Paleohydrogeology at 5- to 50-Kilometer depths of accretionary prism: The Franciscan Complex, California

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 δ¹³C_VPDB = -11.0 to -3.0%, δ¹⁸O_VSMOW = +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.