Morphometric analysis of Chapecó river basin: Searching for vestigial trace of neotectonic on a basaltic landscape at southern Brazil

Abstract

The Chapecó River Basin (CRB) is located in the west of Santa Catarina State (South Brazil) and has a relief modelled by a large basalt lava outpouring that occurred in the Paraná Sedimentary Basin. The CRB is characterised by two plateau compartments resulting from regional denudation and associated with a river network that was formed after the basaltic lava completed its flow phase in the Lower Cretaceous. Elements of structural geology together with weathering of rocks and its products, such as regolith, have been commonly used to analyse and interpret the landforms in CRB, whose fluvial landscape is shaped by bedrock channels and incision valleys. Nevertheless, there is a lack of detailed studies about such structural factors in terms of type and origin, and to what extent they may be proxies of tectonic tensions of an intra-plate character. In the lack of guide levels and deposits that allow a better interpretation of recent tectonic deformations, in this work we aimed to evaluate tectonic-structural factors involved in the landscape evolution of the CRB through measurement and analysis of morphometric indices derived from the ALOS PALSAR DEM. We analysed the following morphometric parameters for 43 sub-basins: the basin asymmetry (FA), hypsometric integral (HI), elongation, local relief, slopes distribution, Hack index for the two main river channels (Chapecó and Chapecozinho), and concavity index of the main channels of sub-basins. The results obtained from the morphometric analysis of the river network indicate the existence of a geomorphological disequilibrium of its relief. This result was observed in the transition from the medium to the low basin and in the base levels of the Chapecó and Chapecozinho rivers. Such disequilibrium is shown by convex and irregular longitudinal profiles, knickpoints, sub-basins with high values of asymmetry and hypsometric integral (HI), as well as a low correlation between slope and channel gradients for the sub-basins analysed. This context is consistent with the neotectonic hypothesis of a regional uplift that provide energy to the system to reactivate faults, tilt blocks, and deepened the incision of the river network, leading to the creation of new knickpoints and a transient river network. From the results and analysis, we can conclude that the largest knickzones and waterfalls in the study area can be considered markers associated with quaternary tectonic forcings, although many of these geomorphological features may have originated simply by joints and fractures in the rocks.