­Forarc dehydration in warm subduction zones provides ample fluids at the depths of episodic slip and tremor

Abstract:

Feedbacks amongst petrologic and mechanical processes along the subduction plate boundary play a central role influencing plate boundary slip behaviors. However, the exact relationships between metamorphic reactions, fluid flow, and deformation mode remain unconstrained. We investigate the role of metamorphic devolatilization reactions in enigmatic slip behaviors of Episodic Tremor and Slip (ETS). ETS, composed of non-volcanic tremor and episodic slow slip events, has been observed below the seismogenic zone of relatively warm subduction zones for the past 20 years. Geophysical and geologic observations show that this portion of the subduction interface is fluid-rich, and many models for these slip behaviors invoke or require high pore fluid pressures. However, whether these fluids are sourced from local dehydration reactions in particular lithologies, or via up-dip transport from greater depths is not known. We present thermodynamic models of the petrologic evolution of four lithologies typical of the plate interface (average MORB, seafloor altered MORB, hydrated depleted MORB mantle, and metapelite) along predicted pressure–temperature (P-T) paths for the plate boundary at several warm subduction segments exhibiting ETS at depths between 25-65 km. The models suggest that 1-2 wt% H₂O is released at the depths of ETS in Jalisco-Colima, Guerrero, Cascadia, and Shikoku due to punctuated dehydration reactions within MORB, primarily through chlorite and/or lawsonite breakdown. These reactions produce sufficient in-situ fluid across this narrow P-T range to cause high pore fluid pressures. Forearc dehydration of hydrated peridotite is minimal for most margins, and metapelite releases H₂O (<1.5 wt%) gradually over a wide depth range compared to MORB. Punctuated dehydration of oceanic crust provides the dominant source of fluids at the base of the seismogenic zone in these warm subduction margins, and up-dip migration of fluids from deeper in the subduction zone is not required to produce ETS-facilitating high pore fluid pressures.