Preliminary Assessment of the Quaternary Deformation in the South Lerma Valley, NW Argentina

GARCÍA, VÍCTOR; ELÍAS, LEONARDO; MONTERO LÓPEZ, CAROLINA

Montgenevre

Conferencia; Young Researchers in Structural Geology and Tectonics Conference; 2018

NW Argentina is located in the retrowedge of the Central Andes, an active subduction orogen driven by the convergence between Nazca and Southamerican plates. In this region, the Eastern Cordillera (EC) geological province consists of a thick-skinned fold-and-thrust belt with widely distributed seismicity and many potentially seismogenic structures. The Lerma Valley is the easternmost of the EC's intermontane basins between 24.5° and 26° SL, and is characterized by a dense vegetal cover and the lack of good outcrops. The main quaternary stratigraphic unit cropping out in the region is the Tajamar Formation, composed by laminated clay and silt rhythmites of millimetric to centimetric white and pink layers alternating with millimetric darker layers (Malamud et al., 1996). These deposits were interpreted as the infill of the ?Lerma lake?, a lacustrine system that could had cover almost the entire area of the Lerma valley (Gallardo et al. 1996, Malamud et al. 1996, etc.). The age of this unit has been tentatively assigned to the upper Pleistocene (ca. 100 ky) mainly based on supposed stratigraphic relationships with older and younger alluvial units (e.g. Gallardo et al. 1996). Evidences of deformation on this unit have been previously reported in the Guachipas town where the strata dips up to 30°W and soft sediment deformation structures (SSDS) were found in several stratigraphic levels (García et al. 2011).Preliminary geological mapping focused on the Quaternary units and structures was carried on the southernmost part of the Lerma valley by means of analysis of high resolution satellite images (0.5 m) and Digital Terrain Models (5 m). One of the most interesting features recognized during fieldwork is the lateral facies change of Tajamar Formation from lacustrine silts to fluvial-alluvial conglomerates towards the mountain front. Also, we recognized that the present-day cultivable soil is in fact the edafized upper section of Tajamar Formation). SSDS are found in several locations on the western side of the valley and typically restricted to the same stratigraphic level. Gentle dipping strata have been documented all around the region allowing us to reconstruct the structural framework on this unit. To constrain the age of the deformation evidences, five samples for Optically Stimulated Luminescence (OSL) were taken from the underlying and overlying layers at a section with SSDS. Additionally, samples for AMS14C dating and Tephrochronology were also taken.