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The Cordillera Blanca detachment in the highest elevations of the Peruvian Andes has been accommodating synconvergent extension since the late Miocene. Stable isotope analysis of synkinematic mica from its exhumed footwall shear zone provides new constraints on deep meteoric-hydrothermal circulation during ductile deformation and regional paleoelevation. Muscovite and biotite that deformed and/or grew synkinematically in the shear zone have δ2H values of −131‰ to −58‰ and −149‰ to −98‰ (versus Vienna standard mean ocean water, VSMOW), respectively. The δ2H value difference between coexisting muscovite and biotite is consistent with equilibrium fractionation of the same fluid at the same temperature. Calculated δ2H values of water (−107‰ to −78‰) in equilibrium with these micas are indistinguishable from those of present-day, deeply circulated (9–11 km) hot spring waters emanating from the fault. Such low-δ2H fluids indicate circulation of meteoric water to the depths of the brittle-ductile transition that cannot be explained by other mechanisms. Average recharge paleoelevation for water entering the shear zone based on hydrogen isotopes was 3400 + 500/–700 m (1σ). This is near, but ~500 m below, the present-day mean elevation of the catchments feeding modern hot springs of 3965 ± 880 m, and ~700 m below the 4200 + 700/–900 m mean recharge elevation derived from δ2H values of modern surface and thermal water. The consistency between modern and ancient fault-assisted hydrothermal systems and elevation suggests that high topography, steep relief, and meteoric-hydrothermal circulation have persisted throughout the history of the Cordillera Blanca detachment system.

Miocene to modern hydrothermal circulation and high topography during synconvergent extension in the Cordillera Blanca, Peru