Dynamic tomography and upper mantle rheology

通过数值模拟分析了地壳和上地幔对与热地幔底辟上升流相关的软流圈对流的动力响应，探讨了其对裂陷、弧后扩张、沉积盆地和造山等地质现象的影响。对于像日本列岛这样的岛弧地区和像盆地、山脉这样的下地壳和上地幔在某些演化阶段都可能表现为韧性层的山脊俯冲区来说，两层对流的力学耦合对于研究这些地质现象的物理机制似乎是非常重要的。地表对地幔底辟上涌的动力响应是活跃的，直到整个地壳有效地表现为弹性岩石圈。对于这种流变状态，可以预测地表隆升，但其幅度取决于岩石圈厚度和地幔柱的空间尺度。随着热地幔底辟上涌时间的延长，地壳的有效黏度降低。如果这两层的有效黏度在10^<19> ~ 10^<21 >pa s，则日本列岛上地幔地震观测到的50 ~ 100 km空间尺度的地幔底流上涌引起的软流层对流可能在1 ~ 10 Ma地质时间尺度上侧向挤压下地壳物质，引起500 ~ 1000 m的地表沉降和数~ 10 km的莫霍面向上迁移。因此，下地壳和上地幔对流之间的耦合是机械的，下地壳物质受到下地壳底部剪切应力的牵引。在这个过程中，原来由下地壳物质占据的空间被地幔物质所取代，导致下地壳变薄。两层间的对流耦合主要是由于莫霍不连续处约500 kg/m^3的密度跃变引起的弛豫模式。因此，在某些地壳下存在热地幔柱的地质条件下，下地壳和上地幔之间的对流耦合状态发生了变化，可以预期由主动裂陷向被动裂陷的转变。少

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