Rheology of the Crust During Syn-convergent Extension: Insights from the Cordillera Blanca Detachment, Peru
Micah Jessup, University of Tennessee, Knoxville
Colin Shaw, Montana State University
Cameron Hughes, University of Tennessee, Knoxville
Models for the rheology of the lithosphere rely on data derived from shear zones that were exhumed in various tectonic settings. The Cordillera Blanca detachment, extending for 200 km along orogenic strike, occurs in the Peruvian segment of the central Andes. It records a zone of syn-convergent extension within the highest elevations (>6,000 m) in this portion of the Andes. The footwall, dominated by leucogranodiorite of the Cordillera Blanca batholith (8 Ma), hosts a shear zone that records a range of deformation conditions that can be used to characterize the rheology of the middle crust during syn-convergent extension.
The detachment strikes northwest-southeast and exhibits a range of dip angles between 10-45° southwest. From deeper to shallower structural positions in the 100-500-m-thick mylonitic zone, solid-state fabric overprints early magmatic fabrics. Kinematic indicators such as mica fish, S-C fabrics, oblique grain shape fabrics, and sigma-clasts consistently record top-down-to-the-southwest sense of shear. The deepest structural positions contain quartz dominated by grain boundary migration recrystallization (>500°C). At intermediate structural positions, quartz recrystallization by subgrain rotation (400-500°C) begins to overprint grain boundary migration. Within the intermediate and shallowest structural positions, localized zones of ultra-mylonite are dominated by smaller grain sizes and record quartz recrystallization by bulging (280-400°C). A zone of cataclasite, including angular clasts composed of mylonitic quartz defines the top of the shear zone.
Preliminary EBSD analysis was conducted on samples representing quartz recrystallization conditions that range from grain boundary migration through bulging. LPO are dominated by prism slip in samples that preserve a range between grain boundary migration and subgrain rotation recrystallization. For samples that predominantly record subgrain rotation, the LPO begins to form a central girdle, indicating a more complex combination of slip systems (prism, rhomb, basal). Paleopiezometric calculations from samples with grain boundary migration and subgrain rotation yield estimates between 32-83 MPa.