Regional scale analysis of landform configuration with base-level (isobase) maps

摘要

Base-level maps (or "isobase maps", as originally defined by Filosofov, 1960), express a relationship between valley order andtopography. The base-level map can be seen as a "simplified" version of theoriginal topographic surface, from which the "noise" of the low-order streamerosion was removed. This method is able to identify areas with possibletectonic influence even within lithologically uniform domains. Base-levelmaps have been recently applied in semi-detail scale (e.g., 1:50 000 orlarger) morphotectonic analysis. In this paper, we present an evaluation ofthe method's applicability in regional-scale analysis (e.g., 1:250 000 orsmaller). A test area was selected in northern Brazil, at the lower course ofthe Araguaia and Tocantins rivers. The drainage network extracted fromSRTM30_PLUS DEMs with spatial resolution of approximately 900 m was visuallycompared with available topographic maps and considered to be compatible witha 1:1,000 000 scale. Regarding the interpretation of regional-scalemorphostructures, the map constructed with 2nd and3rd-order valleys was considered to present the bestresults. Some of the interpreted base-level anomalies correspond to importantshear zones and geological contacts present in the 1:5 000 000 Geological Mapof South America. Others have no correspondence with mapped Precambrianstructures and are considered to represent younger, probably neotectonic,features. A strong E-W orientation of the base-level lines over the inflexionof the Araguaia and Tocantins rivers, suggest a major drainage capture. A N-Stopographic swath profile over the Tocantins and Araguaia rivers reveals atopographic pattern which, allied with seismic data showing a roughly N-Sdirection of extension in the area, lead us to interpret this lineament as anE-W, southward-dipping normal fault. There is also a good visualcorrespondence between the base-level lineaments and geophysical anomalies. ANW-SE lineament in the southeast of the study area partially corresponds tothe northern border of the Mosquito lava field, of Jurassic age, and a NW-SElineament traced in the northeastern sector of the study area can beinterpreted as the Picos-Santa Inês lineament, identifiable in geophysicalmaps but with little expression in hypsometric or topographic maps.