SZ4D International Webinars: Five things about the cold forearc mantle wedge
Kelin Wang - Pacific Geoscience Centre, Geological Survey of Canada
November 20th, 2020 at 11 AM PDT
The forearc mantle wedge plays a critical role in the geodynamics of subduction zones. From five perspectives, I will highlight its thermal, petrologic, and mechanical states and how it affects megathrust slip behaviour. (1) Heat flow data and seismic imaging indicate that the forearc mantle wedge is cold, referred to as the “cold nose”, in sharp contrast with the hot arc-backarc region. With fluids supplied by the dehydrating slab, hydrous minerals form in the ultramafic cold nose. (2) Maintaining the cold and stable thermo-petrologic state requires the forearc wedge to be fully decoupled from the subducting slab and does not participate in mantle wedge flow. This is supported by the lack of seismic anisotropy in the cold nose as inferred from local-earthquake shear wave splitting analysis in the Japan Trench subduction zone. (3) The cold state gives rise to high stiffness, affecting postseismic deformation following megathrust earthquakes. The mechanical contrast of the cold nose with the rest of the mantle wedge is clearly reflected in geodetically observed postseismic uplift between the cold nose and the volcanic arc. (4) In warm-slab subduction zones, a very high degree of serpentinization of the tip area of the mantle wedge is expected to diminish permeability. The hydrological consequence is inferred to foster a geological condition for episodic tremor and slip (ETS) downdip of, but separated from, the megathrust seismogenic zone. (5) In colder-slab subduction zones, serpentinite derived from the base of the forearc mantle wedge affects the mechanics of the megathrust fault zone, resulting in complex seismogenic behaviour downdip of the Moho-megathrust intersection. I will use the 2010 M=8.8 Maule, Chile, earthquake as an example to show how this process may affect coseismic slip, stress drop, and aftershock distribution under the specific P/T condition in this area.
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