METAMORPHIC DEHYDRATION FROM OCEANIC CRUST PROVIDES FLUID SOURCES FOR DEEP SLOW SLIP AND TREMOR IN SUBDUCTION ZONES

The occurrence and distribution of slip behaviors along the subduction plate interface depend on a range of parameters including pressure, temperature, stress, plate boundary structure and lithologies, and fluid content. Feedbacks amongst mechanical and chemical processes during subduction influence these parameters and can result in rheological switches. However, the exact relationships between these processes remain largely unconstrained. We investigate one of these potential feedbacks in the form of metamorphic devolatilization reactions’ relations to 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 data 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 typical subduction lithologies (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 Nankai 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.