RESOLVING RAPID TIMESCALES OF METEORIC WATER-FACILITATED STRAIN-SOFTENING IN THE DUCTILE CRUST: AN EXAMPLE FROM THE CORDILLERA BLANCA SHEAR ZONE, PERUVIAN ANDES

Interactions between meteoric waters and crystalline rocks at conditions consistent with crystal-plastic deformation are commonly evoked as a mechanism of reducing the shear strength of the lithosphere at mid-crustal levels. The mechanism facilitating strain softening is somewhat paradoxical: deep circulation of cool, surface-derived waters can drive rapid cooling, which should lead to embrittlement and effective strain-hardening of quartzofeldspathic crust. Despite this, water-rock interactions, balanced by shear heating and mechanical grain-area reduction, often result in a net reduction in lithospheric strength at depth, potentially prolonging or directly modulating the duration of crystal-plastic shear. Aqueous fluids are often invoked as a weakening agent in shear zones without clear evidence for reactions resulting from water-rock interaction. When fluid-mediated processes are explicitly identified geochemically or petrographically, the timescales they occur along are challenging to determine. We present microstructural, geochronological, and geochemical data demonstrating meteoric fluid enhanced mechanical deformation and metamorphism of feldspar to white mica in a quartzofeldspathic mylonite. Associated reaction-weakening triggered a shift from tectosilicate-dominated rheology to strength control via interconnected mica networks in mylonitic granodiorite from the Cordillera Blanca shear zone. This young, active structure in the Peruvian Andes provides the ideal conditions to constrain the weakening effect of meteoric fluids on ductile shear zones. The proxies applied in this study identify formation of micas during hydrous break-down of feldspar and allow quantification of water-rock ratios enabling reaction-induced strain softening. Pairing these observations with high precision ⁴⁰Ar/³⁹Ar thermochronology from within the shear zone and the undeformed footwall granodiorite indicates that these processes occur on the ≤100 k.y. timescale. Furthermore, in the absence of an external fluid source enabling mica formation, it is unlikely that local shear strain would have been sufficient to exceed shear strength and facilitate along-strike fault propagation in the absence of an external fluid source.