HR: 0800h
AN: T41C-1242    [Abstracts]
TI: Thermochronologic and Sedimentologic Evidence for Variations in Exhumation of the Cordillera Blanca Detachment Fault System, Peru
AU: * Horton, B K
EM: horton@ess.ucla.edu
AF: University of California, Dept. of Earth and Space Sciences, Los Angeles, CA 90095-1567 United States
AU: Giovanni, M K
EM: giovanni@ess.ucla.edu
AF: University of California, Dept. of Earth and Space Sciences, Los Angeles, CA 90095-1567 United States
AU: McNulty, B
EM: bmcnulty@csudh.edu
AF: California State University Dominguez Hills, Dept. of Earth Sciences, Carson, CA 90747 United States
AU: Grove, M
EM: marty@ess.ucla.edu
AF: University of California, Dept. of Earth and Space Sciences, Los Angeles, CA 90095-1567 United States
AB: Thermochronologic and sedimentologic data for the 6-km-high Cordillera Blanca, an active detachment fault system in Peru, indicate rapid along- and across-strike variations in late Cenozoic exhumation. The Cordillera Blanca is composed of an $\sim$8 Ma ($\sim$3 kbar) granodiorite batholith in the Andean hinterland above the subducted flat Nazca slab. The range forms the footwall of a detachment fault striking $330\deg$, oblique to Neogene $082\deg$ Nazca-South America convergence. Previous studies indicate mainly dip slip along the 20-45$\deg$ west-dipping fault and west-down shear sense within a footwall mylonite. Total fault offset is poorly constrained but may exceed 10 km of normal slip. $^{40}$Ar/$^{39}$Ar analyses of granitic footwall rocks collected along several transects reveal considerable variability across and along strike. Across strike, samples 5-10 km from the fault show cooling shortly after $\sim$8 Ma batholith emplacement, 1-2 Myr prior to cooling of samples collected $<$2 km from the fault. These results are consistent with footwall tilting during faulting. Along strike, samples from the central fault segment show rapid cooling from $>$300$\deg$C at 5-6 Ma to below $\sim$150$\deg$C by 2-3 Ma, about 1-2 Myr prior to samples from the south. We tentatively interpret maximum slip along the central fault segment with diminished slip to the north and south, but additional data are required. Investigation of the hanging wall (Callejon de Huaylas) also reveals along-strike variability. New $^{40}$Ar/$^{39}$Ar ages for the $\sim$5.4 Ma tuff at the base of the $\sim$1300 Lloclla Formation (southern segment) within a supradetachment basin are younger than new ages for the $\sim$7.5 Ma Yungay tuff (central segment) that unconformably overlies Jurassic-Cretaceous strata. Although previous workers correlate these two tuffs, the new ages reveal discrepant histories for the central and southern hanging wall. These relationships suggest extensional basin development limited to localized regions along the detachment, implying no direct correlation between slip magnitude and sediment accumulation.
DE: 9360 South America
DE: 8010 Fractures and faults
DE: 8102 Continental contractional orogenic belts
DE: 8107 Continental neotectonics
DE: 8109 Continental tectonics--extensional (0905)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting