Canyon incision and knickpoint propagation recorded by apatite He-4/He-3 thermochronometry

In many regions of the world, deeply incised canyons demonstrate the net effects of physical processes active at Earth's surface in response to surface uplift. Low-temperature thermochronometry can constrain the timing and rates of bedrock incision, which is necessary for relating canyon incision to surface uplift over geological timescales. We analyzed four samples from the Cotahuasi-Ocona canyon system in southwest Peru using He-4/He-3 thermochronometry and we present a new inversion model to identify continuous low-temperature cooling histories that are consistent with the observed data. Derived cooling histories limit the onset of fluvial incision to similar to 13 to 8 Ma. This is in agreement with previously reported interpretations based on a three-dimensional thermal model interpolation to a much larger set of conventional apatite (U-Th)/He ages. However, because He-4/He-3 thermochronometry constrains an independent cooling history for each sample, the results also permit testing of landscape-evolution models with greater spatial variability in exhumation compared to those models that can be tested by a geographically scattered set of conventional (U-Th)/He ages. The different cooling histories of the four samples require asynchronous incision along at least part of the canyon system that is best explained by headward propagation of fluvial incision by knickpoint migration. (C) 2010 Elsevier B.V. All rights reserved.