WHY DO SUBDUCTION THRUSTS LOCK? EVOLVING EFFECTIVE GRAIN SIZES MODULATE CREEP RATES THROUGH PRESSURE SOLUTION (Invited Presentation)

Pressure solution accommodated creep (herein referred to as pressure solution) is an important deformation mechanism within subducting terrigenous sediments. Pressure solution occurs through dissolution of quartz in regions of high stress, diffusion through an intragranular fluid, and (often) reprecipitation in a region of lower stress. Exhumed subduction mélanges exhibit strong pressure solution fabrics within the seismogenic zone, particularly within meta-sedimentary sequences, and fabric strength increases with temperature of deformation. Models of pressure solution show that plate-rate deformation can be accommodated within the seismogenic zone when the grain size of quartz is less than ~100 µm, greater than typical quartz grain sizes in silt to shale. Pressure solution should therefore be able to accommodate all plate-rate motion within the seismogenic zone, but the distribution of subduction zone seismicity demonstrates that it does not. Here, we examine the role of an evolving “effective grain size” in changing the diffusive distance, and therefore the rate of pressure solution, through time in case studies of the Mugi mélange and the Kodiak accretionary complex. Effective grain sizes decrease through cataclasis or by precipitating fine grained silica between micas. These processes increase the rate of pressure solution over short timescales and may explain elevated creep rates preceding large magnitude earthquakes and some post-seismic creep. Effective grain sizes increase by closing diffusive pathways through silica precipitation (silicification), by pore throat closure due to grain shape flattening, or by depleting the sediments of silica through advective fluid transport. These processes inhibit the rate of pressure solution over long timescales causing the plate interface to lock. We propose that increasing the effective grain size within sediment-dominated subduction interfaces is a prerequisite for locking subduction thrusts and initiating an earthquake cycle.