HR: 09:15h
AN: T11G-06    [Abstracts]
TI: Deformation and Exhumation History of the Central Andes, Bolivia: Insights From Thermochronology and Numerical Modeling
AU: * Barnes, J B
EM: barnesja@umich.edu
AF: Deapartment of Geological Sciences, University of Michigan, 1100 N. University Ave, Ann Arbor, MI 48109, United States
AU: Ehlers, T A
AF: Deapartment of Geological Sciences, University of Michigan, 1100 N. University Ave, Ann Arbor, MI 48109, United States
AB: The central Andean plateau is a principal feature of the Andes with a debated uplift history. In Bolivia, geologic evidence has resulted in several proposed end-member plateau uplift scenarios. These scenarios include distributed or eastward-propagating deformation beginning in the late Eocene to late Oligocene (~40-27 Ma) and recent rapid surface uplift of ~2.5-3.5 km during the late Miocene (~10.3-6.7 Ma). We synthesize 52 new thermochronometer samples with model-predicted cooling ages for the previous scenarios. Predicted and observed ages are compared to quantify the exhumation and deformation history across the eastern Andean plateau margin and test the uplift scenarios. We used inverse modeling of apatite fission-track data to resolve sample cooling histories and variations in the onset and duration of Andean deformation. Results from data collected across the Bolivian thrust belt in both the north and south suggest rapid exhumation began (a) during multiple episodes in the late Eocene through Miocene (~40-10 Ma) in the Eastern Cordillera (EC), (b) in the middle to late Miocene (~20-15 Ma) in the Interandean zone (IAZ), and (c) in the Mio-Pliocene (~20-2 Ma) in the Subandes (SA). We used a 2D thermokinematic finite-element model to predict sample exhumation histories and cooling ages. Predicted fission-track ages are cooling-rate dependent and a function of imposed end-member kinematic and erosion histories. A simplified topographic profile of the plateau margin is applied over multiple time steps to simulate plateau growth. Model free parameters include the temporal evolution of topography, thermal boundary conditions, material properties, and the kinematic field. Model results suggest: (a) erosion rates are ~0.1- 0.4 mm/yr in the EC/IAZ and ~0.01->1 mm/yr in the SA, and (b) cooling age variations are sensitive to the plateau uplift history because uplift is coincident with accelerated erosion and sample exhumation. The protracted uplift scenarios predict more complicated along-strike age variations comparable to the observations at >~0.2 mm/yr background vertical exhumation rates. Work in progress is evaluating the sensitivity of these results to all model free parameters.
DE: 1140 Thermochronology
DE: 1815 Erosion
DE: 8011 Kinematics of crustal and mantle deformation
DE: 8102 Continental contractional orogenic belts and inversion tectonics
DE: 9360 South America
SC: Tectonophysics [T]
MN: 2007 Fall Meeting