Constraints on Element Mobility During Deformation Within the Seismogenic Zone, Shimanto Belt, Japan

Subduction interfaces exhibit a variety of slip behaviors, including megathrust and slow earthquakes. Field observations are consistent with crack-seal deformation, in which tensile cracks are sealed by fluid-transported solute. However, there are few constraints on the mass fluxes and length and time scales of such deformation and attendant increases in cohesion within the seismogenic zone. Here, we present a systematic geochemical investigation of mass transport associated with development of crack-seal veins in the Shimanto Belt—an accretionary complex that preserves a record of plate boundary slip behavior at temperatures relevant to the seismogenic zone: 150–350°C. These mélanges show evidence for shear across decameter-scale zones of deformation dominated by anastomosing scaly fabrics and pervasive veins. We use meso- and microstructural observations with geochemical observations of scaly fabrics and crack-seal veins to evaluate the role of silica redistribution in healing fracture porosity along the plate interface and modulating slip behavior. Crack-seal veins contain primarily quartz with albite and calcite, and vein textures provide evidence for partial sealing. Bulk rock analyses determined that the amount of phyllosilicates, specifically illite and chlorite, increases with temperature. A simple mass-balance model based on the immobile chemical component TiO₂ shows a systematic trend in mobility for all three mélanges and increased element mobility as a function of temperature. Scaly fabrics and veins show compositional evidence for locally sourced mineral redistribution. This study supports a model where development of scaly slip surfaces and fracture healing through temperature-dependent mineral redistribution can impact slip behavior and fluid flow along the subduction plate interface.