Effects of frictional melting on seismic slip in subduction-accretion complexes: Insights from high-velocity friction experiments and natural pseudotachylytes

Discovery of pseudotachylytes from exhumed accretionary complexes indicates that frictional melting occurred along illite-rich, argillite-derived slip zones during subduction earthquakes. We conducted high-velocity friction experiments on argillite at a slip rate of 1.13 m/s and normal stresses of 2.67-13.33 MPa. Experiments show the slip weakening followed by the slip strengthening. During the slip-weakening phase, discontinuous melt patches formed at high temperature (~1100 degrees Celsius). The development of melt patches is transient and immediately followed by a formation of a continuous melt layer, most likely due to a high content of least refractory minerals (illite). The formation and shearing of low viscosity melt patches as well as flash heating and thermal decomposition of clay minerals could contribute to the slip-weakening. The subsequent slip strengthening corresponds to the viscous shear of a continuous film of melt. During the strengthening phase, the shear strain rate decreases in association with a progressive widening of the melt layer, implying an increase in the effective viscosity of the frictionally generated melt. The viscosity increase during the slip strengthening is most likely due to dehydration of the melt layer and the adjacent solid (the “dark layer”) that may be subsequently incorporated in the melt layer by thermal erosion. Such dehydration may be specific to clay mineral (illite)-dominated argillite. Our experimental results imply that the frictional melting at shallow depths in subduction-accretion complexes may lead to suppression of seismic slip due to viscous braking if a substantial melt dehydration occurs on a timescale of seismic slip. On the other hand, our experimental data also suggest that the ratio of viscous shear to normal stress progressively decreases with depth, and may eventually become less than the friction coefficient of the fault zone material at greater depths. Compared to experimentally generated pseudotachylytes, argillite-derived natural pseudotachylytes formed at seismogenic depths within subduction-accretion complexes are more hydrous, presumably due to water-saturated environment, relatively impervious fault zone rocks, and longer slip duration in our experiments compared to the typical earthquake rise times. Experimental data combined with field observations suggest that viscous braking could be possible at shallow depths, while melt lubrication is more likely at greater depths; a transition from melt lubrication to viscous braking may be one of the factors controlling the updip limit of the seismogenic zone in subduction-accretion complexes.