HR: 1340h
AN: T53A-0474    [Abstracts]
TI: New Constraints on the Erosion History of the Andean Plateau Inferred From Detrital Thermochronology Across the Northern Bolivian Thrust Belt
AU: * Barnes, J B
EM: barnesja@umich.edu
AF: Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109
AU: Ehlers, T A
EM: tehlers@umich.edu
AF: Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109
AU: McQuarrie, N
EM: nmcq@princeton.edu
AF: Department of Geosciences, Princeton University, Princeton, NJ 08544
AB: Orogenic plateaus are considered important features that influence global climate and mountain building processes. However, the process of orogenic plateau formation remains poorly understood due to a lack of quantitative information about the kinematic histories of plateau uplift. Current models of plateau growth range from broad regional uplift to outward growth from a narrow orogenic wedge. Here we apply detrital thermochronology to quantify the cooling history of the Bolivian thrust belt marginal to the Andean Plateau in an attempt to differentiate between the two end-member models. Fifteen detrital apatite fission track cooling ages were obtained from Precambrian to Mesozoic rocks along an across-strike transect of the Bolivian thrust belt at 15-16$\deg$S. Deconvolution of sample grain-age distributions show that they range from detrital to fully reset. Many of the samples contain mixed ages corresponding to variable provenance and compositional differences in the apatites. Spatial variations in cooling ages from west to east are as follows. Pooled ages in the Andean Plateau range from 101-2.6 Ma with a youngest statistically significant population of grains in mixed-aged samples ranging from 12-2 Ma. Pooled ages in the Interandean Zone range from 30.8-10.5 Ma. The youngest populations in most of these samples range from 19-10.5 Ma. In the Subandean Zone, pooled ages range from 91-6.6 Ma with a youngest statistically significant population of grains in a few samples ranging from 9-5 Ma. Two additional pooled ages of Tertiary basin samples from the plateau and the Subandean Zone are 37.2 and 116 Ma, respectively. These cooling ages are considered detrital, with the older pooled age having a young population at 30.9 Ma. Assuming the youngest grain-age populations in the samples represent cooling associated with the most recent exhumation event, our results suggest that (1) exhumation magnitude increases from east to west along the plateau margin, and (2) Late Miocene to Pliocene cooling throughout the study area may represent a regional tectonic or erosional event associated with increased shortening rates, climate change, or broad plateau uplift.
DE: 8100 TECTONOPHYSICS
DE: 8102 Continental contractional orogenic belts
DE: 1035 Geochronology
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting