of the central and southern sectors of South America, thus allowing the Late Cretaceous tectonic inversion of almost all the Mesozoic extensional systems previously established. During this episode, the master faults of these systems were commonly reactivated, and large inversion anticlines affecting the complete infill of the basin were created. The Cenozoic deformation was marked by three main contractional episodes that occurred during the Paleocene, Eocene, and the Mio-Pliocene, which were characterized by the eastward propagation of large basement thrusts. Many of these thrusts cut previous Mesozoic normal faults, and modified the initial geometry of those Mesozoic back-arc basins currently located in the Chilean Frontal Cordillera. The Andean belt at these latitudes corresponds to a doubly verging deformed belt influenced by the interaction between east-verging basement thrusts and back-thrusts, and, also, by doubly verging inversion anticlines. Therefore, the tectonic vergence in the region is mostly given by the rejuvenation of the ages of deformation. We finally conclude that the structural style of the region is more influenced by the preshortening architecture of the continental margin than by the flat-slab process.

­Acknowledgments This work was supported by Fondecyt project n° 3140557 “Crustal structure and deformation timing along the Chilean flat-slab subduction segment (27°–29°S), Central Andes.” We have benefited from large discussions on Andean tectonic evolution with Drs. Constantino Mpodozis and Reynaldo Charrier. We thank the support of K. Deckart on the U-Pb ages modeling, and S. Villagran and M. Vaccaris for the logistics during field work campaigns. We finally thank A. Folguera for the invitation to participate in this book. We thank J. Martinod and B. Horton for the constructive reviews of this work.

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­Further reading Allmendinger, R.W., Zapata, T.R., 2000. The footwall ramp of the Subandean decollement, northernmost Argentina, from extended correlation of seismic reflection data. Tectonophysics 321, 37–55.