Evidence of Quaternary tectonics along Río Grande valley, southern Malargüe fold and thrust belt, Mendoza, Argentina

Abstract

The Malargüe fold and thrust belt is developed in the Argentinian Andes between 34° and 37° S, through the tectonic inversion of Upper Triassic and Lower Jurassic depocenters of the Neuquén Basin, with an uplift history since the Cretaceous. Evidence of Quaternary deformation has been described in the northern part of it (34–34.5°S), potentially coeval to neotectonic activity along the eastern edge of San Rafael block. To the south, compressional and extensional structures active during the Quaternary were found in the Dorso de los Chihuidos along the Agrio fold and thrust belt front (37.5–38°S). Contrastingly, the southern segment of the Malargüe fold and thrust belt between these two areas with described neotectonic activity is partially covered by Quaternary products of the Payún Matrú volcanic field, that may hide evidence of recent deformation. In this 300 km gap of neotectonic information, the landscape imprint of two individual structures aligned in the mountain front through the Río Grande valley was analyzed. New evidence of neotectonic deformation were recognized, in particular over the western slope of the Cara Cura range, expressed by faulting and folding of Quaternary deposits and lava flows. An 40Ar/39Ar age from a deformed lava flow at the flanks of an anticline in the foothills of the Cara Cura range may suggest at least an upper Pleistocene compressional tectonic activity. Longitudinal river profile analysis revealed anomalies that show some correlation with the neotectonic structures described, especially knickpoints and concavity index changes. Meanwhile normalized steepness index values showed a moderate response to recent deformation. A proposed schematic geomorphic evolution for this segment of Río Grande river is discussed to put the neotectonic activity into the context of landscape formation. All together this evidence supports the idea of an active front through the Río Grande valley during the Quaternary, coetaneous to an active broken foreland to the east in the southern Central Andes.