INFLUENCE OF DUCTILE DEFORMATION MECHANISMS ON SUBDUCTION ZONE SEISMICITY AND FLUID FLOW

Field observations of rocks deformed along the subduction interface and flow laws for grain scale deformation mechanisms together support a conceptual model where the seismogenic zone of megathrusts along sedimented margins is controlled at the updip end by the onset of visible diffusive mass transfer in fine-grained lithologies (DMT) and at the downdip end by the temperature at which plate motions can be accommodated by steady DMT deformation. This model predicts that coarser-grained sandstones deform by cracking throughout the seismogenic zone but would experience visible dislocation creep strain at the downdip end. The field areas are located within the Kodiak accretionary complex and the Shimanto belt of Japan. These terranes expose rocks that were accreted over tens of millions of years along convergent plate boundaries that are currently active. Ten regionally extensive mélanges from these areas were accreted at depths of seismogenesis during subduction of a sediment-rich plate similar to the Cascadia and Nankai margins today. There are generally two different types of behavior recorded by all of these localities: 1) wide (10s to 100s of meters) zones of tectonic mélange that record DMT-assisted noncoaxial strain, producing an anastomosing scaly fabric in mudstones and a pervasive network of veins in sandstone blocks, and 2) narrow (5-20 meters) zones of cataclasis at the top of, or within the melange with localized stringers of ultracataclasite and pseudotachylite. Our model supports the idea that the wide zones record noncoaxial strain during the interseismic period at rates inadequate for the accommodation of plate motions, leading to a slip deficit. The narrow zones of cataclasis record the accumulation of coseismic events. Since deformation in the footwall of the plate interface involves pressure solution, low grade metamorphic reactions, and local redistribution of silica from scaly slip surfaces to cracking blocks with pervasive veins, this process of silica redistribution is a fundamental driver of the interseismic strengthening and permeability reduction that may be critical for earthquake dynamics. Deformation mechanisms thus likely play a role in the changes of state (cohesion, permeability) that control the updip and downdip transitions of the seismogenic zone.