DEFORMATION MECHANISMS RELATED TO STRAIN LOCALIZATION IN THE CORDILLERA BLANCA SHEAR ZONE (CBSZ), PERU

Tectonites preserved in the footwall of the crustal-scale, moderate- to low-angle CBSZ detachment lend insight into processes of strain localization in polyphase granitic shear zones. Microstructural, microtextural, and geothermometric results record evidence for prevalent deformation at moderate to high temperatures with additional signatures of lower temperature, and/or higher strain-rate, ductile and brittle deformation. This study applied microstructural analysis, quartz paleopiezometry, and microtextural analysis of crystallographic preferred orientations (CPOs) to estimate deformation temperature and differential stress for three transects across the CBSZ, an ~200 km long and up to ~360 m wide shear zone in the footwall of the Cordillera Blanca detachment fault in the high Peruvian Andes.

A general relationship of increasing differential stress with decreasing temperature is observed, with recorded stresses ranging from ~15 to ~90 MPa and temperatures from <280 °C to > 600°C. Quartz CPOs dominantly record prism <a> slip with varying contributions of rhomb <a> or basal <a> slip. Two samples record prism [c] slip. Temperatures recorded by asymmetric strain-induced myrmekite are consistent with those suggested by CPOs. Quartz bulging recrystallization and cataclastic microstructures record low temperature and/or high strain-rate ductile and brittle deformation. In summary, our results show evidence for dominantly moderate- to high-temperature deformation as well as lower temperature and/or high strain-rate ductile deformation and brittle processes in the CBSZ.