The Andean Thrust Front Migration and Related Shortening Rates during the Late Pleistocene-Holocene (32° 30′S), Argentina Open Access

Revealing the three-dimensional geometry and evolution of thrust-bounded ranges in frontal deformation zones poses significant challenges in compressional tectonic settings, as these active faults and folds typically involve blind thrusts with limited surface exposures, such as gentle up-bulges and tilted alluvial surfaces [1-4]. Active faulting normally shifts from the mountain-piedmont junction into adjacent basins as footwall splays from the main thrust, and the detailed analysis of these datasets is crucial for understanding the mechanisms through which deformation is accommodated in the mountain orogenic front [2-5] (Figures 1 and 2). Footwall splay migration of younger thrusts has commonly been referred to as a common mechanism in this tectonic setting, although its study is frequently hampered by the youngest thrusts being blind [e.g. 3, 6-10] (Figures 1(c) and 2(c)). Understanding these geometric and kinematic complexities over time directly contributes to the comprehension of mountain-building processes and associated seismogenic sources.

The foreland-facing frontal deformation of the Central Andes makes it challenging to undertake these kinds of studies at a detailed scale, due to geologic and climatic conditions [11, 12]. However, between 30° and 33°S, the morpho-climatic setting of the Precordillera fold and thrust belt (Figures 1(a) and 1(b)) is characterized by arid conditions and favorable stream downcutting, creating an unusual place for the preservation of exposures along the entire Andean chain. This makes it an ideal location to study the characteristics and evolution of the thrust front during the Quaternary [4]. The Southern Precordillera is characterized by the NNW-striking, east-verging LHTS and LPTS, which are dominated by a thick-skinned tectonic style south of 32°30′S [4, 13-20, among others] (Figures 1(b) and 1(c)). The current active orogenic front is characterized by the LPTS, which bounds the eastern edge of the Las Higueras-Las Peñas range (Figures 1(b) and 1(c)). This thrust system overrides deformed Neogene sediments, primarily from the Mariño, Río de Los Pozos, and Mogotes formations, over Quaternary alluvium for more than ~30 km and has concentrated Quaternary deformation along this section of the Andean thrust front [15, 17, 21-23] (Figure 1(c)). The geomorphic signature of the LPTS exhibits overhanging and fold limb scarps, whether it propagates through alluvial deposits or remains as a blind thrust (Figures 2(b) and 2(c)). While its trace is visible and prominent in certain frontal areas, the most recent activity propagates eastward, gradually disappearing beneath the piedmont alluvium along its entire trace [4, 24, 25].

Quaternary shortening rates have been estimated at different specific locations along the Las Peñas thrust front, resulting in contrasting values at different time spans [4, 24] (Figure 1(c)). These results highlight the necessity to refine Quaternary shortening estimation related to these active thrust splays by applying methodologies that align with field observations. Trishear models have been previously developed to reproduce the geometry and kinematics of the structure north of the area here discussed [4, 20, 25]. At La Escondida Creek, the focus of this study, Schmidt et al. [24] estimated shortening rates of 1.2 ± 0.2 and 2 ± 0.4 mm/a for the last 13–16 ka and ~12 ka, respectively, based on vertical fault offsets. In this study, we aim to revisit these observations and recalculate shortening rates by incorporating detailed topographic data and kinematic modeling of propagating thrusts, considering that these calculations should also account for the shortening produced by off-fault folding, which is observable in the field. This research also focuses on documenting the eastward propagation of the LPTS, which results in the incorporation of Quaternary alluvial packages into the hanging wall.

The application of trishear modeling to assess the shortening of active thrusts propagating through Quaternary unconsolidated sediments has not yet been widely used to study the evolution of the mountain front in the Andes or other similar settings. The exceptional preservation of thrust exposures, alluvial surfaces, and their associated stratigraphy in this sector represents an ideal case to apply and test this methodology. The data provided here contribute to discussing slip rate variation through space and time along the LPTS and may enable a better understanding of how the active thrust front is evolving. Moreover, the calculation of slip rate is a crucial parameter for fault source characterization in seismic hazard, and this is even more relevant for the main seismogenic structure of the frontal zone at these latitudes. Therefore, we also aim to populate with new data on the seismogenic parameters of this relevant crustal fault source [12].

The Pampean Flat Slab (27°–33°S) of the Central Andes (Figure 1(a)) concentrates most of the Quaternary deformation located at the easternmost edge of the Precordillera thrust front, where intense shallow crustal seismicity at its southern end defines an area of very high seismic hazard [22, 26-32]. The active orogenic front is characterized by the Precordillera fold and thrust belt, which exhibits different neotectonic styles along its N-S trend [13, 32-35, among others].

North of 32°10′S, the Precordillera fold and thrust belt is characterized by thrusting with opposite vergences giving rise to a triangular zone with two active thrust fronts [16, 17, 36] (Figure 1(b)). South of 32°10′S, east-verging NNW-trending thrusts characterize the Southern Precordillera thrust front [4, 17, 23, 31]. The geometric configuration of this tectonic domain is mainly attributed to the result of the Neogene inversion of Triassic normal faults that formed half-graben structures in the Cuyana rift basin [18, 19, 37-40].

At the Las Higueras-Las Peñas range, both the LHTS and LPTS have been defined as thrust systems due to the recognition of branches associated with the main structure and their geometric complexities [23] and are responsible for active mountain building [22, 23, 41] (Figure 1(b)). The LHTS extends for 90 km, following an NNW-SSE orientation, thrusting Paleozoic and Mesozoic sequences over Cenozoic sedimentary layers in most of its extent, although Mesozoic sequences are also found in the footwall further south [42] (Figure 1(c)). Its trace is recognizable through a slope break along the mountain range [20]. The LHTS has been divided into different sections, where Quaternary activity has been documented at its ends [17, 20, 24, 43]. To the east, the LPTS emplaces Neogene clastic sequences (Mariño, Río de Los Pozos, and Mogotes formations) over Quaternary units [4, 15, 17, 24, 41, 42, 44] (Figures 1(b) and 1(c)). The NW-SE trending of LPTS crops out over 30 km, accommodating a significant part of Quaternary shortening. Its younger splays propagate into the piedmont alluvial plain, deforming Quaternary alluvial levels. These structures are described in the northern creeks of the study area, where eastward-propagating blind structures are inferred from various landform features, including scarps related to folds with different preservation stages [3, 17, 23-25]. Toward its southern end, the LPTS becomes blind, associated with the south-plunging Baños Colorados anticline [15, 17, 45] (Figure 2(a)).

Shortening and dip slip rates have been estimated at different places of the LPTS trace, using dissimilar methodologies (Figure 2(a)). At the Las Peñas Creek, Costa et al. [4] reported shortening rates of 0.27 + 0.11 mm/a across two strands for the last 200 ka through trishear models, using an alluvial surface as a deformation marker [4] (number 1 in Figure 2(a)). Schmidt et al. [24] estimated shortening rates at the La Escondida and Baños Colorados creeks. These results have been built on scarp profiles, considering the pure fragile displacement of alluvial surfaces without incorporating the hanging wall deformation. Ages constrained through cosmogenic nuclide and radiocarbon dating led to shortening rates varying from 1.2 ± 0.2 to 2 ± 0.4 mm/a for the last 13–16 ka and ~12 ka, respectively [24] (numbers 2 and 3 in Figure 2(a)). Hence, there is an order of magnitude difference in shortening rates between the Las Peñas Creek and the La Escondida and Baños Colorados creeks located at a distance of ~5 km among them (Figure 2).

Spatial variation in shortening rates for the last ~3.5 Ma has already been recognized at different space and time scales in the Southern Precordillera, recording an increase in shortening rates southward of east-verging thrusts [42]. A shortening rate of 1.60 mm/a has been estimated through forward kinematic modeling of balanced cross-sections for the last 5.8 Ma between this study area and the Las Peñas Creek latitude. Further south, at the Jocolí Ridges latitude, the derived shortening rate was 1.83 mm/a for the same period [42]. These variations in shortening rates could also be attributed to spatial diverseness in recent times due to internal kinematics heterogeneities, propagation to the east of more recent geological structures, and/or the interaction with other regional faults, combined with a possible temporal variation suggesting an acceleration of activity during the Holocene [4].

For achieving a detailed neotectonic map, we combined diverse sources of satellite images (Bing, Landsat 8 and 9, among others), with scales ranging from 1:50000 to 1:1000, and digital elevation models (Copernicus 30 m and ALOS PALSAR 12.5 m), along with field evidence. Utilizing a surface parameters approach, we attempted to discriminate the main morpho-stratigraphic units characterizing the piedmont landforms assemblage, relevant to analyze Quaternary deformation at the surface. These parameters are sensitive to the exposure time of the alluvial surface to weathering and focused on the characteristics and variation of desert varnish, the thickness of ine-grained sediments, granulometry, microrelief of limestone clasts, roundness and parting and can provide a rough semi-quantitative approach to the relative ages of the alluvial surfaces [46-50]. This resulted in the characterization of the main morpho-stratigraphic units and enabled a first-hand attempt to correlate them in the study area, derived from slightly different alluvial surfaces and their deposits.

We generated a high-resolution DEM (0.2 m) using a UAV (drone DJI Mavic Pro) and controlled by detailed topographic profiles surveyed with a DGNSS (Spectra Precision 60 differential global navigation satellite system), in post-processing kinematic mode (see online supplementary material 1). High-resolution digital elevation models were generated in two different areas at La Escondida Creek. These areas were selected due to the presence of clear and prominent deformation features, such as surface warping, topographic breaks, and anomalous linear features affecting Quaternary alluvial deposits, as well as the availability of cosmogenic ages of alluvial surfaces [51] (Figure 3).

We chose key Quaternary stratigraphic layers related to an exposed thrust splay and Quaternary alluvial surfaces as deformation markers (Figure 3(a)). To estimate the shortening related to thrust splays propagating into the Quaternary alluvial cover, we have conducted trishear models based on these markers, using FaultFold 7 software (https://www.rickallmendinger.net/faultfold). Trishear models have been widely applied in various locations where thrust faults accommodate deformation, combining both bedrock and neotectonic data [e.g. 52-54, among others]. Furthermore, fault propagation folds within Quaternary deposits are consistent with the trishear model and are well-suited for their description [e.g. 4, 20, 55]. Field observations in the study area as well as in other thrusts at the Southern Precordillera indicate that Quaternary layers involved in thrust propagation do not behave as isopach-parallel folds at a decametric scale [4, 20, 23, 25, 43, 45]. Thus, constraining the deformation related to these propagating thrusts solely based on the scarp contour can account for the uplift produced by the fault slip, but may result in discrepancies regarding shortening and total slip estimation related to the structure. Besides, the reconstructed markers of the propagating thrusts in alluvium show a general up-dip loss of displacement, which along with the fold’s shape suggests that the exposed deformation can be modeled as a fault propagation fold. The trishear model [56-58] was used to link the observed folding geometry with kinematically balanced sections through a forward-modeling procedure.

A propagating thrust fault loses slip and terminates upsection by transferring its shortening to a pair of asymmetric folds that develop at the fault’s tip [59, 60]. In the trishear model [56], folds develop progressively within a triangular zone of distributed deformation that expands away from the fault tip. As the tip point propagates, fault propagation folds form contemporaneously [61]. This model successfully reproduces observations found in nature that are incompatible with “kink-band” migration models [59, 62] (Figure 3(b)).

We estimated shortening values on deformation markers with known ages [51] (Figure 3), comparing shortening derived from deformed alluvial layers at the thrust propagation zone and from the thrust-related scarp above. For shortening estimations in the thrust outcrop, an orthophoto mosaic was surveyed through the LiDAR sensor of an iPhone 14 Pro Max and assembled with the 3D Scanner App (https://3dscannerapp.com/) [63], allowing a detailed description of the rectified exposure of a thrust splay (see for example, [64-67]). The 3D Scanner App utilizes the iPhone’s LiDAR sensor, which emits laser beams that bounce off objects and measures the time it takes for the reflected light to return. By employing the principle of triangulation, the scan calculates precise distances. Combined with the iPhone’s position and orientation, it generates a point cloud that recreates the shape, texture, and color of the physical object, constructing a 3D model [63]. For shortening estimations using scarp morphologies, we considered a range of maximum and minimum possible shortening scenarios using different combinations of the parameters, without counting the erosion effect, due to the uncertainties of the forward modeling [57, 68] (Figure 3(c)). We also constrained the very minimum shortening by restoring the line length resulting from profiling the current topography using the open-source software Python (version 3.9.7).

The trace of topographic profiles selected whenever possible preserved the original landscape of the alluvial surfaces to reconstruct the enveloping surface of these deformation markers. The selected cross-sections for shortening estimation resulted from projecting those points to a straight line orthogonal to the thrust traces (Figure 3(a)). The deformation markers chosen for modeling combined a well-preserved original morphology and available dating [51] for shortening estimations. These markers were displayed in the background of the computer screen. Afterward, we considered the fault dip observed in the field and estimated trishear parameters (X and Y positions, total slip, P/S ratio, and the trishear angle), contrasted with the geometric markers through a trial-and-error procedure [57]. Because the P/S ratio is not well known for unconsolidated materials, we considered a wide range between three to seven (see for example [52]). Then, the initial position of the fault tip and the total slip amount were adjusted until the visual best fit with the selected marker was achieved (Figure 3(c)). To envisage the maximum possible shortening accomplished by the modeled thrust, its tip point was located at the intersection between the envisioned projection of the fault plane and the projected maximum height of the hanging wall (Figure 3(c)). The minimum shortening scenario considered a folded geometry similar to the scarp profile and a tip point propagated near the current topography or below (Figure 3(c)).

The alluvial deposits of the study area are part of the coalescent fans of an extensive piedmont alluvial plain, whose complex formation results from the dynamic interplay between endogenous and exogenous processes. At least six Quaternary surfaces of alluvial terraces were recognized at La Escondida Creek (Figure 4). The alluvial surface Q6 has been identified as the oldest morpho-stratigraphic unit, whereas the Q0 surface corresponds to the current streambed. These levels are summarized in Table 1.

The Q6 units correspond to strath and cut-in-fill terraces that bevel the bedrock and are mainly developed on the hanging wall of the LPTS. This unit unconformably overlies Neogene deposits (Figure 4(b)), and its preservation is restricted to the vicinity of the main range slope break (Figure 4 and online supplementary material 2). Q5 and Q4 terrace levels unconformably overlie Neogene deposits (see online supplementary material 2). However, differences in the characteristics of each alluvial surface (Table 1), alongside with differences in the surface deformation coined by thrust propagation, suggest their diachronous origin. The Q4 alluvial surface corresponds to an unpaired strath and cut-in-fill terrace found on the northern margin of La Escondida Creek. The fabric of this unit is characterized by growth strata, recognized by greater bed inclination values at the base that gradually decrease toward the top. The Q3 unit constitutes the youngest terrace deposits affected by thrust splays and associated folding lying on a buried, older, and highly deformed fill terrace (Figure 4 and online supplementary material 2), and the Q2 and Q1 alluvial surfaces represent the younger fill.

The spatial array of Quaternary-active structures at La Escondida Creek allows the discrimination of four morphotectonic domains. These domains are defined as areal units with relatively uniform morphological habits, bounded by west-dipping reverse faults associated with active topography building. From west to east, the first domain (D1) represents the eastern slope of the mountain range, while the other domains are developed in the piedmont area (Figure 4).

The first morphotectonic domain is bounded to the east by the main damage zone conceived to represent the main structure of the LPTS. It is characterized by numerous shear planes of reverse nature, exposing folded Neogene rocks in the hanging wall over undifferentiated sheared Neogene deposits (Figures 2(b), 4(a), and 5(a)). The LPTS has been active since the upper Pliocene, deforming the Mogotes Formation and other Neogene deposits (<3.5 Ma) [41, 42]. Its trace is challenging to map at a detailed scale in the study area due to the intense deformation observed in the Neogene units (Figures 4 and 5). Here, the main trace coincides with the mountain-piedmont junction, and it can be continued to the north since a large fault zone with an intensely sheared zone of approximately 1 m wide was recognized, placing in contact two different Neogene units (Figures 4 and 5(a)).

Domain 2 (D2) is characterized by the remains of old Quaternary alluvium overlying a degraded planation surface in the Neogene unit, showcasing a flattened relief. This domain is bounded to the east by a thrust that overrides Neogene strata over an undifferentiated Quaternary unit, and it is covered by the older Quaternary unit Q6 (Figure 4(b)). Its trace is characterized by a degraded scarp (Figures 5(b) and 5(c)). This fault zone exhibits several imbricated meter-scale thrusts (dipping between 12° and 36° to the west) (Figure 5(c)). The fault’s trace delineates the eastern margin of the Neogene deposits exposed.

Domain 3 (D3) corresponds to an area now incorporated into the LPTS hanging wall as deformation continues to propagate eastward (see Figures 2 and 6). Within this domain, a well-preserved splay (181°/32°) exposes the thrust propagation over Quaternary alluvial layers, controlled by the Neogene strata fabric, showcasing clasts that coincide with the fault’s tip point (see online supplementary material 3).

The easternmost exposed thrust has been recognized in the La Escondida Creek streambed, bounding the eastern limit of the D3 morphotectonic domain and interpreted as the currently active emergent edge of the LPTS (Figures 4 and 6(a); see next section for further information). This thrust might correspond to the easternmost thrust splay already incorporated into the LPTS hanging wall. This interpretation is supported by beveled Neogene bedrock exposures capped by strath terraces along the La Escondida Creek. Several normal antithetic faults affect levels Q5, Q3, and the Neogene strata beneath within domain D3, accommodating the folding of alluvial layers (Figure 4 and online supplementary material 3). The same thrust trace disrupts the Q4 unit until its trace vanishes at the surface to the north (Figures 4 and 6(a)). To the south of the creek, the thrust trace is barely guessed through subtle scarps observed in the high-resolution DEM, where the main trace diverges into three parallel branches related to fold limb scarps, affecting younger and older alluvial surfaces (Q2, Q5; Figures 4 and 6(b)).

The Neogene bedrock does not crop out across the La Escondida river section in morphotectonic domain D4, and secondary metric-scale antithetic thrusts and fold-related scarps accommodate internal deformation in Units Q3, Q2, and Q1 (see online supplementary material 2 and 3). Blind thrust propagation within the piedmont area has been interpreted by previous contributions [4, 42]. Further to the east of the study area, this possibility is also suggested by abrupt changes in stream downcutting, broom-shaped river pattern, sinuosity increase, deflections, convergences, abandoned meanders, and beheaded channels. These drainage anomalies might be related to the Lomas de Jocolí blind thrust [42] (Figure 6(c)).

The natural exposure of a propagating thrust, bounding the eastern margin of domain D3 and disrupting Q3 and Q4 alluvial surfaces has allowed us to link the deformation geometries of alluvial layers with the truncated alluvial surfaces right above. Trishear models of the propagating folds and line length restoration in Q3 key stratigraphic layers, along with forward modeling of the alluvial surface, were applied to constrain cumulated shortening. Alluvial surface Q4 was also chosen for shortening estimation using trishear modeling, providing that this thrust splay has propagated in both alluvial surfaces (Figure 4).

At the northern margin of La Escondida Creek, a natural thrust outcrop is exposed by river downcutting, bounding D3-D4 domains (Figure 7).

The hanging wall of the thrust exposes a lower east-tilted lower alluvial sequence, overprinted by distributed faulting. An eastward-dipping progressive unconformity is highlighted by fine-grained layers, separating it from the upper alluvial sequence (Figure 7(c)). Both alluvial packages exhibit a contractional growth stratal shape. The lower alluvial sequence is only exposed in the hanging wall, whereas the upper alluvial sequence can be recognized in both hanging wall and footwall fault blocks (Figure 7(c)). The thrust zone exposure reveals two main semi-parallel fault strands, which have faulted and folded the sequences described above. These splays record multiple deformation events, as indicated by up-dip loss of stratigraphic layers (Figures 7 and 8).

A photolog-assisted interpretation attempted to identify geometric markers correlatable through both thrust walls (Figure 8(a)). Although key stratigraphic layers are challenging to trace, the vertical slip component of the upper alluvial layers was constrained by comparing it against the near-field scarp amplitude tight above (Figure 8). Accordingly, we estimated possible values of maximum and minimum shortening on each selected key stratigraphic layer (green and red), based on trishear models and line length retrodeformation of these layers (Figure 8, Tables 2 and 3).

A trishear model has been done for this thrust scarp, with a near-field amplitude of 5.6 m, aiming to compare with the results derived from the folded alluvial key stratigraphic layers, as mentioned in the previous section. Trishear parameters are displayed in Figure 9. The ramp angle (28°) resulted from the averaging measurements conducted on several fault surfaces exposed in the logged thrust outcrop (Figure 7(c)).

The chosen models were those matching the best fit with the scarp silhouette at both thrust walls. Ramp angle, trishear angle, and P/S ratio control the resulting geometry of the selected geometric marker, whereas X and Y location, total slip, and initial height of the undeformed bed impact the tip point location. The selected trishear models (Figure 9) show minimum and maximum values of 2.90 and 5.06 m of shortening, respectively.

The thrust affecting the Q4 alluvial surface was modeled through trishear folding to analyze the deformation of the same thrust addressed in the previous section. The easternmost structure affecting the Q4 unit exhibits a monocline-like scarp with a far-field scarp of 17 meters. For modeling trishear folding compatible with the scarp amplitude, we considered the near-field scarp (8.1 meters; Figure 10) and the tip point above the surface considering the same morphogenesis of the Q3 scarp, supporting the continuity of the structure to the north (Figure 4). The lack of an outcropping fault plane forces us to run trishear models assuming different envisioned scenarios as to the fault tip position, varying from considering the tip near to and above the topographic surface. We also assumed that the surface deformation driven by thrust slip resulted in an overhanging collapsed scarp. Therefore, we used the scarp amplitude to constrain possible fold geometries, but not the current scarp profile providing that the fold frontal limb has been eroded. Similar approaches have been undertaken at LPTS scarps further to the north [4, 25]. The selected trishear models corresponded to those that best fit the alluvial surface marker on both thrust walls. We used the same known ramp angle as in Q3 (Figure 10(b)) resulting in shortening values ranging from 8.56 to 10.33 m. The age data considered for rate estimation correspond to an adjacent same Q4 surface (Figure 4 and online supplementary material 2).

The spatial array of morphotectonic domains recognized documents the shifting of the thrust front toward the piedmont. The main thrust zone in bedrock divides domains with distinct morphological habits (D1/D2 boundary). Toward the east, a barely exposed beveled bedrock in the proximal piedmont is bounded by a thrust splay with degraded morphology, indicating its incorporation into the LPTS hanging wall (D2/D3 boundary). This interpretation is supported by the recognition of east-directed propagating splays in Q3, Q4, and Q5 alluvial surfaces (D3/D4 boundary; Figures 4-7). Furthermore, the active propagation of the active front further east can also be inferred from drainage anomalies, along with other features such as breaks and warping in the topography of the alluvial surfaces (Figure 6(c)). This is also supported by regional balanced sections that suggest the presence of a blind structure to the east [42] (Figure 1(c)).

Shortening rate estimations of the stratigraphic layers of the thrust outcrop related to the Q3 unit showcase slighter lower values of trishear models as to line length retrodeformation (Unbalanced: 0.48 to 1.81 mm/a vs. trishear: 0.50 to 1.13 mm/a, Table 3). This might be related to modeling assumptions [57, 62] or the synchronicity between sedimentation and deformation, making it challenging to generate a reliable model. Shortening rates derived from the thrust-related scarp coined in unit Q3, using the homonymous alluvial surface as a geometric marker (available age 3.3 ± 1.3 ka) resulted in values ranging from 0.63 to 2.53 mm/a. These scattered values and errors are mostly attributed to analytical uncertainties in age determination and alluvial surface reconstruction, although they overlap with the shortening rates derived from the Q3 trench stratigraphy (0.5 to 3.11 mm/a; Figures 8, 9, and 11 and Table 3).

In summary, assessing shortening rates is very challenging for several reasons. First, the recurrence interval of the studied fault remains unknown, although analytical and empirical criteria suggest it likely spans several thousand years [69, 70]. Hence, it is unclear whether the time window provided by the available age datums encompasses at least one complete seismic cycle of the most recent event. This uncertainty is particularly critical for the younger age datum (3.3 ± 1.3 ka), as the resulting rates may represent open or apparent rates, which could overestimate the actual shortening rates [1]. Additionally, the large uncertainties associated with the younger age datum and the different scenarios considered for shortening calculations result in a wide range of possible rates. Therefore, although we are confident that the shortening rates here estimated bracket possible minima and maxima values, it is difficult to assess a preferred value. However, based on the results obtained using the Q4 alluvial surface as a geometric marker, we favor an active LPTS splay shortening rate of <1 mm/a, and likely ~ 0.5–1.0 mm/a. Although the fault scarp in this surface is less preserved, the uncertainties associated with age and slip determinations are lower for this marker. Furthermore, the results overlap with the lower rates derived from modeling other markers (<1 mm/a). The age range determined for the Q4 alluvial surface (12.6 ± 0.2 ka) is more likely to encompass at least one complete seismic cycle. Moreover, the lack of a clear imprint on Q2 and younger alluvial surfaces suggests that the lower rate results are more consistent with the kinematic conditions for this thrust splay.

The higher shortening rates range here assessed might fall within the same order of magnitude as those calculated by Schmidt et al. [24], but are near an order of magnitude larger than those of Las Peñas Creek [4] for the last 200 ka. Schmidt et al. [24] calculated a shortening rate in the alluvial unit here identified as Q4, using scarp amplitude and the measured ramp angle. However, this result should be regarded as very minimum, because the scarp amplitude considered by these authors involved diachronous alluvial units in the hanging wall and footwall (Q4 and Q3 units respectively, as described here). These considerable discrepancies in outcomes emphasize the challenges involved for reliable outcomes. Still, we estimated shortening rates for Q4 and Q3 units separately but driven by the same thrust for better estimations (Figures 4 and 9-11, and Table 3), resulting in values discussed above. Spatial rate variation along the LPTS is difficult to address because rates were not analyzed for the same time span in the different creeks. Nevertheless, the local thrust framework at the Las Peñas Creek where Costa et al. [4] estimated shortening rates of 0.27 mm/a could be similar to the D2-D3 domain boundary thrust limit in the La Escondida Creek, as both override Neogene strata over Quaternary sequences (Figure 11). Therefore, it might be possible that the active thrust bounding D3 and D4 domains, described at La Escondida Creek, is not exposed or yet developed at the Las Peñas Creek, suggesting that the active edge thrusting lies in the piedmont domain (D3-D4 domains).

An increase in shortening rates toward the south has been suggested by Richard et al. [42] for a larger timescale (5.8 Ma), also involving the nearby Salagasta and Lomas de Jocolí thrusts (Figure 1(b)). Nevertheless, the different space and time scales and the punctual nature of data preclude a rigorous comparative analysis, leaving open the uncertainty of whether there exists a spatial variation in the southern sector of LPTS during the Late Pleistocene.

The LPTS shows clear evidence of propagation toward the piedmont of the Southern Precordillera, allowing the detailed study of splays and fault scarps.

We recognized clear evidence of four morphotectonic domains resulting from the eastward shifting of active thrusting. Domain D1 corresponds to the range slope and is limited to the east by the principal thrust zone at the main slope break, which is currently inactive. Domain D2 is cored by beveled Neogene deposits overlaid by a thin cap of the oldest Quaternary alluvium preserved. An emergent thrust overriding Neogene strata over younger Quaternary alluvium marks the eastern boundary of this domain, characterized by an evolved hanging wall collapsed scarp. This feature represents the southernmost expression of Neogene sedimentary rocks overriding Quaternary deposits, which until now was only recognized to the north (Las Peñas, El Infiernillo, El Cóndor, and Las Mañeras creeks) [4, 25]. Domain D3 is bounded to the east by a thrust splay propagating into Holocene alluvial layers here labeled as Q4 and Q3 units (12.6 ± 0.2, 3.3 ± 1.3 ka). Domain D4 is associated with blind thrust splay propagation and affects only Quaternary units at the surface.

The shortening rate estimation of the D3-bounding thrust yielded scattered results. Our best assessment favors considering shortening rates < 1 mm/a for this frontal thrust, likely well-depicted by the results obtained using trishear models of the Q4 alluvial surface as a deformation marker (0.80 ± 0.14 mm/a).

Our field data strongly support a shifting in thrust activity toward the east, conveying suspicions that another active blind may lie below the piedmont surface. Regardless of the kinematic complexities in space and time that thrust systems pose, it is possible that shortening rate estimation at different (and less active) LPTS domain boundaries at the Las Peñas Creek [4], as to the data here discussed, may explain shortening rates variability.

The findings presented in this study are documented in both the main article and the Supplementary Material. Further inquiries may be directed to the corresponding author.

The authors declare that there is no conflict of interest regarding the publication of this paper.

The financial supports were provided by the Universidad Nacional de San Luis CyT project [03-0323]. PICT 2019-00974 and Ubacyt 20020190100234BA directed by Andres Folguera. Strategy II Project of National Council for Scientific and Technical Research (CONICET).

The original version of this manuscript benefited significantly from a thoughtful review by Dr. F. Martínez. We acknowledge LAS tools for providing the license and thank Bodo Bookhagen and Ananya Pandey for their support in generating high-resolution elevation models. The authors thank the National Council for Scientific and Technical Research (CONICET) for their further support. This is the contribution number R-505 of IDEAN.

This article includes figures as supplementary material, as detailed below. Further information is provided in each figure caption.

Supplementary Material 1: Satellite image of the study area showcasing the topographic surveys conducted using a UAV flown at an average altitude of 100 m, and a DGNSS, which was also used to verify and properly calibrate the data obtained with the drone.

Supplementary Material 2: Topographic profiles of the Q3, Q4, Q5, and Q6 alluvial surfaces. The portion of each profile used for forward modeling is marked with a dashed violet square.

Supplementary Material 3: Normal and antithetic faults, highlighting the associated folding affecting the Quaternary deposits and the underlying Neogene bedrock.

Supplementary Material 4: Raw image of the trench analyzed and interpreted.