Field and Isotopic Evidence for Fluid Mobility in the Franciscan Complex: Forearc Paleohydrogeology to Depths of 30 Kilometers

Seth J. Sadofsky, Gray E. Bebout

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19 Citations (Scopus)


In the Franciscan Complex exposed in the California Coast and Diablo ranges (representing depths of up to 30 km in a Mesozoic to Early Tertiary accretionary prism), differences in porosity, permeability, and rheology impacted geometries and scales of fluid mobility and the deep entrainment of pore water (chemically evolved seawater). Carbon and O isotope compositions of abundant, texturally diverse CaCO3 veins (most veins with σ13CVPDB = -11.0 to -3.0‰, σ18OVSMOW = +12.0 to +18.5‰) are in part consistent with control of fluid isotopic compositions by relatively local-scale exchange with large volumes of metaclastic host-rocks. Although this apparent local-scale equilibration, observed for some veins, complicates assessment of external sources for the fluids that produced these veins, many veins with elevated σ18O (relative to calculated rock-buffered values) could reflect up-dip flow of H2O released at greater depths and previously equilibrated with similar lithologies at higher temperatures (and sluggish reequilibration of the rocks with these externally derived fluids). In the Coastal Belt, differences in vein σ13C in adjacent coherent graywacke and shaley mélange zones may be due to preferential infiltration of the more permeable mélange zones by deeply derived CH4-bearing fluids, perhaps in part during exhumation and cooling. A number of variations in vein isotopic composition in individual exposures can be attributed to vein formation over a range of increasing T during underthrusting, then decreasing T during exhumation, and related to varying rheology (affecting permeability) within intercalated highly disrupted shales, sandstone-shale sequences (with interbedding at mm to cm scales), and more massive metagraywacke. Calculated fluid σ18O for veins in the Franciscan units peak metamorphosed at lower temperatures (Coastal Belt) spans the range of fluids venting in active accretionary prisms and producing forearc serpentinite seamounts (mostly 0 ± 3‰). In general, higher flux of aqueous fluid from depth in accretionary prisms, preventing reequilibration with rocks along its path, should favor expulsion of fluid with σ18O higher than that of seawater that could be traced in fluids along fault structures at shallower levels. Salinities of fluid inclusions in vein quartz are mostly lower than that of seawater, consistent with delivery of "fresher" aqueous fluids toward the surface along structural heterogeneities. Recent work on volatile and trace element contents of the Coast and Diablo ranges exposures of the Franciscan Complex indicates significant loss of structurally bound H2O (i.e., not pore water) due to clay-to-mica transitions, but at shallower levels than those for peak metamorphism of all units studied here (i.e., at <5 km). Much of the fluid flux through these rocks involved fluids liberated at greater depths, precipitating calcite and quartz along down-T, down-P paths, and mobilizing trace elements (e.g., B, Cs, perhaps also K, Rb, As, and Sb) liberated at higher temperatures. Calcite cement in the Coastal Belt (representing very shallow offscraping, likely to <5 km) is absent in higher-grade rocks representing deeper subduction of similar rocks, consistent with significant loss of carbonate cement during decarbonation reactions at 5-10 km depths (evidenced by significant depletions in Ca and Sr in the higher-grade rocks; Sadofsky and Bebout, 2003).

Original languageEnglish
Pages (from-to)1053-1088
Number of pages36
JournalInternational Geology Review
Issue number12
Publication statusPublished - Dec 2004
Externally publishedYes

ASJC Scopus subject areas

  • Geology


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