HR: 0830h
AN: T31C-0856    [PDF]
TI: Assessing Mechanisms of Exhumation in the Cordillera Real, Bolivia, Based on Regional Structural Mapping and Thermochronology
AU: * Gillis, R J
EM: bgillis@ess.ucla.edu
AF: Dept. of Earth and Space Sciences, Univ. of California, Los Angeles, CA 90095-1567 United States
AU: Horton, B K
EM: horton@ess.ucla.edu
AF: Dept. of Earth and Space Sciences, Univ. of California, Los Angeles, CA 90095-1567 United States
AU: Grove, M
EM: marty@argon.ess.ucla.edu
AF: Dept. of Earth and Space Sciences, Univ. of California, Los Angeles, CA 90095-1567 United States
AB: In the Eastern Cordillera of the central Andes, the change from lower elevation, more subdued relief south of 18$\deg$ S to higher elevation and relief to the north corresponds with increased precipitation northward along the eastern margin of the central Andean plateau. Although distinguishing between climate- and shortening-related uplift or their interplay is difficult, constraining the kinematic history of shortening in a fold-thrust orogen is a critical first step in understanding climate-tectonic interactions. An ideal area to investigate the possible link between climate and high topography is the Cordillera Real in the Eastern Cordillera of Bolivia, where elevations over 6000 m decrease dramatically eastward towards the Andean foreland (several thousand meters over a few tens of kilometers) and some of the highest precipitation in the Andes is concentrated along its eastern margin. Here, new 1:50,000 scale geologic mapping, 40Ar/39Ar and apatite fission track analyses are being integrated to better constrain the kinematic history of fold-thrust structures in the Cordillera Real. Although granitic plutons in the Cordillera Real and Quimsa Cruz have been previously considered to be relatively undeformed, geologic mapping has revealed that they are deformed by regional faulting which continues into the surrounding Paleozoic wallrocks. 40Ar/39Ar biotite cooling ages ranging from 107.2 Ma to 38.3 Ma are interpreted to have been thermally reset at approximately 40 Ma. Muscovite cooling ages between 200 Ma and 42.9 Ma exhibit a disturbed age spectra, possibly due to retention of excess argon, and increasing age discordance with the biotite results westward along the transect. Apatite fission track analyses in the Cordillera Real yield ages nearly indistinguishable within error from 16.2$\pm$1.3 Ma to18.2$\pm$1.1 Ma despite 1555 m of total vertical relief between the samples. This is inconsistent with simple vertical cooling through denudation. Two of the most northwestern samples, which are separated by 760 m of vertical relief, yield a hangingwall age of 16.2$\pm$1.3 Ma for the higher sample and a footwall age of 8.57$\pm$0.7 Ma across an east-directed thrust fault. A similar age relationship is observed in the Ar/Ar data, with the hanging wall yielding an age of 40.7 Ma and a footwall age of 38.3 Ma. By first constraining the kinematic history of the Cordillera Real through regional structural mapping and thermochronogy, the role of climate in exhumation of the Eastern Cordillera can be better assessed.
DE: 8010 Fractures and faults
DE: 8015 Local crustal structure
DE: 8102 Continental contractional orogenic belts
DE: 8110 Continental tectonics--general (0905)
DE: 9360 South America
SC: Tectonophysics [T]
MN: 2003 Fall Meeting