Denudation and retreat of the Serra do Mar escarpment in southern Brazil derived from in situ‐produced 10Be concentration in river sediment

Abstract

AbstractThe Serra do Mar escarpment, located along the southeastern coast of Brazil, is a high‐elevation passive margin escarpment. This escarpment evolved from the denudation of granites, migmatites and gneisses. The granites outcrop in the form of a ridge along the escarpment crest, due to its differential erosion (‘sugarloaf’ hills) from the surrounding lithologies. Several studies suggest that the passive margin escarpments are actively retreating toward the interior of the continent. However, no prior study has calculated the long‐term denudation rates of Serra do Mar to test this hypothesis. In this study, we measured the in situ‐produced 10Be concentration in fluvial sediments to quantify the catchment‐wide long‐term denudation rates of the Serra do Mar escarpment in southern Brazil. We sampled the fluvial sediments from ten watersheds that drain both sides of the escarpment. The average long‐term denudation rate of the oceanic side is between 2.1‐ and 2.6‐fold higher than the rate of the continental side: 26.04 ± 1.88 mm ka‐1 (integrating over between 15.8 ka‐1 and 46.6 ka‐1) and 11.10 ± 0.37 mm ka‐1 (integrating over between 52.9 ka‐1 and 85.4 ka‐1), respectively. These rates indicate that the coastal base level is controlling the escarpment retreat toward the continental high lands, which is consistent with observations made at other high‐elevation passive margins around the globe. The results also demonstrate the differential erosion along the Serra do Mar escarpment in southern Brazil during the Quaternary, where drainages over granites had lower average denudation rates in comparison with those over migmatites and gneisses. Moreover, the results demonstrate that the ocean‐facing catchments have been eroded more intensely than those facing the continent. The results also reveal that drainage over the granites decreases the average denudation rates of the ocean‐facing catchments and the ‘sugarloaf’ hills therefore are natural barriers that slowly retreat once they are exhumed. Copyright © 2013 John Wiley & Sons, Ltd.