Late Quaternary tectonics controlled by fault reactivation. Insights from a local transpressional system in the intermontane Lerma valley, Cordillera Oriental, NW Argentina

Abstract

To improve the knowledge about the Quaternary tectonic landforms, their potential level of activity, and the associated inherent seismic hazards in the Cordillera Oriental of northwestern Argentina we analyzed the Lomas de Carabajal area along the western border of the Lerma valley. In this region, Pliocene to Pleistocene synorogenic conglomerates are folded and faulted with a NNW-SSE trend, locally showing high angle dipping, and an en-échelon arrangement characterized by wavelengths of <1 km. Late Pleistocene conglomerates unconformably overlay those sediments and are folded following the same pattern as the Neogene deposits, displaying growth-strata geometries near faults. This configuration is compatible with local left-lateral transpressional tectonism driven by ENE-WSW buttressing against the NW oriented border of a Cretaceous extensional basin (Alemanía sub-basin). Optically Stimulated Luminescence analyses of sandy-silty layers interbedded within the folded late Pleistocene conglomeratic sequence implies uplift rates of 0.83–0.87 mm/a during the last 30–40 ky. Nearby, a WNW-striking, 3-m-high fault scarp disrupts loessic deposits, dated by AMS 14C in ca. 10 ky, providing a mean uplift rate of 0.30 mm/a for Holocene times. Our data unambiguously show that shallow crustal deformation in the intermontane Lerma valley is ongoing. This study bridges the gap between instrumental seismicity records and long-term deformation features associated with mountain building in this sector of the Andes.