HR: 1340h
AN: T23C-1541    [Abstracts]
TI: Quantifying transient erosion of orogens with detrital thermochronology from syntectonic basin deposits: insights from the central Pyrenees, Spain
AU: * Rahl, J M
EM: rahlj@wlu.edu
AF: Washington and Lee University, Department of Geology, Lexington, VA 24450, United States
AU: Ehlers, T A
EM: tehlers@umich.edu
AF: University of Michigan, Dept of Geological Sciences 2534 CC Little Bldg 1100 North University Ave, Ann Arbor, MI 48109-1005, United States
AU: van der Pluijm, B A
EM: vdpluijm@umich.edu
AF: University of Michigan, Dept of Geological Sciences 2534 CC Little Bldg 1100 North University Ave, Ann Arbor, MI 48109-1005, United States
AB: Detrital thermochronology represents a powerful tool with which to constrain the exhumational history of mountain belts. In steadily eroding mountain belts, the long-term erosion rate may be calculated from low-temperature thermochronometer ages in syn-orogenic sediments. The difference between a thermochronomter age and the depositional age – the "lag-time" – is a function of the erosion rate, with faster erosion rates associated with shorter lag-times. Although widely applied, this approach systematically leads to errors if the erosion rate of the source region is not steady. Geologically reasonable variations in erosion rate create perturbations in the thermal field of a mountain belt that distort the expected relationship between lag-time and erosion rate. In such cases, field data are best interpreted by numerical models that account for the evolution of the thermal field with time. In this study we integrate new detrital thermochronometer samples with a 1D transient thermal finite element model to characterize the exhumation history of the orogen over ~15 m.y. The thermal model predicts cooling rate dependent thermochronometer ages in a stratigraphic section as a function of variable source region erosion histories and thermophysical material properties. New thermochronometer data are presented from a syn-orogenic foreland basin deposit exposed in the Spanish Pyrenees (Sierra de Sis conglomerate). A several km thick section of conglomeratic deposits preserves approximately 15 million years of erosion in the core of the orogen, from ~42 to 27 Ma. Apatite fission-track and apatite and zircon (U-Th)/He data from this sequence therefore provide an opportunity to constrain the long-term evolution of the source region. Apatite fission-track data, including track-length distributions, indicate that conglomeratic clasts from throughout the section record rapid cooling between 48 and 42 Ma. Apatite and zircon (U-Th)/He data also show generally invariant ages throughout the section, consistent with rapid exhumation of the source terrane during the mid to late Eocene. (U-Th)/He ages show a similar trend, although significant larger variability in grain-ages is present. Thermochronometer ages are constant throughout the stratigraphic section, requiring that the lag-times increased throughout time. For example, apatite fission-track lag-times increase from ~0-2 m.y. at the base of the section to about 15 m.y. near the top. This increase in lag-time implies a gradual but significant reduction in the erosion rate of the source terrane. Results from numerical models of a wide range of transient erosion histories suggest a significant decrease in the erosion rate of the source terrane from around 1.0 mm/year to ~0.5 to 0.2 mm/yr. This conclusion is consistent with the results from bedrock thermochronometric studies from the source region that similarly indicate a decrease in erosion rate in the late Eocene and early Oligocene, demonstrating the potential of the analysis of syn-orogenic deposits.
DE: 8169 Sedimentary basin processes
DE: 8175 Tectonics and landscape evolution
SC: Tectonophysics [T]
MN: 2007 Fall Meeting