DENSE VEIN ARRAYS IN SUBDUCTION MELANGE EXHUMED FROM THE SEISMOGENIC ZONE: EVIDENCE FOR DIFFUSION OR ADVECTION?

Fault zone processes in the footwall of the subduction interface are characterized within rocks exhumed from the seismogenic zone and used to inform models of interseismic deformation, coseismic slip, and fluid flow. Ten subduction mélange units from the Shimanto belt of Japan and the Kodiak accretionary complex in Alaska depict a pervasive scaly fabric of anastomosing slip surfaces that record pressure solution during interseismic simple shear parallel to the plate boundary. Based on a pressure solution flow law, this deformation occurs at rates too slow to accommodate plate motions for the temperature conditions estimated for the seismogenic zone (150-350˚C), but fast enough to promote tapering of a slip rate deficit to zero with increasing depth. These same rocks contain widespread quartz-calcite-albite-chlorite veins. The geochemically determined volume loss of the scaly fabric is approximately the same as the volume of precipitated vein material, consistent with pressure solution as the dominant deformation mechanism along the plate interface between earthquakes. Paleotemperature estimates and stable isotopes of veins are consistent with veins forming in the seismogenic zone at temperatures of 100-250˚C, and potentially in concert with overpressures related to fluid release from the smectite to illite transition. The pressure solution flow law is incorporated into MEFISTO, the Mineralization, Earthquake, and Fluid flow Integrated SimulaTOr, which models temperature dependent interseismic healing with reductions in permeability, combined with restoration of background permeability and strength during earthquakes. In this model, silica supersaturation and precipitation rates are tracked through time during updip advection of fluid punctuated by earthquake fluid pressure transients. Given that the underthrusting sediments traverse the 100-250 degree window along the plate interface in less than 2 million years, the precipitation rates calculated in the model due to fluid advection are not capable of generating the vein volumes that are observed in subduction melange. Thus, the formation of dense vein arrays, which contributes to healing and permeability reduction, is largely driven by local pressure solution, or dissolution-precipitation creep, rather than advection of silica-saturated fluids.