Paleohydrogeology at 5‐ to 50‐kilometer depths of accretionary prisms: The Franciscan Complex, California

Abstract

High‐P/T metamorphism of the Franciscan Complex represents evolving subduction‐zone processes at depth. Varied peak P‐T conditions (100–350°C and 3–10 kbar) promote differences in porosity, permeability, and rheology, potentially impacting fluid mobility and entrainment of seawater. Local‐scale isotopic buffering of CaCO3 veins by exchange with adjacent host‐rocks obscures assessment of fluid sources (most vein δ13CVPDB=−11.0 to −3.0‰, δ18OVSMOW=+12.0 to +18.5‰). Some veins with elevated δ18O (relative to rock‐buffered values) may reflect up‐dip flow of H2O released at greater depths. Differences in vein δ13C in adjacent coherent greywacke and shaley mélange zones of lower‐grade rocks may be due to preferential infiltration of mélange zones by deeply derived CH4‐bearing fluids or to vein formation over a range of T. Calculated fluid‐δ18O for lower‐T veins spans the range of fluids venting in active accretionary prisms and producing forearc serpentinite seamounts. Calcite cement in the Coastal Belt is absent in higher‐grade rocks, suggesting that cement is lost to decarbonation reactions like those invoked to elevate fluid alkalinity in Marianas seamounts.