Insights from the geological record of deformation along the subduction interface at depths of seismogenesis

Abstract

AbstractSubduction interfaces are loci of interdependent seismic slip behavior, fluid flow, and mineral redistribution. Mineral redistribution leads to coupling between fluid flow and slip behavior through decreases in porosity/permeability and increases in cohesion during the interseismic period. We investigate this system from the perspective of ancient accretionary complexes with regional zones of mélange that 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 the removal of fluid-mobile chemical components, whereas stronger sandstone blocks preserve veins that contain chemical components depleted in mudstones. These observations support local diffusive mass transport from scaly fabrics to veins during interseismic viscous coupling. Underthrusting sediments record a crack porosity that fluctuates due to the interplay of cracking and precipitation. Permanent interseismic deformation involves pressure solution slip, strain hardening, and the 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 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.