Abstract. Base-level maps (or "isobase maps", as originally defined by Filosofov, 1960), express a relationship between valley order and topography. The base-level map can be seen as a "simplified" version of the original topographic surface, from which was removed the "noise" of the low-order streams erosion. This method is able to identify areas with possible tectonic influence even within lithological uniform domains. Base-level maps are usually applied in semi-detail scale (e.g., 1:50 000 or larger) morphotectonic analysis. In this paper, we present an evaluation of the method's applicability in regional-scale analysis (e.g., 1:250 000 or smaller). A test area was selected in Northern Brazil, at the lower course of the Araguaia and Tocantins rivers. The method provided results consistent with the scale of the data used as topographic base and with the drainage network (1:1 000 000). Some of the base-level anomalies interpreted correspond to important faultlines and geological contacts present at the 1:5 000 000 Geological Map of South America. Others have no correspondence with mapped structures and are considered to represent more recent morphotectonic features. The E–W inflexion of the lower Tocantins is considered as a major drainage capture, originated by an E–W, southward-dipping normal fault. The base-level map also presented a good correlation with anomalies in geophysical data, which shows that the method is sensitive enough to detect features with little topographic expression.
Regional scale analysis of landform configuration with base-level maps
