Deciphering the Crustal Structure of the Lerma Valley (NW Argentina): A Multi-Method Seismic Investigation

Abstract. We investigated the crustal structure beneath the Lerma Valley in northwestern Argentina using data from a local seismic network deployed between 2017 and 2018. This geologically complex transition zone between the Eastern Cordillera and the Sierras Subandinas is characterized by moderate to high seismicity (INPRES, 2024), yet remains largely understudied despite its strategic location within the Andean orogen (Jordan et al., 1983; Allmendinger et al., 1997). Its passive orogenic setting and evidence of inherited structures (Ramos, 2008; Mon and Salfity, 1995; Kley and Monaldi, 2002) make it a natural laboratory for exploring intraplate deformation and foreland basin evolution (Pérez et al., 2016; Tassara et al., 2018). We combined local and teleseismic receiver functions with ambient noise tomography (ANT), jointly inverting Rayleigh wave phase velocities to obtain 1D shear-wave velocity profiles. The results reveal a stratified crust with four main discontinuities at ∼ 53–43, 35–30, 10–8, and 1.5–1.2 km, corresponding to the Moho, mid- and lower-crustal boundaries, and the base of the sedimentary basin. A southward-dipping Moho is evident from CCP migration and T-component phase shifts. Velocity profiles also show a north–south contrast: lower velocities (1–2.5 km/s) in the south indicate thicker, less consolidated sediments, while the north exhibits more competent crust (up to 3.5 km/s). The final model comprises five layers, including three sedimentary and two crystalline crustal units. We also introduced a layer-dependent κ correction, revealing a trend from 1.65 at the Moho to 2 in upper layers. These results provide new geophysical constraints on the crustal architecture and tectonic evolution of this underexplored Andean region.