AGU Fall Meeting 2009

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Thermochronometer and Numerical Modeling Constraints on Canyon Incision and Topographic Evolution, SW Peru (Invited)
T. F. Schildgen¹; G. Balco²; D. L. Shuster²; T. A. Ehlers³; K. Hodges⁴; K. X. Whipple⁴
1. Institut fur Geowissenschaften, University of Potsdam, Potsdam, Germany.
2. Berkeley Geochronology Center, Berkeley, CA, USA.
3. Institut fur Geowissenschaften, Universitat Tubingen, Tubingen, Germany.
4. School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA.

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 rates of bedrock incision, which are necessary for relating canyon incision to surface uplift over geological timescales. Combined with studies of the incision process and potential delays between uplift and river incision, deep canyons can preserve one of the best records of long-term surface uplift.

The Cotahuasi-Ocona canyon system in SW Peru is one of the deepest canyons in the world, having incised to depths of over 3 km beneath the western margin of the surrounding Altiplano surface. Apatite (U-Th-Sm)/He thermochronology data from 31 samples in transects collected along both the valley bottom and the valley walls show a large change in slope (0.02 to 0.2 km/Ma) when plotted on an age versus depth (below surface) plot. We applied coupled three dimensional thermal, erosion, and age-prediction models to quantify the range of topographic evolution scenarios consistent with the data. Chi-squared misfits from comparison of 210 simulations with the data, as well as independent geological constraints, suggest an onset of up to 3 km of incision between ~8 and 11 Ma, and incision ending by between ~5 and 2.21 +/- 0.02 Ma.

These results, however, are limited in that they did not incorporate the potential effects of knickpoint propagation through the canyon system and associated temporal variations in incision along the canyon bottom. We present results from four samples analyzed with apatite 4He/3He thermochronometry and a new inversion model to identify continuous low-temperature cooling histories that are consistent with the observed data. Derived cooling histories constrain the onset of fluvial incision to ~13 to 8 Ma. This is in agreement with the best-fit results inferred from the thermal modeling of bulk apatite (U-Th-Sm)/He ages. However, because 4He/3He thermochronometry constrains an independent cooling history for each sample, the results also permit more robust tests of individual incision models. Thus, the combination of these two approaches helps to more narrowly define the topographic evolution of the region.

Results from combining the two data sets identify different cooling histories suggesting asynchronous incision along at least part of the canyon system that is best explained by headward propagation of incision. The apparent rate of knickpoint propagation (~6 to 12 km/Ma) between two sample sites may indicate that asynchronous incision occurred along the entire drainage system. Although geologic and geomorphic evidence upstream from the two sites supports more rapid propagation of the incision signal upstream from the sites, we have not yet tested whether the region downstream from the sites was characterized by slow knickpoint propagation. A more extensive set of samples analyzed with 4He/3He thermochronometry would allow us to directly test whether or not incision was time-transgressive across a much greater length of the canyon.

Contact Information
Taylor F. Schildgen, Potsdam, Germany, 14476, click here to send an email