Tremor and Slip: Subducting Plate Fluids Channeled to Forearc Mantle Corner

Abstract:

In three well-studied subduction zones, Cascadia, SW Japan, and S. Mexico, there is a strong continuous band of slow slip and associated seismic tremor, approximately parallel to the margin. At all three, the subducting oceanic crust is young and hot such that we expect strong fluid expulsion at fairly shallow depth. In each, the landward limit of the megathrust seismogenic zone is concluded to be thermally controlled to an unusually shallow depth and there is a gap of 30-70 km downdip between the seismogenic zone and the main Episodic Tremor and Slip (ETS); there is not a continuous transition from unstable to conditionally stable sliding. Seismic rupture occurs mainly offshore, ETS lies onshore. To explain the location, we find that the ETS band lies over the forearc mantle corner in each of these subduction zones and argue that rising fluids responsible for ETS are focused to rise above this corner. Hydrated minerals in the subducting oceanic crust and uppermost mantle are dehydrated with down-dip increasing temperature, and seismic tomography data indicate that the fluids have serpentinized the overlying forearc mantle. Laboratory data show that such serpentinite has low permeability and likely blocks vertical expulsion. Fluid flow beneath the forearc mantle is restricted to updip in the underlying permeable oceanic crust and subduction shear zone. At the forearc mantle corner these fluids are released upward into the more permeable overlying forearc crust. Indications of this fluid flux comes from, (a) high electrical conductivity just above the corner seen in magnetotelluric data, (b) low Poisson’s Ratios (and Vp/Vs) found above the corner that may be explained by a concentration of silica which has exceptionally low Poisson’s Ratio. The fluids should be silica saturated and precipitate silica with decreasing temperature and pressure above the corner. We note several other possible implications of the large amounts of fluid expelled above the corner. (1) The role of these fluids in the formation of large concentrations of quartz veins seen in outcrop sections. Very large amounts of fluid are required since the solubility of quartz is small. (2) A role of these focused fluids in the formation of mesothermal gold; huge amounts of focused fluid are inferred to be required, with as yet no satisfactory proposed source.