Electrical resistivity tomography applied to the study of neotectonic structures, northwestern Precordillera Sur, Central Andes of Argentina

Abstract

The Precordillera, which is one of the main morphotectonic units along the Pampean flat-slab subduction segment of the Central Andes of Argentina, is characterized by its high neotectonic activity. Thus, the study of the Late-Cenozoic to Recent deformation along this zone directly contributes to the seismic hazard characterization of the region. The Paleozoic and Mesozoic geological history of the southern sector of the Precordillera (Precordillera Sur) resulted in an inhomogeneous basement whose mechanical anisotropies seem to control the Andean structural features. As the northwestern edge of the Precordillera Sur and its piedmont sector shows no Cenozoic rocks, it is not possible to use them as a tool to reconstruct the tectonic deformation of this age. However, the presence of a regional Pre-Cenozoic tilted erosion surface can be use as a structural marker. The Andean tectonic deformation is here represented by some scattered Cenozoic to Recent structural highs carved in Paleozoic rocks and Quaternary sediments and by drainage modifications within the Quaternary alluvial fans. The aim of this contribution is to present a geophysical study of two piedmont neotectonic structural features: the Yalguaraz structural high and a subtle drainage incision anomaly. We studied the shallow subsurface character of these structural features by means of electrical resistivity tomographies in order to evaluate the subsurface presence of the erosion surface, its structural fabric and its relationship with Late-Cenozoic to Recent tectonic structures. This methodology showed to be effective and useful in the study of such features. We interpreted the Yalguaraz fault as a subvertical NE-dipping reverse fault, related to a fault scarp retreat of about 50?70 m. Furthermore, its NW-trending could be associated with mechanical anisotropies, likely Paleozoic in age. According to analyses of aerial photographs, the oldest aggradation level of the study zone (Early?Middle Pleistocene) could be affected by the activity of the Yalguaraz fault. The youngest Quaternary deposits (Holocene) do not show surface as well as subsurface evidence of deformation. In the case of the studied Quaternary drainage incision anomaly, the performed electrical resistivity tomography corroborates the relationship between these elements and incipient tectonic deformation.