CASCADIA SUBDUCTING PLATE FLUIDS CHANNELLED TO FOREARC MANTLE CORNER: ETS AND SILICA DEPOSITION

In this study we first show that on the Cascadia subduction thrust, there is a 50 km gap downdip between the seismogenic zone and the Episodic Tremor and Slip (ETS). There is not a continuous transition from unstable to conditionally stable sliding. Seismic rupture occurs to a depth of 15-20 km, mainly offshore for this hot subduction zone. ETS lies at a depth of 30-35 km, mainly onshore. We then argue that that ETS is generated by rising fluids that are focussed above the forearc mantle corner. There is a remarkable correspondence between the center of ETS and this corner along the whole margin. Hydrated mineral assembledges in the subducting oceanic crust are dehydrated with down-dip increasing temperature, and seismic tomography data indicate that these fluids have strongly serpentinized the forearc mantle. Laboratory data show that such forearc mantle serpentinite has low permeability and updip flow may be restricted to the underlying permeable oceanic crust. At the forearc mantle corner these fluids are released upward into the more permeable overlying forearc crust. An indication of this fluid flux comes from low Poisson’s Ratios found above the corner that may be explained by a concentration of silica which has exceptionally low Poisson’s Ratio. The rising fluids should be silica saturated and precipitate silica with decreasing temperature as they rise above the corner. A similar correspondence of the forearc mantle corner and ETS is observed for SW Japan and Mexico.