4.a. The Niquizanga area

In this area, two ~5 km long, basement ridges are observed (Figs 5, 7a): the western Niquizanga and the eastern La Posta ridges. These two ridges are asymmetric, with a western steep slope bordered on the west by reverse faults while their eastern slope corresponds to the Neogene unconformity. At both ridges, the basement overthrusts the Neogene sediments. These layers present a syncline geometry in the footwall, with a shallow E-dipping limb (e.g. 07°E close to site 17; Fig. 5) and a steep to vertical W-dipping one (e.g. 90° close to site 2; Fig. 5), attesting for W-verging kinematics for the fault. These faults are at least 6 km long and cannot be extended along more than 10 km. Based on their ~200 m high escarpment and the 10°E dip of the erosion surface below the Neogene sediments, the maximum fault displacement can be estimated to up to 500 m. This Niquizanga bounding fault reactivated an inherited pre-Andean shear zone (Niquizanga shear zone) that was active as an extensional structure during the Silurian (Mulcahy et al. Reference Mulcahy, Roeske, McClelland, Jourdan, Iriondo, Renne, Vervoort and Vujovich2011).

Figure 7. Cross-sections through both investigated areas. See Figures 5 and 6 for location. (a) Cross-section through Niquizanga area. Two faults are mapped and dip about 40°E, with Neogene sediments between them. The faults are parallel to the inherited foliation in the basement. (b) Cross-section through Ampacama area. Three main thrusts are represented with dips about 30°E. Along one of the faults, the fault plane is parallel to the basement foliation, while elsewhere it is strongly oblique. Between the basement thrust, the Neogene sediments mainly dip about 30°E. A fourth fault is suspected in the western part, but no Tertiary sediments crop out to confirm it.

In the basement, small faults, located in the hangingwall of the main W-verging thrusts, are characterized by brittle deformation mainly parallel to the inherited metamorphic foliation. Microtectonic observations in the basement highlight an ENE–WSW contraction (sites 2, 6, 17, 23, 47; Fig. 5). This is consistent with the microtectonic observations in the sediments (ENE–WSW to ESE–WNW contraction, site 32; Fig. 5). Thus, it is most likely that the microfaults observed in the basement have the same age as the deformation seen in the late Tertiary – Quaternary sediments. Moreover, the shortening described above is also consistent with the overall N–S to NNE–SSW strike of the Pie de Palo anticline (Fig. 5), which is a late Tertiary–Quaternary feature. This also suggests that the brittle deformation observed in the basement is late Tertiary – Quaternary in age.

In addition, small-scale folds affecting both the sediments (site 32; Fig. 5) and the metamorphic foliation (sites 2, 6, 23; Fig. 5) are oriented NNW–SSE to N–S. These data also strongly suggest that all the mentioned deformation (both in the basement and the Cenozoic strata) is Plio-Quaternary in age and consistent with an overall E–W shortening.

These folds and faults affecting the basement (Fig. 8) generally present W-verging kinematics consistent with the kinematics of the two main faults. Moreover, the folds are brittle in the sense that their hinges are very localized and consist of few fractures while the limbs are very planar (Figs 8b, c, 9a). The folds also present kink geometries (Fig. 8a). These folds induce striations along the foliation planes (Fig. 9b), whose direction (roughly E–W) is also consistent with the E–W shortening. Finally, in the basement, one may also observe larger folds (metric-scale or more; Fig. 9c). These folds and their wavelength imply strong disharmony and several levels of decoupling inside the basement.

Figure 8. Field photographs of folds affecting the basement rocks. The folds display very brittle style with (a) kink geometries (site 2, Fig. 5), and (b, (c) sharp, fractured hinges with planar limbs (sites 10 and 17, respectively, Fig. 5), while the inherited ductile deformation can still be observed as boudins (c). Note that the fold axes are approximately N–S and that preferential vergence of the folds is systematically a W-vergence.

Figure 9. Field photographs of folds affecting the basement rocks. (a) Fold with sharp and fractured hinge and (b) associated foliation-parallel slip (site 2, Fig. 5). Fold and slip indicate an E–W shortening. (c) Larger-scale fold in the basement. Note the W-verging kinematics.

Parallel to the metamorphic foliation, breccias are also very frequent. They consist of brecciated elements from the basement attesting for shearing, with little or no cement (Fig. 10a). The same feature can be observed with a higher amount of calcite cement (Fig. 10b, c). In thin section, one may clearly identify the original metamorphic rocks and newly formed calcite (Fig. 11c, d).

Figure 10. Foliation-parallel breccias. (a) With no cement (site 23, Fig. 5). (b, c) With more cement (site 10, Fig. 5).

Figure 11. Microphotographs. (a) Micaschist (site 4, Fig. 5) with foliation underlined by quartz and micas. (b) Quartzo-feldspathic rocks with a high amount of chlorite and no clear anisotropy (site 96, Fig. 6). (c, d) Breccias and calcitic cement in the brecciated layers (site 23, Fig. 5).

Brecciation, shearing, and small-scale folding seem strongly controlled by the anisotropy of the basement. Indeed, these structures are linked to levels of decoupling/décollement characterized by mica-rich layers (Fig. 11a) where brittle folds are rooting. Moreover, faults such as the La Posta Fault (Fig. 5) are clearly controlled by the inherited basement foliation: at its southern end, the fault turns systematically parallel to the inherited foliation. At larger scale, the La Posta fault also follows the inherited foliation: in the northern part, both the fault and the foliation are striking NNE–SSW; in the southern part, both the fault and the foliation are N–S.

On the eastern side of the section, the Neogene sediments (Fig. 7a) are gently dipping ~5–10°E. A 720 m thick Neogene series has been described in the Niquizanga area (Cuerda, Varela & Iniguez, Reference Cuerda, Varela and Iniguez1983), south of the La Posta Ridge (Fig. 5). Accounting for the map width of these Neogene sediments and their average dip just to the north of the Niquizanga ridge, we calculated a rough estimate of 600–700 m for the total Neogene thickness including the torrential conglomerates that are conformably topping the Neogene strata. This estimate is similar to the thickness calculated by Cuerda, Varela & Iniguez (Reference Cuerda, Varela and Iniguez1983), indicating that the Neogene and basement brittle deformation, which are observed in the Niquizanga area, were formed at less than 1 km depth. Finally, no tapering structures or progressive unconformity were observed within the Neogene series. Nevertheless, the Late Neogene, Río del Camperito Formation (Cuerda, Varela & Iniguez, Reference Cuerda, Varela and Iniguez1983), which is coarser than the underlying Niquizanga Formation (see Section 3), contains metamorphic clasts from the Sierra Pie de Palo basement. These clasts indicate that these coarser Late Neogene sediments are coeval with the onset of the basement uplift.

4.b. The Ampacama section

If this area is comparable to Niquizanga in the sense that it displays several W-verging reverse faults (Fig. 6), Neogene beds have steeper dips and the basement ridges are more numerous in the Ampacama area. Indeed, we mapped seven faults: four in the northern part affecting the basement and the Neogene deposits; two in the sedimentary cover affecting both Neogene beds and Quaternary alluvial strath terraces; and one in the southern part affecting the basement and the Neogene. These faults are NNW–SSE to N–S and dip at about 40°E (Fig. 7b) and even 76°E for the southernmost one. The faults observed in the sedimentary cover may affect the basement at depth. Although this cannot be observed, it is suggested because these faults stand in the southward prolongation of the easternmost basement ridge. Most of these faults are at least 3–4 km long and have displacement up to 500 m.

The faults in both the basement and the cover witness an E–W shortening (Fig. 6), which is consistent with the microfaults reported for the southern Niquizanga area (Fig. 5).

In this area, it is noteworthy that several basement faults are parallel to the inherited metamorphic foliation, while some others are strongly oblique, or even perpendicular (Fig. 7b). Where the faults are oblique to the foliation, the latter is not characterized by a strong mineralogical segregation, i.e. the foliation is particularly not underlined by mica-rich layers (Fig. 11b).

The sedimentary layers are folded in this area, mainly as synclines between the basement ridges. Along their western limb, the dips are about 20–40°E, while in the eastern limb they can be up to 60–70°W. These geometries attest for significant asymmetries, which are also consistent with the W-verging basement thrusts.

Further west, in the Pie de Palo range, recent basement fault zones are also suspected (westernmost fault with a question mark; Figs 6, 7b). These faults are characterized by intensively fractured and faulted rocks, with a clear W-verging kinematics. However, due to the lack of Neogene sediments, their age cannot be strictly constrained. It is noteworthy that, in this area in particular as well as a few others, the basement foliation is not E-dipping, but is rather vertical to W-dipping. Further south, the foliation is also clearly folded: indeed, the orientation changes a lot in a rather restricted area (Fig. 6).

In the southernmost part of Ampacama, the sedimentary layers have dips increasing westward (from 20 to 65° and even vertical in a few places; Fig. 6, southern part). In the area where the Neogene layers are steeper (50–62°E), a sub-vertical basement slice (89°E) crops out. On its western side, a very steep (62°E) W-verging fault can be observed at the contact with the Neogene (Fig. 6). In this area, where the Neogene beds show steeper dips than in the southern Niquizanga area, the basement reverse faults affecting the Neogene also tend to be steeper.

As in the Niquizanga area, the Neogene series is composed of two sedimentary formations that are conformably capped by Early Quaternary alluvial conglomerates (Fig. 6). No detailed study is available on this Neogene series. Using the same kind of method as in the Niquizanga, we calculated two rough estimates for the Neogene maximum thickness: a first one of ~900 m to the east of the eastern basement ridge and a second one of ~1100 m on the southern part of the Neogene outcrop. These two estimates suggest that the thickness of the Ampacama Neogene series is on the order of 1000 m, hence slightly thicker than the Niquizanga Neogene formations. Nonetheless, the Ampacama thickness estimates also indicate that the Neogene deformation seen within the basement was formed under very shallow depth conditions. As in Niquizanga, the Neogene layers are characterized by lower dips on the eastern side of the section (20–35°E) while they show steeper (40–65°E) dips in the western part (Fig. 7a). No progressive unconformities or sedimentary tapering were observed in the field. Nevertheless, as in Niquizanga, the Upper Neogene Rio del Camperito Formation is coarser and contains clasts of metamorphic rocks derived from the Sierra Pie de Palo. This indicates that the basement of the Pie de Palo range was actively uplifting and under erosion during the Upper Neogene.

4.c. Synthesis

To sum up, here are our main observations. At two locations, small (in both length and displacement) W-verging reverse faults are mapped east of the Sierra Pie de Palo: these faults have lengths ranging between 2 and 6–7 km but can hardly reach 10 km; their displacements are usually less than c. 500 m. Compared to the length of the Pie de Palo Range, these faults are typically second-order structures. Most of these faults reactivated the inherited metamorphic foliation and/or shear zones, especially along mica-rich levels with an oriented fabric (Fig. 11a). Along these planes, close to and above the reverse faults, we observed foliation-parallel slip, breccias and folded foliation planes in between. The kinematics of these structures is consistent with both the microfaults measured within the sediments and the faults showing basement over sediments, as well as the overall geometry of the Pie de Palo range. This kinematics is also similar to that obtained from the inversion of earthquake focal mechanisms (Siame et al. Reference Siame, Bellier, Sebrier and Araujo2005). It is noteworthy that this top-to-the-west kinematics is most likely not inherited from pre-Andean times, as Palaeozoic top-to-the-east kinematics and associated lineations were observed in the Niquizanga shear zone. Thus, the reverse, top-to-the-west movement is most probably Neogene.

These W-verging faults have displacement ranging between a couple of hundred metres and 500 m. In some places (Fig. 6, southern part), the upward fault propagation is accompanied by gentle folds within the Neogene. Displacements in the Quaternary strath terraces are always much lower than within the Neogene formations. The northward prolongation of the Niquizanga basement ridge is at the limit of identification within the Quaternary alluvial surface. On the other hand, the surface of the Quaternary alluvial does not appear affected by the northward prolongation of the La Posta Ridge, while the underlying Neogene is still tilted (Fig. 5). In the Ampacama area, where the Neogene beds are more deformed, a tier of Middle to Late Quaternary strath terraces is conspicuously gently folded, the highest, hence oldest, surface terrace showing steeper dips than the younger ones. Nevertheless, the Quaternary terraces’ displacements and dips are much smaller than those observed within the Neogene. All these data provide clear evidence that these W-vergent reverse faults are typically early within the deformation timing of the Sierra Pie de Palo. Moreover, the fact that steeper fault dips are always associated with the steeper Neogene dips also strongly suggests that these second-order faults are early in the folding timing of the Sierra Pie de Palo

In between the two described areas, neither basement rocks nor reverse faults are clearly identified along the central-eastern side of the Sierra Pie de Palo. The only dubious candidate for such a faulted basement ridge could stand in the Casas Viejas area (31.38°S, 67.81°W) (Fig. 3b). However, it is difficult in this area to distinguish between the Neogene beds and a highly hydrothermal-altered basement. Either such basement ridges are present, but buried below the Neogene beds, or they did not develop in the same way in the central part of the fold as in the areas of Niquizanga and Ampacama. It is worth mentioning that similar range-facing scarps, striking NW–SE, are also observed within Quaternary alluvial fans of the NE corner of the Pie de Palo range (31.09°S, 67.8°W), 5–7 km to the SE of the Pajaritos fault (Figs 2, 3).

All these observations suggest that E–W shortening was accommodated at the surface in several places by shear along foliation planes and inherited shear zones. In the next section, we detail how these newly created shear zones reactivating the foliation may evolve through time, how the Sierra Pie de Palo developed and what implications this has for the crustal structure.

5.a. Foliation-parallel shear

We showed above that most of the small reverse faults observed along the eastern side of the Sierra Pie de Palo initiated as foliation-parallel shear zone, controlled by (i) the inherited anisotropy of the foliation and (ii) the presence of low-strength mineral phases such as micas.

Micas are weak mineral phases, and few experimental works have underlined that mica-rich aggregates are weaker than other rocks (such as quartzo-feldspathic ones) over a broad range of mid-crustal depths (Shea & Kronenberg, Reference Shea and Kronenberg1992). Although the deformation at Sierra Pie de Palo occurred at much shallower depth than mid-crust, we may assume that the micas also behave as weak phases at shallow depths. In this case, they would allow accommodation of a large amount of strain, hence localizing the deformation and constituting shear zones.

Probably more significant than the intrinsic mechanical properties of these mineral phases, their anisotropy has been shown to control the rock strength. Indeed, in rock mechanics experiments, the weakening is controlled by the orientation of micas relative to the stress orientation (Shea & Kronenberg, Reference Shea and Kronenberg1993). This means that the softening is controlled by both the presence of micas (or any other phyllosilicate) and, more importantly, their arrangement (e.g. Bos & Spiers, Reference Bos and Spiers2001, Reference Bos and Spiers2002; Niemeijer & Spiers, Reference Niemeijer, Spiers, Bruhn and Burlini2005; Jefferies et al. Reference Jefferies, Holdsworth, Wibberley, Shimamoto, Spiers, Niemeijer and Lloyd2006; Collettini et al. Reference Collettini, Niemeijer, Viti and Marone2009; Niemeijer, Marone & Elsworth, Reference Niemeijer, Marone and Elsworth2010; Holdsworth et al. Reference Holdsworth, van Diggelen, Spiers, de Bresser, Walker and Bowen2011).

Hereafter, we propose a field-based conceptual model for the evolution of mica-rich foliated basement rocks deformed at low depths (Fig. 12). Initially, the foliation has overall a shallow dip (i.e. before Late Neogene folding), then it is activated as reverse shear zones (Fig. 12b) on weak mica layers. At small scale, these weak layers act as local décollement, and thin quartzo-feldspathic layers start forming asymmetric folds consistent with the sense of shear (Fig. 12b). If deformation continues, the folded thin quartzo-feldspathic layers evolve as breccias subsequently more or less cemented by calcite (Fig. 12c). Ultimately, this zone can turn into a localized zone of shear. Thus, at larger scale, once this zone can accommodate significant deformation, it may act as a décollement level for larger folds involving a thicker pile of metamorphic rocks (Figs 9c, 12d). This model accounts for the discrete shear zones/faults, large as well as small folds observed in the field.

Figure 12. Conceptual model of shear zone development. (a) Inherited metamorphic foliation with both quartzo-feldspathic and mica-rich layers. (b) Shearing induces disharmony with folding of thin quartzo-feldspathic layers embedded between mica-rich layers that most likely accommodate foliation-parallel shear. Layer-parallel shortening of the upper quartzo-feldpsathic layer is represented for balancing purposes, although this has not been clearly observed in the field. (c) Breccias resulting from the breakdown of the folded quartzo-feldspathic layer. The shearing localizes in this brecciated zone. (d) Ultimately, this brecciated layer may evolve as a new local décollement allowing the development of larger folds affecting the upper quartzo-feldspathic layers.

This model assumes that the deformation seen in the hangingwall of the main reverse faults can be interpreted as ‘immature’ structures that formed the reverse faults. Thus, foliation-parallel slip was localized along the main reverse faults as well as distributed in their hangingwall (and footwall?). Additionally, some reverse faults are simply the reactivation of inherited shear zones (see above, Niquizanga thrust). The reactivation of other inherited Pampean thrusts/shear zones was already suggested from seismic line interpretation in the W-verging model (Vergés et al. Reference Vergés, Ramos, Meigs, Cristallini, Bettini and Cortés2007).

As mentioned in Section 4, these structures formed at very shallow depth, i.e. c. 1 km, based on the estimated thickness of the Neogene sediments around Sierra Pie de Palo. This explains why only very brittle structures are observed, such as very localized shear zones, faults, brittle folds with very localized and fractured hinges, planar limbs, and breccias (Figs 8–10).

At such shallow depth, the basement rocks are usually deformed in a very localized mode, i.e. with large faults or a series of smaller ones (e.g. Scholz, Reference Scholz2002; Handy, Hirth & Hovius, Reference Handy, Hirth, Hovius, Handy, Firth and Hovius2007). For the Pie de Palo Range, along with the small thrusts mapped in the field, the main feature is the anticline shape of this range, especially highlighted by relicts of the pre-Neogene erosional surface (Siame et al. Reference Siame, Sebrier, Bellier, Bourlès, Costa, Ahumada, Gardini and Cisneros2015). In the next section, we discuss the mode and kinematics of such a basement shortening.

5.b. Fault vergence at the Sierra Pie de Palo

The deformation of the eastern Sierra Pie de Palo consists of several W-verging reverse faults. These faults, which have small lengths (2–8 km) and small displacements (less than 0.2 to 0.5 km), are narrowly spaced (0.5–3 km) and correspond mostly to foliation-parallel, brittle shear zones that start developing within weak foliation levels of the basement during the early stage of folding. Consequently, their significance needs to be discussed in the framework of the folding models which have been proposed for the Pie de Palo anticline, i.e. W-verging fault-bend fold (Ramos, Cristallini & Pérez, Reference Ramos, Cristallini and Pérez2002; Vergés et al. Reference Vergés, Ramos, Meigs, Cristallini, Bettini and Cortés2007) or E-verging fault-propagation fold whose blind thrust branches at depth on a W-verging thrust (Siame et al. Reference Siame, Bellier, Sebrier, Bourlès, Leturmy, Perez and Araujo2002; Siame, Bellier & Sebrier, Reference Siame, Bellier and Sebrier2006) or not (Kadinski-Cade and Reilinger, Reference Reilinger and Kadinsky-Cade1985), especially in terms of the mechanism that distributes deformation in the basement.

According to Ramos, Cristallini & Pérez (Reference Ramos, Cristallini and Pérez2002), the W-verging Niquizanga Fault (at Niquizanga Ridge; Figs 5, 7), which is located at the rear of the Pie de Palo fault-bend fold, is a rather steep reverse fault ramping off below the eastern South Bermejo Basin, at more than 20 km depth. Such a fault contradicts the long-term topography (the southern Bermejo Valley should be uplifted and the Sierra Pie de Palo should be subsiding) and could only fit with an out-of-sequence structure. However, this option disagrees with our observations because the W-verging faults have short lengths, appear tilted with the sedimentary bedding and can thus hardly ramp off at 20 km. Moreover, their displacement pre-dates or more likely is coeval with the folding, hence partly early in the deformation timing of the Pie de Palo Anticline.

Another alternative associated with a fault-bend fold might correspond to what was proposed for the Alpine basement units in the Pyrenees Axial Zone (Casas et al. Reference Casas, Oliva, Roman-Berdiel and Pueyo2003). There, steep Meso-Cenozoic faults affect both the basement and its Mezosoic cover. They are interpreted as faults initiated in the basement where it passed above a flat–ramp–flat system. Indeed, in such a case, the structural configuration and kinematics implies internal deformation within the basement (Casas et al. Reference Casas, Oliva, Roman-Berdiel and Pueyo2003). Such a model could be contemplated for the Pie de Palo anticline assuming a fault-bend fold (Ramos, Cristallini & Pérez, Reference Ramos, Cristallini and Pérez2002; Vergés et al. Reference Vergés, Ramos, Meigs, Cristallini, Bettini and Cortés2007). Where the basement passes above a flat–ramp or a ramp–flat transition, there might be either contraction or extension in the upper unit, respectively. The W-verging reverse faults observed in the field might then witness this later configuration. However, this would imply that they formed at depth, as the flat–ramp transition is in the deep crust. This is unlikely given the brittle structures observed in the field.

Considering that these small W-verging faults are observed in the eastern part of the Pie de Palo anticline, which corresponds to a forelimb accounting for the shape of the erosional surface (Fig. 3), makes it necessary to test the E-verging fault propagation fold model. In such a model, the W-verging faults could represent intercutaneous structures such as have been observed at several places (e.g. Mackay et al. Reference Mackay, Varsek, Kubli, Dechesne, Newson and Reid1996). If so, the slip on an E-verging blind basement fault would be distributed on several W-verging small basement faults. However, one would expects such an intercutaneous wedge structure to develop rather late in the deformation timing. Thus, this option remains possible, but not very likely because our observations suggest these small W-verging faults started to develop early in the sequence of shortening.

Considering that these small W-verging reverse faults are located in the forelimb of the Pie de Palo anticline, where the dips are steepest (30–90°), one can surmise that they are linked to a mechanism of folding. This fault pattern being due to foliation-parallel slip resembles the bedding-parallel slip in the flexural-slip fold model. The role of basement pre-existing fabric has been highlighted in several basement-cored folds (Schmidt, Genovese & Chase, Reference Schmidt, Genovese, Chase, Schmidt, Chase and Erslev1993). The mode of folding described above may belong to ‘mode 2’ of Schmidt, Genovese & Chase (Reference Schmidt, Genovese, Chase, Schmidt, Chase and Erslev1993). However, in this latter work, the fabric allowed the deformation to be distributed in the basement in the sense that the zone of deformation below the forelimb is large. Nonetheless, the mode of deformation is not folding s.s. It rather corresponds to a distributed mode of faulting that induced a fold in the overlying sedimentary cover.

Such a zone of deformation in the basement has been described in a few places and often interpreted as a triangular shear zone, large toward the basement top, rooting and thinning downward in the upper tip of a thrust. Such a model has been generalized for the sedimentary rocks by Erslev (Reference Erslev1991) as a trishear mode of folding and applied for the basement to explain distributed faulting below the basement/cover interface (Bump, Reference Bump2003; Garcia & Davis, Reference Garcia and Davis2004). If the fault upper tip is initially located in the basement, there might be, near the fault tip, a triangular zone of distributed deformation active until the fault potentially propagates to the basement/cover interface. In this zone, the basement may deform by pervasive faults and fractures, either pre-existing or newly created (Garcia & Davis, Reference Garcia and Davis2004); this induces a curved basement/cover interface because of distributed deformation. If such a model applies for Sierra Pie de Palo, an E-verging basement thrust should terminate upward with a distributed zone of shear characterized by W-verging small faults.

Such switch of vergence makes the trishear model unlikely for the Sierra Pie de Palo, although it cannot be ruled out. Thus, finally, the small W-verging faults more probably witness a foliation parallel-slip mode of folding associated with an E-verging fault-propagation fold, similar to what has been proposed for sedimentary rocks (Suppe & Medwedeff, Reference Suppe and Medwedeff1990). Above the blind thrust tip, a fold developed to accommodate shortening. In this fold, the mode of folding, i.e. the way the distributed deformation is accommodated, is characterized by slip along inherited foliation planes (or inherited shear zones). Even if our observations cannot provide any kind of evidence to root a trishear model, one cannot totally discard it where the E-verging fault tip propagates at depth. In such a case it would coexist with the foliation-parallel slip model we propose for the Pie de Palo anticline.

Interestingly, unfolding the Neogene Pie de Palo anticline reveals a pre-existing series of antiforms and synforms (Fig. 13a) suggesting that the Neogene and Quaternary shortening should have amplified inherited basement antiformal geometries (Fig. 13b): in the western part of the sierra, the inherited foliation appears folded, with dips much higher than the erosional surface (Figs 3, 7, 13). In the western Pie de Palo limb, several pre-Andean E-dipping thrusts are mapped and most of them are associated with E-dipping foliation (e.g. Mulcahy et al. Reference Mulcahy, Roeske, McClelland, Jourdan, Iriondo, Renne, Vervoort and Vujovich2011; Van Staal et al. Reference Van Staal, Vujovich and Currie2011). However, W-dipping foliation can also be observed in some places (e.g. Mulcahy et al. Reference Mulcahy, Roeske, McClelland, Jourdan, Iriondo, Renne, Vervoort and Vujovich2011; Van Staal et al. Reference Van Staal, Vujovich and Currie2011), especially at the sierra top (Ramos & Vujovich, Reference Ramos and Vujovich2000). Thus, in the western limb, a series of antiforms and synforms of foliation is inherited, while at the sierra top and in the eastern limb a single asymmetric antiform is inherited from a pre-Andean event (Fig. 13). Thus, we propose that the foliation-parallel slip mechanism for folding is relevant and could be the main folding mechanism determining the present-day shape of the Pie de Palo anticline. In such a case, as mentioned above, this mechanism would somewhat resemble the flexural-slip model, hence, foliation-parallel slip in the basement would play the role of bedding-parallel slip in the cover. To accommodate the shortening in a flexural-slip fold, there is partitioning between bedding-parallel slip and layer internal deformation (fractures, veins, pressure-solution, twinning). In our model of the foliation-parallel slip mode of folding, there is also a partitioning between slip along inherited foliation planes (reverse faults; Figs 7, 12, 13) and internal deformation of basement rocks (here, fractures mainly; Figs 8 and 9). Such deformation is most likely similar to that observed in the field: ‘brittle’ folds with planar limbs and localized hinges (Figs 8 and 9).

Figure 13. (a) Cross-section of the Pie de Palo (see Fig. 3 for location) and (b) its restoration. In (a) the foliation traces are from field observation and geological maps (see Fig. 3) for the surface ones. East of Pie de Palo, at depth, the inherited foliation is projected southward from seismic lines located NE of Pie de Palo (Zapata, Reference Zapata1998). Seismic foci in the northern part of the Pie de Palo (Regnier et al. Reference Regnier, Chatelain, Smalley, Chiu, Isacks and Araujo1992) were projected on the section (dots). Pal. for Palaeozoic; Neog. for Neogene.

It is noteworthy that the reverse faults interpreted as witnessing flexural-slip mechanism are spaced a few hundred metre to a couple of km apart (Figs 5, 6). This fits well with the fact that the displacements along these faults are on the order of about a few hundred metres (Fig. 7), as predicted by Alonso (Reference Alonso1989).

In some places where the foliation was previously intensively folded, at smaller scale than the basement antiform (see spatial variations of foliation orientation in Fig. 5), new faults were laterally propagated and created oblique to the local foliation, to ensure a kinematics consistent at fold scale.

Therefore, the W-verging small reverse faults seen along the eastern side of the Pie de Palo anticline agree with an E-verging fault propagation fold. Moreover, the dip of the Neogene series in the eastern Pie de Palo is very high at some places (more than 60°; Fig. 6), which does not fit well with a fault-bend fold model where such dips should represent the dip of the W-verging thrust. Thus, although the W-verging model cannot be totally discarded we have shown that structural data permit the suggestion of an alternative model with a major crustal blind thrust and an associated fault-propagation fold (Figs 13, 14). Moreover, this surmised major crustal blind thrust is quite consistent with local microseismic data (Regnier et al. Reference Regnier, Chatelain, Smalley, Chiu, Isacks and Araujo1992) showing a W-dipping alignment of foci (average dip around 30–35°E), especially in the northern part of Pie de Palo (Figs 4a, 13, 14). Two mechanisms can fit with this latter structural model: intercutaneous faults or foliation-parallel slip mechanism, this second mechanism being much more likely. Finally, the Neogene Pie de Palo fold appears to have developed on inherited basement antiformal and synformal geometries.

Figure 14. Crustal-scale cross-section of the Sierra Pie de Palo, the Sierra Valle Fértil and the Precordillera (see Fig. 1b for location). (a) From Vergés et al. (Reference Vergés, Ramos, Meigs, Cristallini, Bettini and Cortés2007); (b) in Siame et al. (Reference Siame, Sebrier, Bellier, Bourlès, Costa, Ahumada, Gardini and Cisneros2015); and (c) this study. In (c), the front of the Precordillera has been modified after Alonso et al. (Reference Alonso, Gallastegui, Rodriguez Fernandez and Garcia-Sansegundo2014). The asterisk near the star along the western crustal E-verging thrust represents an alternative solution for the 1944 focus (from Meigs and Nabelek, Reference Meigs and Nabelek2010). The 1977M earthquake focus have been translated 30 km to the west as suggested in Regnier et al. (Reference Regnier, Chatelain, Smalley, Chiu, Isacks and Araujo1992).

6.a. Crustal implications

We have proposed that the dominant folding mechanism should be foliation-parallel slip and that it induced the formation of small reverse faults, the foliation being not parallel to the basement/cover interface. Such interpretation has also been proposed for other Andean structures (e.g. Alonso et al. Reference Alonso, Rodriguez_Fernandez, Garcia-Sansegundo, Heredia, Farias and Gallastegui2005). Alternatively, these small structures may also be due to intercutaneous wedge processes. However, in both cases the resulting crustal thrust should be E-verging. It is noteworthy that an E-verging blind thrust below the Sierra Pie de Palo is opposite to the main kinematics of the Pampean province.

According to seismological data, we consider here that the E-verging blind thrust below the Sierra Pie de Palo should root at the base or below the upper seismogenic crust, i.e. at ~40 km (Fig. 14c) as already proposed by Siame et al. (Reference Siame, Sebrier, Bellier, Bourlès, Costa, Ahumada, Gardini and Cisneros2015) for a slightly different structural interpretation (Fig. 14b). In our model, the thrust to the SW of the Sierra Pie de Palo (Tulum Fault) is considered as a back thrust branched at depth on the main E-verging thrust. This configuration is in agreement with the seismicity (Figs 13, 14), especially considering that the 1977 earthquake should be located 30 km west (Fig. 14c) of its initial location (Fig. 14a, b) as stated in Regnier et al. (Reference Regnier, Chatelain, Smalley, Chiu, Isacks and Araujo1992). The 20°E dip of the erosional surface (backlimb west of the Tulum W-verging thrust, below the Neogene sediments of the Tulum valley; Figs 2, 13a, 14) thus approximately represents the dip of the E-verging thrust at depth, consistent with the model of fault-propagation fold.

The E-verging thrust ramps off a deep crustal décollement (either at the base of the seismogenic crust or in the lower crust; Fig. 14c) as well as the E-verging basement thrust that activated the Eastern Precordillera (Meigs et al. Reference Meigs, Krugh, Schiffman, Vergés and Ramos2006). This Precordillera is W-verging because of the change of vergence in the décollement at the base of the cover (Fig. 14c). This is consistent with the interpretation of Vergés et al. (Reference Vergés, Ramos, Meigs, Cristallini, Bettini and Cortés2007), Meigs et al. (Reference Meigs, Krugh, Schiffman, Vergés and Ramos2006) and Rockwell et al. (Reference Rockwell, Ragona, Meigs, Owen, Costa and Ahumada2014), if we consider that the 1944 earthquake is due to a displacement at the cover base where the thrust vergence changes (Fig. 14c) at 9–11 km (Alvarado & Beck, Reference Alvarado and Beck2006; Meigs et al. Reference Meigs, Krugh, Schiffman, Vergés and Ramos2006). Alternatively, this 1944 earthquake could have ruptured deeper (~25 km) on the E-verging basement ramp (Meigs & Nabelek, Reference Meigs and Nabelek2010), which is located below the Eastern Precordillera (Smalley et al. Reference Smalley, Pujol, Regnier, Chiu, Chatelain, Isacks, Araujo and Puebla1993) (Fig. 14c).

These two E-verging structures, which are located below the Eastern Precordillera and the Pie de Palo Anticline, stand between the thin-skinned Western and Central Precordillera to the west and the Pampean W-verging Sierra de Valle Fértil to the east. It is noteworthy that the crust below the Sierra Pie de Palo and the Valle Fértil is considered as different (e.g. Ramos, Reference Ramos2004): the crust below Valle Fértil and further east is Pampia, while below Sierra Pie de Palo and the Precordillera it is Cuyania. Thus, there might be a link between these different crusts and the different vergences in these domains: in the Cuyania domain, the thrusts are E-verging at the surface (except in the Eastern Precordillera) while in the Pampia crust the thrusts appear predominantly W-verging.

6.b. Lithospheric structure

In the North American Laramide province of the Rocky Mountains, the presence of large basement uplift (similar to the Sierras Pampeanas; e.g. Jordan & Allmendinger, Reference Jordan and Allmendinger1986) is probably due to subcrustal shear, crustal buckling and faulting above a décollement in the lower crust, or crustal buckling and faulting superimposed on a lithosphere buckling (Erslev, Reference Erslev, Kalstrom and Keller2005; Yonkee & Weil, Reference Yonkee and Weil2015; see a synthesis in Lacombe & Bellahsen, Reference Lacombe and Bellahsen2016). In all cases, the driving force arises from the push of a flat slab transmitting stresses far in the interior of the overriding plate. In the Andean foreland of NW Argentina, both vergences are found in the mountain range and are most likely controlled by inherited structures (Ramos, Cristallini & Pérez, Reference Ramos, Cristallini and Pérez2002).

The E-verging thrusts beneath the Sierra Pie de Palo and the Eastern Precordillera could be considered as a ‘normal kinematics’: they are the eastward thick-skinned propagation at depth of the E-verging thin-skinned system of the Precordillera. As classically considered, the flat slab geometry induced stresses to be propagated further down in the crust and to the east. The W-verging Pampean structures (such as Valle Fértil) are strongly controlled by structural inheritance, i.e. E-dipping inherited Triassic normal faults that are reactivated during the Neogene and Quaternary (Ramos, Cristallini & Pérez, Reference Ramos, Cristallini and Pérez2002).

The Cuyania crust below the Sierra Pie de Palo is very thick (52–60 km), while the thickness of the Pampean crust, further east, decreases eastward from ~50 km to 35 km (Ammirati et al. Reference Ammirati, Alvarado, Perarnau, Saez and Monsalvo2013; Ammirati, Alvarado & Beck, Reference Ammirati, Alvarado and Beck2015). Moreover, these authors interpreted the Cuyania lower crust as eclogitized. As the shortening is rather low in this region (Fig. 13), it cannot only explain the eclogitization; it should rather result from the initial thickness of the Cuyania crust. Thus, it is likely that Cuyania was already quite thick, at least in its eastern part, before the Neogene shortening.

Below the Precordillera, the crust is thicker (~65 km; e.g. Ammirati et al. Reference Ammirati, Alvarado, Perarnau, Saez and Monsalvo2013). Considering the amount of shortening in the Precordillera (135 km), the Cuyania crust must have been significantly underthrusted below the Andean Cordillera, i.e. below the Chilenia upper crust (Cristallini & Ramos, Reference Cristallini and Ramos2000). Thus, it is likely that, for balancing purpose, the Chilenia lower crust was underthrusted below Cuyania (Cristallini & Ramos, Reference Cristallini and Ramos2000) (Fig. 15). This process is coeval with the Tertiary formation of the Principal and Frontal Cordilleras as well as the Western and Central Precordilleras. During this process, the Chilenia lower crust was possibly eclogitized as well as the Cuyania lower crust further east.

Figure 15. Lithospheric-scale section of the Andes modified from Cristallini and Ramos (Reference Cristallini and Ramos2000) (see Fig. 1a for location). The crustal structure at Pie de Palo and below the Precordillera were modified according to the present study. The Moho and lower crustal structure (eclogitized root) are from Ammirati et al. (Reference Ammirati, Alvarado, Perarnau, Saez and Monsalvo2013) and Ammirati, Alvarado & Beck (Reference Ammirati, Alvarado and Beck2015). The Eastern Sierras Pampeanas structure is from Ramos et al. (Reference Ramos, Cristallini and Pérez2002) and Perarnau et al. (Reference Perarnau, Gilbert, Alvarado, Martino and Anderson2012).

When the slab dip decreased, at c. 10 Ma, the stresses were transferred to the Cuyania and Pampia crust. It is noteworthy that it may also correspond to the period when the crust below the Cordillera and Precordillera became too thick and too strong because of its eclogitized root. The shortening propagated through a deep décollement between the seismogenic crust and the eclogitized root, in the Cuyania crust at ~40 km depth, and furthermost to the Sierra de Córdoba region within the Pampia / Río de la Plata crusts at c. 35 km depth.