Abstract

VARGAS, Mariana et al. Morphometric and morphotectonic analysis of two collinear and opposite intermountain river basins in the Precordillera, Central Andes of Argentina. Bol. Soc. Geol. Mex [online]. 2020, vol.72, n.1, e111019.  Epub Dec 22, 2020. ISSN 1405-3322.  https://doi.org/10.18268/bsgm2020v72n1a111019.

The Gualilán and La Cantera river basins are located in the Central Precordillera of the central-western Andean ranges of Argentina. Both river basins lie in a N-S tectonic depression. The regional drainage in the depression is controlled by two thrust-fault systems. To the west lies a system of thrusts associated with the Invernada mountain range. The eastern area is controlled by the La Cantera thrust system. Both basins, collinear, opposite and divergent, with an elongated shape in the N-S direction, are separated by a smooth water divide or “portezuelo.” suggesting that in the past they comprised a single intermountain valley. The objective of this study is to make a morphometric and morphotectonic characterization of these river basins, complemented with an analysis of a stream piracy process. From this analysis, the Gualilán river basin suggests a low tectonic activity for the structures that cross the basin, while those of the La Cantera River have clear evidence of Quaternary tectonic activity. The average slope of the La Cantera river basin is steeper than that of the Gualilán river basin. Morover, the Gualilán longitudinal river profile shows several knickpoints, mainly in its upper section, while the La Cantera longitudinal river profile also shows knickpoints in its upper section but has the main slope breaks in its lower section. The La Cantera fault system would has contributed to generate these variations in the slope along both rivers, product of the late Pleistocene to Holocene and probably historical activity. This paper also describes a reverse fault with horizontal displacement trending E-W, coincident with the headwaters of both N-S rivers, which would also influence the dynamics of these basins. The high gradient of the main channel and the drop in elevation between the headwaters and the outlet of the La Cantera River with respect to the Gualilán River are factors that directly favor upstream erosion. Easily eroded rocks (sandstones, siltstones, and conglomerates of the Neogene) also favor the headward erosion. Erosion would culminate in a future process of piracy of the endorheic basin of the Gualilán River, and the deviation of its flows towards the captor (La Cantera River), down to the mouth of the San Juan River, which is the base level of the region. These capture processes seems to be frequent in many of the longitudinal tectonic valleys of the Precordillera, whose cause of rejuvenation is considered in this work as dynamic (tectonic).

Keywords : River basin; Stream piracy; Neotectonics; Precordillera.

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