INSIGHTS FROM THE GEOLOGICAL RECORD OF DEFORMATION ALONG THE SUBDUCTION INTERFACE AT DEPTHS OF SEISMOGENESIS

Subduction interfaces are loci of interdependent seismic slip behavior, fluid flow, and mineral redistribution. Mineral redistribution affects fluid flow through variations in crack porosity/permeability, and earthquake physics through variations in cohesion. We investigate this system from the perspective of ancient accretionary complexes, where regional mélange zones record noncoaxial strain during underthrusting adjacent to the subduction interface. Deformation of weak mudstones is accompanied by low-grade metamorphic reactions, dissolution along scaly microfaults, and removal of fluid-mobile chemical components, whereas stronger sandstone blocks preserve veins that contain minerals with chemical components depleted in mudstones. These observations support local diffusive mass transport from scaly fabrics to veins during interseismic viscous coupling. Permanent interseismic deformation involves pressure solution slip, strain hardening, and development of new shears in undeformed material. In contrast, coseismic slip may be accommodated within observed narrow zones of cataclastic deformation at the top of many mélange terranes. A kinetic model implies interseismic changes in physical properties in less than hundreds of years, and a numerical model that couples an earthquake simulator with a fluid flow system depicts a subduction zone interface where patterns of slip are governed by feedbacks between fluid production, permeability, hydrofracturing, and aging via mineral precipitation. During an earthquake, interseismic permeability reduction is followed by coseismic rupture of low permeability seals and fluid pressure drop in the seismogenic zone. Updip of the seismogenic zone, there is a post-seismic wave of higher fluid pressure that propagates trenchward. Thus, the processes that define the subduction interface system can be calibrated through the effect of silica redistribution on earthquake clustering, earthquake size distributions, and coseismic fluid flow transients.