Indícios morfotectônicos e estruturais do relevo da bacia do Rio Chapecó (SC)

Abstract

The drainage network is an important tool for understanding the regional geomorphology. It expresses in a sensitive and rapid way the transformations that occur over time on the geological scale. Rivers are features that demonstrate the occurrence of deformation processes, especially those related to endogenous factors. The hydrographic basin of the Chapecó River – BHRC – constitutes a plateau relief modeled on the volcanic coverings (basic and acidic) that ended the sedimentation of the Sedimentary Basin of Paraná throughout the west of Santa Catarina. The geomorphological characteristics of this area of more than 8000 km² have, for decades, account for structural factors intervening in its morphogenesis and in the surface conditions of the materials that constitute this landscape characterized by rivers in predominantly rocky beds and incised valleys, however, more detailed explanations which would be the responsible factors and to what extent they may in fact be markers of inherited or still prevailing intra-plate tectonic tensions. This research aimed to identify the role of lithostructural and tectonic factors in the morphogenesis of the relief of the Chapecó River Basin, and, in addition, to analyze and understand the work of the drainage network based on the morphometric parameters analyzed and opposed to the dissection of the relief of the bowl. Through the spatial analysis of morphometric indices, an attempt was made to identify anomalies that could reveal the neotectonic influence on the relief of the region. The following morphometric parameters were applied and analyzed: asymmetry factor, hypsometric integral, Hack index, elongation index, circularity index, correlation of gradients of channels and slopes, analysis of longitudinal profiles and their concavity factor, analysis of average slopes and scan profiles. These indices were jointly applied to a sample of 43 sub-basins, tributaries of the two main channels of the BHRC, Chapecó and Chapecozinho rivers. The results obtained by the morphometric and qualitative analysis of the drainage network indicate the existence of a condition of geomorphological imbalance characteristic of the current morphogenetic phase of its relief. This condition is marked by the existence of convex and irregular longitudinal profiles, knickpoints, sub-basins with high values of asymmetry and HI and low correlation between slope and channel gradients for the 43 sub-basins analyzed. This context is consistent with the manifestation of neotectonic factors related to a regional uplift capable of providing energy to the geomorphological system to move faults, tilt blocks and induce the incision and rejuvenation of the drainage network with the generation of new knickpoints and the constitution of a transient drainage network. Many of these knickpoints and waterfalls can be considered as markers of more or less vertical tensions linked to minimally Quaternary tectonic forcing, although many other of these features may have originated only by joints and fractures in the rocks.