In August 2017, a temporary seismic network, composed of thirteen seismic stations, the Lerma Valley Ring Installation of Seismometers LEVARIS, was installed in the studied area. The network spanned the central and northern regions of the valley (-24.558,-25.272;-65.623,-65.318) and operated for a total of thirteen months. Prior to this deployment, there was only one permanent short-period station within the valley, managed by Argentine agency INPRES code SLA,.The dimensions of our temporary network spanned approximately 80 km in the north–south direction and 30 km in the east–west direction, with seismic stations strategically located to ensure safety, accessibility, and minimal interference from anthropogenic noise sources. The seismic stations were equipped with a DATA-Cube3 type digitizer paired with a Lennartz 3D/5s sensor. One of the installations (2Q.09A in Fig. ) used a Mark L-4C-3D short-term seismic sensor. In all cases, instruments were buried at an approximate depth of 60 cm. The Data-Cube3 digitizers were set to a sampling rate of 100 Hz, and the seismic stations were powered by batteries connected to solar panels.

In order to study the various discontinuities of the crust below the grater Lerma Valley and to derive local velocity models, we employed three methods: receiver functions analysis (teleseismic and local), ambient noise cross-correlation tomography, and joint inversions (forward modeling). The first two methods involved processing the raw data from the LEVARIS network see Sect. , to produce receiver functions and dispersion curves. Then latter results were combined to be inverted using forward modeling and fitting the data to the S-wave velocity model, thus obtaining a representative crustal model. In the following sections we present and briefly describe each method and also provide a complete description of the parameters used in the data processing.

The solution obtained from the H-k analysis showed to be stable and constrained in depth for both teleseismic and local receiver functions station stacks, which resulted in good results, for the deepest discontinuities at ∼53-43 and ∼36-30km. However, the k values fluctuated considerably for the local receiver functions for the layers above the shallower ∼10-8 and ∼1.5-1.2km discontinuities. In Fig.  we present the results for the Moho defined from the teleseismic receiver functions for stations 01A and 05A. These two stations were selected because they show representative results, however the results for the other seismic stations are available in pickle format.

We see in Fig.  that the depth of the Moho discontinuity and the k parameters are well constrained. The vp/vs ratio is the one expected for the Moho's region, being ∼1.75. For receiver functions calculated from local events the estimated Moho depths for the seismic stations shown in Fig.  are similar but the average vp/vs ratios are slightly lower (see Table ).

Figure  shows the receiver functions for station 01A for the radial Q and transversal T components, marking the Moho conversion times for Ps on the Q component at about 5 seconds. The traces were then stacked using a binning of 15° in backazimuth with an overlap of 5°. In the T components there is a clear azimuthal conversion near 200° at 2.5 s, evidently more present in the local RFs.

Figure  shows the acausal and causal parts of the ambient noise cross-correlation traces in terms of the inter-station distance, where there is a clear one-sided tendency towards positive times. This, in principle will appear to be a contraposition to the homogeneity assumption of the ambient noise wavefield on which ANT is based. However, showed that even in this scenario of a dominant noise direction the cross-correlations are not significantly affected (less than 10 %). On the other hand, this also points to clear difference between the northern and southern sectors.

A combination of phase and group velocities of Rayleigh waves was obtained from the cross-correlations. The maps, computed for periods of 10 s, showed similar characteristics within the above time span (see Fig. ). However, the quality of the phase velocities proved to be more consistent for shorter periods, particularly between 1.6 and 2.2 s. As a group, the velocities in all cases have an unstable (sharp oscillatory) behavior in the processed periods.

For the period of 2 s (see Fig. a and b), a weak zone of relatively slow velocities appears between the area enclosed by the stations 09A-07A-08A and 12A-06A-10A. This zone is only visible in the phase velocity maps. On the other hand, the group velocity shows a zone of relatively high velocity in the line formed by stations 12A-11A-10A and a zone of relatively low velocity in the area bounded by stations 06A-05A-07A-08A-09A. For the period of 3 s (see Fig. c and d), two distinct zones appear in both group and phase velocities: A zone of high relative velocity in the area bounded by stations 01A-02A-04A-05A-06A-03B, which will be called the northern sector, and a zone of relative low velocity between stations 11A-07A-08A-10A, which will be called the southern sector. The maps for the period of 4 s (see Fig. e and f), show for the high velocity zone an increase in contrast and extension in the group velocity map, and a decrease in extension and contrast of the low and high velocity zones in the phase velocity map. Finally, for the period of 5 s (see Fig. g and h), the two zones of high and low velocity, show a decrease in the velocity contrast.

We observe that the best model derived from the joint inversion of RFs and dispersion curves reproduces the model proposed by with the main difference that velocity of the shallowest layer are decreased to lower values and the discontinuities at 35 and 46 km change slightly to increased depths.

In Fig. , we present the inversion results for stations 01A, 05A, 07A and 10A. All four stations share similar depth and velocity characteristics, though subtle differences emerge. Notably, the upper layers in the northern region (station 01A) exhibit slightly higher S-wave velocities compared to those in the south, while the lower layers show consistently lower velocities compared to the reference model across all four stations without significant variation.

The receiver function fits are reliable for the selected stations, with station 05A displaying the best fit. At this station, the model closely follows the observed data, capturing not only the shape but also the amplitude of all maxima and minima.

Moreover, the results from the evolutionary algorithm inversion (see Fig. ) align well with those from the joint inversion in the middle of the upper layers, between 3 and 5 km depth. A primary distinction is the presence of a low-velocity layer with shear wave velocities between 0.7 kms-1 and 1 kms-1, that varies in thickness, from 0.7–1 km, being thicker for station 10A (see Fig. ).

The combined velocity model for all stations comprises five distinct layers: an upper sediment layer (0.8 km thick), below it a consolidated sediment layer (3.7 km thick), followed by a lower consolidated sediment layer (2 km thick), then an upper crustal layer (32 km thick), and finally a lower crustal layer (10 km thick). The Moho is located at a depth of 48–49 km.

The results presented in the previous section (see Sect. ) highlight the complexity of the crustal structure in the Lerma Valley on multiple levels. In this trend, the discontinuities identified through the H-k analysis of the receiver functions (both teleseismic and local) align well with previously proposed regional crustal models, e.g. by . Specifically, regarding the depth of the Moho, all stations showed constrained and stable solutions at 48±5km, a feature that aligns closely with the findings of , who positioned the Moho depth at 46 km in the Santa Bárbara system. Similarly, the corrected vp/vs ratios remained in the range of 1.5–1.8, with a mean value of 1.65. This value also agrees with the results of , who attributed this ratio to a dry felsic composition in the lower crust. However, this low vp/vs ratio is stable also for the upper discontinuities in the teleseismic receiver function results, as teleseismic signals, due to their long-period frequency, are less sensitive to minor changes in layer velocities. In contrast, local receiver functions reveal a gradual increase in the vp/vs ratio from 1.64–2.0, spanning from the Moho upward. This behavior is also present in , where a vp/vs ratio of about 2 is measured for the second layer.

In addition to the dry felsic layer identified above the Moho region, expressed by the low vp/vs ratio of about 1.65, there is a shift in azimuth in the T components of the receiver functions indicates a dip along the north–south axis, centered around 200°. As shown in Fig.  for both local and teleseismic data, this feature suggests a gradual change in the Moho surface. Similar observations have been reported in New Zealand , where azimuthal analysis of receiver functions revealed Moho dips associated with variations in the geometry of the subducting plate.

Moving to the middle crust discontinuities, the one at an average depth of 30 km appears well defined in teleseismic receiver functions but more dispersed in local receiver functions, likely represents the mid-lower crustal boundary. This finding is consistent with the velocity model of . Further, a discontinuity at 15 km depth, exclusive to local receiver functions, likely marks an upper detachment horizon with an extensive fracture network occupying the middle crust. Notably, similar features have been proposed on different scales by , , and . This distinction by the local receiver functions is due to the high-frequency content in the spectra of Zapla cluster events .

Within the upper crust, a discontinuity at approximately 8 km depth is clearly imaged in the teleseismic receiver functions and at one station in the local dataset, whereas shallower interfaces between 5 and 1 km depth are consistently observed in both RF types. The deeper of these features likely reflects a first-order rheological contrast within the upper crust, which is preferentially resolved by long-period signals and may correspond to the transition from consolidated basement to overlying sedimentary sequences. Comparable depths for the sediment–basement transition (6–10 km) have been reported in seismic reflection and refraction studies across the Eastern Cordillera and adjacent foreland basins .

In contrast, the shallower discontinuities are interpreted as marking the structural basement of the basin, represented by the Puncoviscana Formation (see Sect. ), overlain by the Santa Victoria and Mesón Groups, the Salta Group, and younger Orán Group and Quaternary deposits. Stratigraphic and geophysical constraints indicate that the cumulative thickness of these sedimentary units commonly ranges between 3 and 7 km in the Lerma Valley and surrounding regions .

Independent constraints from ambient-noise cross-correlation tomography reveal two seismic velocity domains in the uppermost crust, characterized by a low-velocity zone (∼1.75kms-1) and an underlying high-velocity zone (∼3.5kms-1). Similar velocity contrasts and thicknesses (1–4 km for low-velocity basin fill overlying higher-velocity sedimentary or metasedimentary units) have been documented in surface-wave and refraction studies in the Andean foreland and Eastern Cordillera . While a direct lithological attribution of these velocity contrasts remains non-unique, their depth range and magnitude are consistent with reported differences between poorly consolidated Quaternary sediments and the more competent, quartz-rich units of the Santa Victoria Group . We therefore interpret these zones as reflecting, at least in part, the transition from low-density basin fill to a mechanically stronger, higher-velocity sedimentary basement, acknowledging that alternative compositional and structural controls may also contribute to the observed seismic response.

This feature directly corresponds to the differences observed between the northern and southern basin, as noted by . The northern section lacks outcrops of the Salta Group, which dominate in the southern division controlled by the COT lineament (see Sect.  and Fig. ).

The Common Conversion Point plots (Fig. ) provide compelling evidence for the presence of multiple seismic discontinuities, consistent with those listed in Table  and shown in Fig. . These features appear as continuous zones across all profiles, supporting the interpretation of laterally coherent crustal structures. In the north–south (CCP) profile A-A′, three prominent interfaces–located at approximately 47 km (the Moho), 30 and 10 km depth–are identified in both local and teleseismic receiver functions. The improved sharpness of these features in the local RFs highlights their higher resolution and sensitivity to fine-scale crustal layering, consistent with previous findings on the advantages of local RFs .

Directly related to the discontinuities previously discussed, detachment zones, such as the one present at about 15 km depth, play a key role in accommodating crustal shortening and deformation in orogenic systems, particularly within the Andean orogen. In the Eastern Cordillera, these zones are commonly associated with mid-crustal decoupling, where strain is partitioned between upper and lower crustal levels, often facilitated by the presence of weak, extremely fractured layers . Such detachment structures have been invoked to explain the style and distribution of deformation in the Eastern Andes, where thick-skinned tectonics transitions to more complex, distributed strain at depth .

Within this regional tectonic framework, our receiver function (RF) analysis reveals a distinct detachment horizon at approximately 15 km depth, observed exclusively in the local RFs. This feature likely reflects mid-crustal shearing or the presence of an extremely fractured zone, processes commonly associated with deformation and metamorphism in active orogens . The localized expression of this horizon is consistent with interpretations of widespread mechanical decoupling and intra-crustal strain partitioning documented in other Andean foreland systems .

In contrast, the Moho, evident in both local and teleseismic profiles, exhibits a clear southward-deepening trend, reaching depths greater than 50 km. This pattern may indicate crustal underplating or lithospheric flexure associated with ongoing convergence and crustal thickening . Similar Moho deepening has been reported in seismic studies across the central Andes and is often linked to magmatic additions or lower crustal flow in response to long-term tectonic loading . These structural features are further supported by receiver function and seismic tomography results, which reveal significant heterogeneities in crustal structure tied to the evolution of the Andean orogen .

In the east–west oriented profiles B-B′ and C-C′ (see Fig. ), which cross the southern and northern segments of the study area, the same discontinuities are observed. However, in the northern profile, these features appear more diffuse. This may indicate lateral heterogeneity in crustal composition or increased attenuation due to structural complexity or varying seismic properties .

Importantly, the higher frequency content of the local receiver functions significantly enhances structural clarity in the east–west profiles (see Fig. ), emphasizing the utility of high-resolution RF analysis for imaging crustal discontinuities. The combined use of local and teleseismic data provides a more comprehensive image of crustal architecture and reveals important spatial variations that contribute to our understanding of the geodynamic evolution of the region .

The models derived from the joint and SWD inversions (see Fig. ) closely aligns with that obtained from the receiver functions and phase velocity dispersion curve, delineating four primary boundaries at depths of 47, 36, 6, and 4 km. These interfaces, first identified by , correspond well with the discontinuities observed in the teleseismic receiver functions (see Sect. ). However, a notable discrepancy exists in both depth and shear wave velocity: our model systematically indicates lower velocities and greater depths across all discontinuities.

It is important to note that joint inversion results are inherently non-unique, and the final velocity structure may depend on the choice of the initial model. To assess this sensitivity, we tested alternative starting models. A preliminary inversion using a reference model derived from local VELEST results was performed; however, this approach proved unstable, producing poor fits and yielding unreliable results, including negative velocity gradients in the lower crust. Such artifacts are geologically implausible in our study region and were therefore excluded from further consideration. In contrast, the preferred starting model led to stable and consistent solutions that satisfactorily fit both receiver functions and dispersion data, suggesting that the main structural features we report are robust despite the non-uniqueness of the inversion.

In addition, it should be noted that the upper layers of the model mentioned above, down to five kilometers, include a low velocity layer of about 0.7 kms-1. This feature can be attributed to the Tajamar Formation, for the southern stations only (see Sect. ). The extent of this unit will be relevant, since it is conformed by fine-grained limos that are expected to suffer liquefaction when water oversaturates during strong motion produced by high magnitude events .

To further support the interpretation of the main discontinuities identified in this study, it is useful to place them within a broader geophysical context. Crustal interfaces at depths of ∼40-50km, interpreted as the Moho, have been consistently documented across the central Andes using a variety of seismic techniques, including receiver functions, refraction profiles, and gravity-constrained models . Similarly, mid-crustal discontinuities near ∼25-35km depth have been widely reported and are often associated with rheological transitions, changes in crustal composition, or zones of partial melt and fluid accumulation . Shallower discontinuities, particularly those located between ∼10 and 20 km depth, are frequently interpreted as detachment horizons or zones of mechanical decoupling within orogenic crust, reflecting complex deformation processes and strain partitioning . The depths and characteristics of the discontinuities observed in this study are therefore consistent with first-order crustal structures reported throughout the Andean system.

In addition to regional-scale observations, similar multi-layered crustal architectures have been identified in other tectonically active regions worldwide, reinforcing the interpretation of the observed interfaces. For instance, studies in the Tibetan Plateau and other continental collision zones reveal comparable patterns of Moho topography, mid-crustal layering, and upper crustal detachments, often linked to crustal shortening, underplating, and channel flow processes . In these settings, variations in seismic velocity across discontinuities are not uniquely indicative of tectonic boundaries but may also reflect changes in lithology, anisotropy, temperature, or fluid content . This highlights the importance of integrating multiple geophysical constraints, as done in this study, to reduce ambiguity and provide a more robust interpretation of crustal structure.

The final Fig.  provides a synthetic summary of the structures inferred from this study, integrating all results into two lithospheric-scale profiles across the Lerma Valley. One profile is oriented north–south and the other east–west, allowing the three-dimensional geometry of the subsurface to be visualized through two orthogonal sections. These profiles combine constraints from receiver functions, H-κ stacking, and inversion-derived velocity models into a unified structural framework. The principal seismic discontinuities are explicitly traced, illustrating variations in crustal thickness and the geometry of deeper interfaces across the study area.

Seismic velocities are indicated along both sections, and the main velocity gradients are emphasized to highlight vertical and lateral heterogeneities within the crust and upper mantle. The comparison between the north–south and east–west profiles reveals structural asymmetries and along-strike variations that are not evident in individual station-based results alone. By condensing the dataset into these two orthogonal cross-sections, the figure provides an integrated view of the lithospheric architecture beneath the Lerma Valley and serves as a structural reference for the geodynamic interpretation discussed above.