SPECTRUM OF MEGATHRUST SLIP BEHAVIOR RECORDED WITHIN ANCIENT ACCRETIONARY COMPLEXES: THE ROLE OF SILICA REDISTRIBUTION

Mechanical behavior of the subduction interface reflects the relationship between physical processes, such as fracturing and fluid flow, and chemical processes, such as pressure solution and mineral redistribution. These relationships are informed by observations from shear zones in the Kodiak and Shimanto ancient accretionary complexes that are consistent with a record of slow slip and quasi-dynamic fault motion, including: 1) scaly shears that record diffusive mass transfer and linear viscous flow; 2) repeated antitaxial and syntaxial cracking and sealing, with cyclic behavior rather than continuous creep; 3) distributed shearing on many slip surfaces, with strain hardening on individual features; and 4) small crack apertures suggesting repeated low stress drop failure events. Development of scaly slip zones is a weakening mechanism due to loss of cohesion, but each slip surface subsequently hardens because of increases in normal stress associated with hydrofracturing or by the activation of a hardening mechanism such as pressure solution. A kinetic model is constructed to estimate the time required to seal a single fracture event and the processes that dictate slip speed and rupture propagation. Vein sealing is driven by diffusive redistribution of Si from solid-solid surfaces to undersaturated cracks. Our calculations predict that cracks heal on secular time scales, and that temperature exerts a primary control on the rate of crack-seal deformation. Rates are likely faster due to contributions from salinity, and from reactions, including the breakdown of albite to illite within scaly shear zones and the growth of albite within open cracks. We propose a model for propagating, slow ruptures that move at rates dictated by shear processes within a zone of finite thickness. Stress rises at the front of a propagating slow slip instability, leading to plastic failure along scaly slip surfaces in the footwall. Development of scaly folia causes initial weakening, but slip surfaces harden due to progressive increase in contact area across the fracture mesh. Slow slip events are thus analogous to Volterra dislocations in crystals or self-healing slip pulses, a distinction that could explain the difference in scaling laws for slow slip events and regular earthquakes.