The influence of lithology on hillslope development in the area of the Jacupiranga Alkaline Complex and its surroundings (São Paulo, Brazil)

Abstract

The area of the Jacupiranga Alkaline Complex and its surroundings is characterized by a variety of slope forms. Their spatial differentiation and the differences in relative relief and hilltop heights are a function of lithology. Significant contrasts exist, particularly between the ultramafic rocks of the Alkaline Complex and the quartz-rich rocks of the surrounding Precambrian basement. Although climatic conditions are uniform, the physical and chemical properties of the weathering products formed on the various rock types are very different. Through their properties, they affect the hillslope processes and thereby influence the hillslope development. Slope-forming processes on deeply weathered ultramafic rocks are slow mass movements and chemical denudation. On the more resistant ultramafic rocks, wash processes appear to be more important. Wash and rapid mass movements predominate on gneisses, granites and mica schists. The geomorphological development of the Jacupiranga Alkaline Complex can be explained in terms of a long, continuous downwearing. The large differences in elevation within the Alkaline Complex suggest that the development towards a more differentiated relief may extend well back into the Tertiary. The mutual adjustment of slope forms, processes and lithological controls favoured the development of rock-dependent summit heights, where heights are determined by the steepness of the slopes and the spacing of the drainage lines. Climatic fluctuations in the Pleistocene failed to produce any fundamental change in the long-term development of the hillslopes as their duration was too short with respect to the relaxation time of the hillslopes or their influence was not great enough to alter the overall trend of slope development.