Fluid and stress state at the base of the subduction seismogenic zone

Friday, November 9, 2018 - 3:30pm
ILC S131

The base of subduction seismogenic zones host slow slip events and tectonic tremor, phenomena that, despite extensive investigation, remain poorly understood. Slow slip in particular is thought to increase shear stress on the seismogenic portion of the plate boundary, potentially triggering earthquakes. However, because we do not understand the mechanisms responsible for slow slip and tremor, incorporating them into current models of seismic hazard is impossible. Because geophysical studies indicate this is a fluid-rich environment, high pore fluid pressure is proposed to control fault slip mechanisms, even though current constraints on the fluid environment of this area remain indirect and incomplete. We examine rocks from an exhumed subduction interface to quantify fluid pressures and stress state while also isotopically characterizing these pore fluids at subseismogenic conditions. Brittle and viscous deformation structures reveal near-lithostatic pore fluid pressures and low differential stresses that fluctuated in time. Triple oxygen isotopes indicate that pore fluids were drawn from the same source, consistent with surface waters modified by substantial water-rock interactions or metamorphic fluids liberated during dehydration. Together, these results support slow slip and tremor mechanisms which require or are enhanced by low differential stress and high pore fluid pressures.

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