Morphotectonic analysis of two axial tributary basins of the San Juan river controlled by the Precordillera fold and thrust belt, Central Andes of Argentina

Abstract Analysis of several morphotectonics tools (longitudinal river profiles, knickpoints, swath profiles, and geomorphic indices) together with geomorphological, lithological, and structural data provided objective and quantifiable elements to improve the knowledge of the Quaternary tectonic activity in the region. In this study, we analyzed and compared two large intermountain basins located in the Precordillera of the Central Andes of Argentina. Local distortions in longitudinal river profiles and river patterns may reflect resistant lithology, an increase in shear stress, or differential surface uplift. Both basins have similar shapes, although one mirrors the other producing a large-scale broom-shaped drainage pattern. The values of the Asymmetry Factor and Transverse Topographic Symmetry Factor show opposite signs and almost identical values, suggesting that both basins have mirror symmetry independently of order and scale. The asymmetry in both basins would be a product of the progressive migration of the Andean orogenic front towards the east. Knickpoints were mainly associated with resistant rocks, faults with the scarp facing upstream, or both. In the reaches where the rivers flow across areas with presumed higher rates of uplift, we observed a reduction in the width of the active channel belt by lateral confinement and river pattern changes. The results presented in this paper constitute one of the first attempts to apply geomorphic indices to large drainage basins in the Precordillera fold-and-thrust belt.