Geomorphic expression of shear zones in Southern Brazilian and Uruguayan Shields

Abstract

Earth's surface is a result of complex tectonic and erosional processes shaping landscapes and producing transient signs of different evolutionary stages. These transient signs are formed by a gradual adjustment of rivers to an equilibrium stage through channel incision and uplift. Processes have different magnitudes according to lithological contrasts and base-level changes that result in distinct disequilibrium phases of bedrock rivers. This integrated study of geomorphic indices in bedrock rivers of the southernmost Brazilian and Uruguayan Shields is developed to identify key signs of transience associated with surface processes and compare with contrasting drainage basin results. The study area is composed of igneous-metamorphic rocks of Precambrian age of the Dom Feliciano Belt amalgamated during the Proterozoic-Phanerozoic boundary in the Brasiliano Orogeny. Digital elevation models (DEM) are used to extract hypsometric indices, χ-plots for each basin, and an interpolated Ksn map through interactive MATLAB tools emphasizing erosional stages and uplifted regions. Hypsometric integral values varying from 0.20 to 0.45 and a mean concavity of 0.5 show an overall eroded landscape, however, knickzones, positive χ-curves and Ksn anomalies of up until 45 reveal transient landscape signs in the area. Combined hypsometric, χ-plots and Ksn anomalies reveal distinctive patterns of erosion and uplift components with knickpoints aligned to structural signatures of lineaments. In the Sul-rio-grandense Shield (SRGS), these patterns point to a rugged and reworked relief controlled by shear zones, whereas the Uruguayan Shield (UYS) shows a denudated and more stable topography. Major tectonic events are well constrained in the Dom Feliciano Belt, but the timing and driving forces of the geomorphological features remains difficult to estimate because of the long geological history involved and intraplate context.