HR: 0900h
AN: T11G-05    [Abstracts]
TI: A Slow and Steady Growth of the Andean Plateau?
AU: * Ehlers, T A
EM: tehlers@umich.edu
AF: University of Michigan, Department of Geological Sciences, Ann Arbor, MI 48109-1005, United States
AU: Poulsen, C J
EM: poulsen@umich.edu
AF: University of Michigan, Department of Geological Sciences, Ann Arbor, MI 48109-1005, United States
AB: Quantifying the timing and rates of central Andean mountain building and plateau formation have previously been limited by (1) sparsely available age constraints on the timing of deformation, and (2) a lack of constraints on the elevation history of the plateau. Recent thermochronometer and paleoaltimetry studies from the plateau and adjacent fold and thrust belt have started to address these limitations. In particular, paleoaltimetry results suggest a rapid 3.7 +/- 0.4 km increase in plateau elevation between 10.3-6.7 Ma, based on stable and clumped isotope, and paleobotanical techniques. This interpretation is based on the assumption that the paleoclimate at the time the paleoaltimetry proxies were deposited was similar to modern. In this study, we evaluate the influence of changing Andean plateau height on South American paleoclimate and interpretations of plateau elevation from paleoaltimetry data. A series of experiments are presented using regional (RegCM) and Global (GENESIS) General Circulation Models (GCM) to characterize changes in Andean precipitation amount, surface temperature, and wind direction (vapor source) as a function of changing plateau elevation. Results indicate that South American and Andean climate changed significantly in response to plateau growth. More specifically, lowering of the plateau to below 0.50-0.75 of its present day elevation results in an 700 mm/yr decrease in mean annual precipition over large portions of the Andes. Plateau lowering also results in 6 to 10 C increase in temperatures (after correction for a change in lapse rate) over the central Andean Plateau. Finally, the prevailing wind direction and the vapor source for precipitation progressively changes from the equatorial Atlantic to South Pacific as plateau elevation decreases. Taken together, these changes in paleoclimate would have enriched the oxygen isotopic concentration of precipitation by an estimated -11.4 to - 16.6 per mil, and resulted in an apparent paleo-elevation that is up to several kilometers lower than the true elevation at the time of rainfall. We conclude that the apparent rapid rise of the Andean Plateau from climate sensitive paleoaltimetry data may be an artifact of large changes in paleoclimate. Thus, the simple model of a slow and steady rise of the Andean Plateau associated with crustal thickening may be viable.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1854 Precipitation (3354)
DE: 3344 Paleoclimatology (0473, 4900)
DE: 8108 Continental tectonics: compressional
DE: 8177 Tectonics and climatic interactions
SC: Tectonophysics [T]
MN: 2007 Fall Meeting