DEFORMATION OF THE CRUST AND UPPER-MANTLE BENEATH THE BACK-ARC CAUSED BY THE SUBDUCTING PLATE

We study the deformation of the crust and upper mantle beneath the back-arc region caused by the movement of the subducting plate based on simple 2-D fluid dynamical models. Time evolution of the amount of the crust dragged into the mantle is calculated as a function of the viscosity of the crust and mantle. If the viscosity of the crust is 1/100 times less than that of the mantle, the ''subduction'' of the crust is suppressed because of the occurrence of the secondary flow within the crust. However, for a crust with temperature-dependent viscosity model, the decoupling-zone, whose viscosity is less than that of the surrounding mantle by a factor of 1/10 to 1/100, along the subducting plate is found to be necessary to keep the back-arc crust intact. The buoyancy of the crust is not the essential controlling factor of the fate of the back-arc crust. The deformation ellipse of the lower crust is stretched horizontally by a factor of ten during 10 m.y., and is larger than those of the other places. This result suggests that reflection planes, found by the seismic exploring, at the depth of 20-40 km may be caused by the flow-induced deformation of the heterogeneous rocks in the lower crust.