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

High-P/T metamorphism of the Franciscan Complex represents evolving subduct ion-zone processes at depth. Varied peak P-T conditions (100-350 degreesC a nd 3-10 kbar) promote differences in porosity, permeability, and rheology, potentially impacting fluid mobility and entrainment of seawater. Local-sca le isotopic buffering of CaCO3 Veins by exchange with adjacent host-rocks o bscures assessment of fluid sources (most vein delta C-13(VPDB) = -11.0 to -3.0 parts per thousand, delta O-18(VSMOW) +12.0 to +18.5 parts per thousan d). Some veins with elevated delta O-18 (relative to rock-buffered values) may reflect up-dip flow of H2O released at greater depths. Differences in v ein delta C-13 in adjacent coherent greywacke and shaley melange zones of l ower-grade rocks may be due to preferential infiltration of melange zones b y deeply derived CH4-bearing fluids or to vein formation over a range of T. Calculated fluid-delta O-18 for lower-T veins spans the range of fluids ve nting in active accretionary prisms and producing forearc serpentinite seam ounts. Calcite cement in the Coastal Belt is absent in higher-grade rocks, suggesting that cement is lost to decarbonation reactions like those invoke d to elevate fluid alkalinity in Marianas seamounts.