Quaternary deformation and seismicity at the easternmost Malargüe fold and thrust belt, Mendoza, Argentina.

Abstract

The Malargüe fold and thrust belt (FTB) develops in the Argentinian Andes between 34º and 37º S, through the tectonic inversion of depocenters from the Upper Triassic and Lower Jurassic since Upper Cretaceous times, with a main shortening phase in the Miocene. Different works in the last decades have focused on characterizing, mainly through recognition of syntectonic strata, these phases of mountain building, although only few have addressed the youngest deformational stages at the orogenic front area. The present work is aimed to show direct and indirect evidence of recent deformation in the eastern limit of the Malargüe FTB, between the Sierra de Palauco and Sierra de Reyes anticlines, along the Río Grande valley, a segment that lacked these kinds of studies and was considered as entirely shaped in the last 15 Ma based on synorogenic and thermochronological data. Direct evidence on the activity of the easternmost Malargüe FTB includes faults in unconsolidated fluvial deposits, folded Quaternary basalts, lacustrine deposits with strong internal deformation, drainage anomalies, etc., all aligned with the orogenic front, to the east of the Palauco, Cara Cura and Reyes structures. We also encountered Quaternary monogenetic cones and lavas that deviated and dammed a westward drainage network coming from Quaternary uplifts and affluent to the Río Grande river. Moreover, a local seismic recording array allowed detecting clusters of crustal seismicity potentially linked to the recognized structural trends. Particularly, south of the Sierra de Palauco two previously unknown crustal seismogenic structures were illuminated. Thus, considering that the orogenic front in the area is thought to be westerly retracted, both the neotectonic evidence found and these seismic data constitute an improvement in the state of knowledge of the Quaternary in this region. Finally, as the Las Malvinas fault system (slightly to the north of the studied area), provoked great damaging earthquakes in historical times, a more detailed neotectonic and paleoseismological study of the region becomes necessary.