A PRESSURE SOLUTION FLOW LAW FOR THE SEISMOGENIC ZONE: APPLICATION TO CASCADIA

The Shimanto belt of Japan and the Kodiak Accretionary complex in Alaska expose rocks that were deformed during underthrusting along the subduction interface at conditions characteristic of the seismogenic zone. In these cases, interseismic strain is accommodated by simple shear in mudstones through slip on an anastomosing network of striated surfaces, which forms a scaly fabric. The dominant deformation mechanism in these systems is dissolution-precipitation creep, with diffusive mass transfer from slip surfaces to cracks. To investigate the impact of this ductile deformation on the evolution of slip deficit in subduction zones, we leverage a linear viscous constitutive relationship derived from natural mudstones (Kenis et al., 205) to derive a diffusive mass transfer (pressure solution) flow law that includes solubility as a function of pressure/temperature, grain size, and activation energy. This flow law is general and can be applied to any tectonic environment, but it is particularly well suited for making predictions about the interseismic behavior of subduction zones—an environment where the sedimentary inputs, plate kinematics, slip deficit, and the coupling ratio can be observed or constrained. We apply the flow law to the Cascadia subduction zone, where the thermal structure, geometry, relative plate velocity, and GPS velocity field is known. We find that the flow law can explain the observed tapering of the slip deficit accumulation rate in the forearc of the margin, with intriguing potential explanations for details of the tapering such as increasing grain size near the downdip end of the seismogenic zone or decreasing width of the plate boundary shear zone. The process of diffusive mass transfer from scaly fabrics to cracks likely plays more than a passive role (i.e., reducing the slip deficit with increasing temperature) by promoting interseismic strengthening and permeability reduction.