Morphotectonic and Gravimetric Analysis of the La Burra Valley, Central Precordillera, San Juan, Argentina

This work describes new evidence of Quaternary tectonic activity in the La Burra intermountain valley in the Central Precordillera, San Juan Province, Argentina. Quaternary structures are located in the eastern piedmont of the Sierra de la Crucecita, and western piedmont of the sierra de Talacasto (30o 54’–31o S and 68o 47’–68o 55’ W), about 70 km northwest of the city of San Juan. By means of morphotectonic analysis, the main deformation features have been identified and located in both piedmonts, named from north to south: Las Crucecitas and Vertientes faults (in the eastern piedmont of the Sierra de La Crucecita) and the Western Talacasto fault system (in the western piedmont of the homonymous range). The faults affect alluvial deposits from Late Pleistocene to Holocene ages. They are defined by a series of features, typical of compressive environments such as: slope scarps (eastern piedmont of the Sierra de La Crucecita) and counter-slope scarps (western piedmont of the Sierra de Talacasto), fluvial strath terraces restricted to the hanging block of faults, sagponds, aligned springs, diverted and obstructed drainage, deeply incised and sinuous streams in the hanging blocks, alluvial fans with anomalous slopes, among other features that evidence their Quaternary activity. Six Quaternary alluvial levels were identified, from the most recent Q1 to the oldest Q6, according to their relative elevation, presence or absence of desert varnish and pavement and degree of incision. The faults affect Q6 to Q2 alluvial levels in coincidence with the east-verging thin-skinned structural style typical of the Central Precordillera. This deformation is related to the regional tectonics that have been active in the Precordillera area.