DEFORMATION AND METAMORPHISM IN THE ROOF OF THE CORDILLERA BLANCA BATHOLITH, PERU

The Cordillera Blanca, located in the Ancash region of Peru, constitutes the highest elevations in the northern Andes and sits along the boundary between the fold-thrust belt and hinterland. The Cordillera Blanca also contains the largest collection of tropical glaciers in the world, and the foundation of this research project lies in mapping the rocks that have been exposed during deglaciation over the past several decades. The Cordillera Blanca consists of a granodiorite batholith that is approximately 200 km in length, and 20 km in width. The batholith is overlain by a suite of weakly metamorphosed to unmetamorphosed Mesozoic sedimentary rocks, represented in the mapping area by the Jurassic Chicama Formation. Previous work indicates the batholith was emplaced at relatively shallow depths during Neogene time. The western margin of the batholith is bounded by the Cordillera Blanca detachment, a west-dipping, low-angle, synconvergent normal fault. Mylonitic fabric development in the granodiorite is associated with the detachment, but at the high elevations of the study area, the granodiorite is relatively undeformed. Regional mapping indicates that the overlying Chicama Formation contains outcrop- to km-scale folds and faults with geometry that is consistent with general fold-thrust relations in the Western Cordillera. In detail, the fold-thrust structures of the Chicama Formation are complex and have likely been tilted in association with slip on the Cordillera Blanca detachment. Previous estimates of tilting based on geobarometry of the granodiorite have large uncertainties, and our observations indicate that there is a large amount of structural relief (≥1 km) on the batholith contact zone. The lower Chicama Formation has a pervasive foliation and mineral lineation that are discordant with the batholith contact. Along with the structures and fabrics documented in the field, preliminary microstructural analysis and metamorphic petrology of the lower Chicama are inconsistent with a simple history of emplacement, contact metamorphism, and tilting. Future work requires more systematic sampling transects and will focus on estimating conditions of metamorphism in the Chicama as well as the age of deformation fabrics.