DEFORMATION DURING SYN-CONVERGENT EXTENSION ALONG THE CORDILLERA BLANCA SHEAR ZONE, PERU

The Cordillera Blanca Shear Zone (CBSZ), Peru accommodates deformation associated with exhumation of a granodiorite batholith across the brittle-ductile-transition along an active, ~200 km long, WSW dipping low-angle normal detachment fault situated above the Peruvian flat-slab segment of the Andean margin. The CBSZ provides a record of changing differential stress, temperature, and deformation mechanisms during exhumation of the footwall batholith and thus provides an outstanding case study for evaluation of mid-crustal rheology in a young, active, syn-convergent extensional setting. We characterize deformation across a transect through the CBSZ along Quebrada Gatay in the central part of the range using EBSD analysis of quartz lattice preferred orientations, microstructural analysis, and two-feldspar thermometry of asymmetric strain-induced myrmekite. A general trend from relatively low to high differential stress occurs from structurally deep to shallow positions. Deepest structural positions record quartz grain boundary migration recrystallization (T > 500 °C), prism <a> slip, and complex feldspar histories including bulging recrystallization, myrmekite formation, and brittle fracture. Intermediate structural positions record dominant quartz subgrain rotation recrystallization (400-500 °C) and prism <a>, with some rhomb <a>, slip. Deformation temperatures of deep and intermediate samples derived from two-feldspar thermometry in asymmetric myrmekite agree with temperature estimates from quartz recrystallization microstructures. Shallower positions record dominant bulging recrystallization in quartz (280-400 °C) and contributions of basal <a> and rhomb <a> slip, with little to no asymmetric myrmekite. Brecciation of mylonitic fabrics and cataclastic overprinting occur at shallowest positions along the detachment. Overall, deformation conditions recorded in the Gatay transect of the CBSZ suggest progressive strain localization along the detachment as the footwall batholith was exhumed through the brittle-ductile-transition.