Remnants of a rift-related, low-angle detachment system are spectacularly exposed in the continental Err nappe and occur discontinuously in the underlying oceanic Platta nappe in southeast Switzerland. Rocks along the detachment fault exhibit systematic changes in mineralogy, fabrics, and structures that are related to deformation and hydration of granite to form cataclasite and subsequent gouge. Structural, mineralogical, and geochemical data indicate a continuous evolution of fluid flow and mass transfer along the detachment fault. Mass-balance calculations together with stable isotope data suggest (1) an altered seawater source and local interaction with mantle rocks; (2) limited fluid-rock interaction and mass transfer during initial cataclasis at deeper crustal levels and temperatures of 150 to 200 °C; (3) increased fluid flux and mass transfer at slightly lower temperatures (100–150 °C) during gouge formation at shallower crustal levels; and (4) strongly localized fluid-rock interaction at low temperatures (65–80 °C) near the sea floor at a final stage of detachment faulting.

We propose a model in which seawater-dominated fluids penetrated and locally interacted with mantle rocks before they were caught and channelized along large-scale detachment faults. Flow direction is upward along the detachment from higher to lower temperatures, suggesting that the driving force of fluid flow was thermal fluid convection associated with mantle exhumation. Our study suggests that this large-scale detachment fault is a good analog for extensional structures along present-day, nonvolcanic, rifted margins. Such structures along ocean-continent transitions are important in controlling fluid pathways and mass transfer between mantle and continental crust.

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