EXAMINING STRAIN GRADIENTS ALONG A MYLONITIC EXTENSIONAL DETACHMENT FAULT: MICROSTRUCTURAL ANALYSIS OF THE CORDILLERA BLANCA DETACHMENT, NORTHERN PERU

The Cordillera Blanca detachment is a ~200-km-long normal fault that separates the lowlands of the Callejon de Huaylas from the >6000m peaks of the Cordillera Blanca Andean subrange. The footwall of the fault is a well-preserved shear zone that accommodates exhumation of the granodiorite Cordillera Blanca batholith, and is exposed in drainages transecting the foothills of the western Cordillera Blanca. This shear zone is a rarely preserved example of synconvergent, arc-normal extension and exhumation through the brittle-ductile transition zone. The ductile regime demonstrates variable strain gradients preserved in the progression of quartz, feldspar, and mica deformation behavior and serves as the focus of this study. The transition from undeformed granite to ultramylonite is well exposed and accessible, providing an ideal setting for examining strain partitioning and microstructural development in response to increasing deformation. This study examines thin sections that represent progressive mylonitization of footwall granodiorite from undeformed, phenocrystic igneous grains to compositionally banded and recrystallized ultramylonite. Undeformed Cordillera Blanca batholith is an Na-rich, metaluminous two-feldspar bearing leuco-granodiorite with a variant magmatic fabric. Protomylonitic Cordillera Blanca batholith includes poikilitic feldspar, quartz grain boundaries that suggest recrystallization by grain boundary migration, and minor recrystallized white mica and biotite. Mylonitic granodiorite preserves strain-induced myrmekite formation, quartz recrystallization by grain boundary migration and subgrain rotation, and well-developed mica fish that preserve a top-down-to-the-southwest shear sense. Ultramylonite from near the detachment surface demonstrates the highest local strain preserved in fine-grained bands of feldspar, recrystallization by grain boundary bulging and subgrain rotation in quartz, mica shear bands, and pervasive coarser-gained mica fish. The progression of these microstructures records increasing strain partitioning from the batholith to the detachment surface and allows for approximation of deformation temperatures and levels of strain localization.