CONDITIONS OF STRAIN LOCALIZATION ALONG THE CORDILLERA BLANCA SHEAR ZONE (CBSZ), PERU

The CBSZ occupies the footwall of an active moderate- to low-angle detachment system accommodating syn-convergent extension in the high Andes of Peru. It deforms an ~8.2 Ma leucogranodiorite batholith and preserves a strain gradient from cataclasite and ultramylonites nearest to the detachment to undeformed granodiorite towards the batholith core. We describe strain localization processes and variations in these processes along strike of the shear zone though geothermometry and microstructural, microtextural, and paleopiezometric analysis of three transects across the strain gradient.

Transects presented here span ~40 km along strike of the CBSZ. Each transect records similar deformation mechanisms, although shear zone thickness ranges from ~45 m to ~450 m. Generally, increasing differential stress correlates with decreasing temperature in all transects. Recrystallized quartz paleopiezometry yields differential stresses between ~15 and ~90 MPa. Highest stresses occur in the central transect, which exposes a ~450 m thick shear zone, at 100-150 m structurally below the detachment surface. At similar structural positions, pseudotachylite overprints ductile deformation fabrics. Recrystallized quartz crystallographic preferred orientations (CPOs) from all transects record dominant prism <a> slip with varying lesser contributions of rhomb <a> or basal <a> slip. Prism [c] slip is observed at structural depths of ~35 m in the southern transect and ~200 m in the northern transect. Results suggest pervasive deformation throughout the shear zone is accommodated via dislocation creep in quartz at moderate- to high-temperatures (T > 400 °C), as supported by quartz CPOs, recrystallization microstructures, and two-feldspar thermometry on strain-induced microstructures. Microstructural analysis also suggests that at least locally, deformation is accommodated by fracture and diffusion-precipitation creep in feldspar and grain-size-sensitive creep in quartz aggregates and polyphase aggregates.