Long-term Canyon Incision From Low-Temperature Thermochronology

River incision is one of the most dramatic landscape responses to the growth of orogenic plateaus. However, using incision as a proxy for surface uplift is fraught with problems. Few methods exist that record long-term incision directly, and it is often difficult to quantify the lag time that may exist between surface uplift and the onset of incision, particularly in arid regions where knickpoints may propagate slowly through a river system, or where major changes in climate have occurred. Approaches that can quantify long-term incision in multiple locations along a river offer the best potential for giving meaningful data that can be corrected for lag time. Low-temperature thermochronometers can record transient thermal adjustment induced by incision in areas where incision exceeds the initial depth of the closure temperature. We present apatite (U-Th)/He data from 31 samples collected in Cotahuasi-Ocoña Canyon, Peru, which cuts to depths of over 3 km through the western margin of the Altiplano plateau. Interpretation of the canyon incision history from cooling ages is complicated by a southwest to northeast increase in temperatures at the base of the crust due to active subduction, as well as by three-dimensional variations in sample cooling histories that depend on the style of landscape evolution. We address these complications with coupled finite-element thermal, landscape evolution, and thermochronometer age-prediction models to quantify the range of topographic evolution scenarios consistent with observed cooling ages. Geological evidence for early canyon depths of at least 200 m and comparison of 210 model simulations to observed cooling ages and regional heat flow determinations identify a best-fit history with 0.2 to 0.5 km of incision starting prior to ~14 Ma and 2.7 to 3.0 km of incision starting between 8 and 11 Ma. Young thermochronometer ages and a 40Ar/39Ar age on a valley- filling volcanic flow imply that incision ended between ca. 5 and 2.21 ± 0.02 Ma. Initial results from 4He/3He dating of a subset of the samples suggest that even higher precision in the cooling history and interpreted canyon incision can be derived with this method. Furthermore, the potential for the entire incision-induced cooling signal to be recorded in a single sample implies that the timing of incision can be deduced for multiple locations along the valley bottom, allowing for tests of knickpoint propagation through the system.