DEPTH OF THE DECOUPLING PLATE INTERFACE AND THERMAL STRUCTURE UNDER ARCS

Subduction proceeds as thrust movements at the boundary between the subducting plate and the rigid overlying plate. At depth below this mechanically decoupling plate interface, the subducting plate couples with the overlying mantle, and flow is induced in the mantle wedge. This induced flow is the main factor that controls thermal structure under arcs. Depth of this decoupling to coupling transition at the plate interface is estimated from observed surface heat flow in the northeastern Japan arc using a two-dimensional numerical simulation of thermal structure. The estimated depth is about 70 km. The termination depth of thrust earthquakes agrees well with the depth of the coupling transition, which indicates that these earthquakes occur at the decoupling plate interface. The mechanism of this decoupling plate interface is discussed in this paper. It is believed that water associated with the subducting oceanic crust reduces the frictional stress at the plate interface and thrust movement thus becomes possible. Petrological studies show that dehydration reaction in the subducting oceanic crust terminate at about 2.0 GPa and have little temperature dependence. Termination depths of thrust earthquakes estimated from seismic studies are nearly the same (60 approximately 80 km) for various arcs. This indicates that depth of the decoupling to coupling transition is controlled by the termination depth of dehydration reaction in the subducting oceanic crust. Across-arc heat flow profiles observed in various subduction zones show similar features; heat flow is very low in the forearc region and increases steeply at the volcanic front. Considering that the depth of the slab is about 100 km at the volcanic front for various arcs, this heat flow feature is closely related to the depth of the slab. Thus it is concluded that thermal structure under arcs is mainly controlled by the depth of the pressure dependent dehydration reactions in the subducting oceanic crust.