EVIDENCE FOR METEORIC FLUID INFILTRATION INTO A YOUNG, ACTIVE MID-CRUSTAL SHEAR ZONE: δ2H OF MICA FISH FROM THE CORDILLERA BLANCA SHEAR ZONE, PERUVIAN ANDES

The introduction of meteoric water into mid-crustal shear zones via surface connected faults effects the thermal, chemical, and mechanical properties of the deforming crust. Infiltration of meteoric water into ductile shear zones is evoked to amplify changes in rock properties such as grain-size reduction processes, metamorphic reactions, or hydraulic weakening, thus playing an important role in strain localization and shear zone development. These processes are widely recognized in inactive shear zones, but direct links between fault-fluid activity in actively exhuming shear zones and modern fault-driven hydrothermal systems have been elusive. The Miocene-modern Cordillera Blanca detachment fault is a syn-convergent, trench parallel extensional detachment fault that bounds the southwestern flank of the highest topography in the Peruvian Andes. The exhumed footwall of the detachment fault preserves the top-down-to the southwest mylonitic Cordillera Blanca shear zone (CBSZ). The fault system hosts an active hydrothermal network in which fluids circulate to depths of 9-11 km. Here, we present hydrogen stable isotope (δ²H) values of synkinematic muscovite and biotite from 18 samples of mylonitic granodiorite collected along structural depth transects across the CBSZ from three positions along strike. δ²H of muscovite and biotite range from -131 ‰ to -58 ‰ and -142 ‰ to -98 ‰, respectively. Microstructural characteristics indicate that mica fish formation is bracketed by deformation temperatures between 390 ˚C and 550 ˚C, consistent with previous constraints. Equilibrium water δ²H values calculated with these temperatures range from -107 ‰ to -35 ‰. These data indicate that meteoric water was present in the CBSZ during ductile flow at all examined transects. In the thickest segment of the CBSZ, low δ²H water values occur at structurally high In narrow segments of the shear zone low δ²H is present at all examined structural depths. These relationships imply that meteoric fluids derived from high altitudes, perhaps similar to modern, infiltrated the central and southern CBSZ forming a fault-circulated hydrothermal network like that observed along the modern fault, providing a link between historic hydrothermal processes and those active today.