RHEOLOGY, KINEMATICS, AND THERMAL CONDITIONS DURING SYN-CONVERGENT EXTENSION ON THE CORDILLERA BLANCA DETACHMENT, PERU

Footwall rocks exhumed from depth along the Cordillera Blanca shear zone preserve a record of crustal strength and rheology during syn-convergent extension at the highest elevations of the Peruvian Andes. Structural and microstructural analysis of the shear zone provides a record of kinematics, temperature, and stress conditions during syn-orogenic extension parallel to the direction of plate convergence.

The 200-km-long Cordillera Blanca detachment formed largely within granodiorite batholith. The top of the detachment is brecciated, whereas, structurally lower positions record a degree of solid-state fabric that decreases towards undeformed granodiorite in the footwall. Data from several transects across the detachment located at different positions along strike are compared to create a conceptual model for the evolution of the detachment during exhumation. Dynamically recrystallized quartz records an early stage of grain boundary migration that is overprinted to variable degrees by subgrain rotation and bulging recrystallization at higher structural positions. Kinematic indicators from the mylonite show top-down-to-southwest shearing in all transects. Lattice preferred orientation data from dynamically recrystallized quartz indicate contributions of prism, rhomb, and basal <a> slip. Two-feldspar geothermometry using compositions from asymmetric strain induced myrmekite yield temperatures (424-479 ºC) during shearing. Estimates of differential stress using quartz paleopeizometry are between 20-58 MPa. These data are the foundation for a generalized conceptual model that conveys a progressive exhumation history on the Cordillera Blanca detachment.