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Paleohydrogeology at 5‐ to 50‐kilometer depths of accretionary prisms: The Franciscan Complex, California
Author(s) -
Sadofsky Seth J.,
Bebout Gray E.
Publication year - 2001
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2000gl008533
Subject(s) - geology , seamount , forearc , accretionary wedge , geochemistry , vein , metamorphism , subduction , metamorphic rock , meteoric water , mineralogy , petrology , hydrothermal circulation , seismology , tectonics , psychology , psychiatry
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 CaCO 3 veins by exchange with adjacent host‐rocks obscures assessment of fluid sources (most vein δ 13 C VPDB =−11.0 to −3.0‰, δ 18 O VSMOW =+12.0 to +18.5‰). Some veins with elevated δ 18 O (relative to rock‐buffered values) may reflect up‐dip flow of H 2 O released at greater depths. Differences in vein δ 13 C in adjacent coherent greywacke and shaley mélange zones of lower‐grade rocks may be due to preferential infiltration of mélange zones by deeply derived CH 4 ‐bearing fluids or to vein formation over a range of T. Calculated fluid‐δ 18 O 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.