Dynamic tomography and upper mantle rheology

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Dynamic responses of the crust and upper mantle to the asthenospheric convective flow related to hot mantle diapiric upwelling were numerically evaluated in order to examine the implications for the geological phenomena such as rifting, back-arc spreading, sedimentary basin and mountain building. For the island arc areas such as the Japanese Islands and for the ridge subducting areas such as Basin and Range region, where both lower crust and upper mantle may behave as ductile layers in some evolutional stages, the mechanical coupling of convections in two layers seems to be very important for examining the physical mechanism of these geological phenomena. Surface dynamic response to mantle diapiric upwelling is active until the whole crust effectively behaves as an elastic lithosphere. For this rheological state, surface uplift is predicted although its magnitude depends on the thickness of lithosphere and on the spatial scale of mantle plume. However, the effective viscosity of the lo … More wer crust may decrease with increasing to duration of hot mantle diapiric upwelling. If the effective viscosities of these two layers are 10^<19> - 10^<21> Pa s, the asthenospheric convective flow induced by mantle diapiric upwelling with the spatial scale of radius 50-100 km, which has been seismically observed in the upper mantle for the Japanese Islands, may laterally squeeze out the lower crustal material followed by surface subsidence of 500-1000 m and by upward migration of Moho discontinuity of several to 10 kilometers in the geological time scale of 1-10 Ma. Thus the coupling between the convections in the lower crust and upper mantle is mechanical, and the lower crustal material is dragged by the shear stresses operating at the base of lower crust. In this process the space originally occupied by lower crustal material is replaced by mantle material, leading to the thinning of lower crust. This convective coupling in two layers is mainly attributed to the relaxation modes due to the density jump of about 500 kg/m^3 at Moho discontinuity. Thus in some geological conditions having hot mantle plume beneath the crust, the transition from active to passive rifting can be expected as the result of change of convective coupling state between the lower crust and upper mantle. Less