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

GARCIA, VICTOR H.; MONTERO, C.; ELIAS, L.

Montgenevre

Simposio; International Meeting of Young Researchers in Structural Geology and Tectonics; 2018

University of Leeds

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 (Fig. 1) 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. This work pretends to summarize the preliminary results obtained after a year of remote sensing-based mapping and field work and the Quaternary evidences of deformation in the southern Lerma Valley That will allow us to interpret the recent structural evolution of this region and characterize the potentially seismogenic sources.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, Malamud, 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 (Gallardo, Malamud, etc.). Evidence of deformation on this unit has been previously reported in the Guachipas town where the strata dips up to 30°/W and soft sediment deformation structures (SSDS) were found to several stratigraphic levels (Garcia 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 more interesting features recognized during fieldwork was the lateral facies change of Tajamar Formation from lacustrine silts to fluvial-alluvial conglomerates towards the mountain front (Figure XX). Also, we recognized that the present-day cultivable soil is, in fact, the edafized upper section of Tajamar Formation (Figure XX). SSDS are found in several locations on the western side of the valley and typically restricted to the same stratigraphic level (Figure XX). Gentle dipping strata have been documented all around the region allowing us to reconstruct the structural framework on this unit (Figure XX). To constrain the age of the deformation evidence, 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.