Strong tectonic and weak climatic control on exhumation rates in the Venezuelan Andes

Abstract

We studied the relationships among present-day relief, precipitation, stream power, seismic energy, seismic strain rate, and long-term exhumation rates for the Venezuelan Andes. Average long-term exhumation rates were determined for seven large catchments in the Venezuelan Andes from fi ssion-track analysis of detrital apatite. A quantitative comparison between eight new detrital apatite fi ssion-track (AFT) age distributions presented here and previously published bedrock AFT age patterns shows that detrital AFT ages can be used for predicting exhumation patterns across the mountain belt. Catchment-averaged exhumation rates estimated from the raw data range from 0.48 ± 0.02 km m.y. -1 to 0.80 ± 0.26 km m.y. -1 Accounting for variable sediment yield and assuming that short-term sediment production rates scale with long-term exhumation rates, these rates vary from 0.33 ± 0.07 km m.y. -1 to 0.48 ± 0.08 km m.y. -1 No variation in rates is observed between the northwestern and southeastern fl anks of the mountain belt, despite a threefold increase in precipitation from the northwest to the southeast. Long-term exhumation rates are strongly correlated with relief in the different catchments, weak or negative correlations exist with precipitation data or present-day erosion indexes, while the correlation with seismic energy released by earthquakes is weak to moderate. This lack of correlation may be caused by the insuffi cient temporal range of the available precipitation and seismicity data, and the different time scales involved in the comparison. Long-term exhumation rates are, however, strongly correlated with seismic strain rates (which take the temporal earthquake magnitude-frequency scaling into account), suggesting that the moderate correlation with seismic energy is indeed related to the different time scales and that tectonic control on exhumation is signifi cant. In contrast, given that precipitation patterns in the Venezuelan Andes should have been installed during Miocene times, we suggest that decoupling of relief and exhumation from present-day climate explains the lack of correlation between exhumation and precipitation. © 2012 Geological Society of America.