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Dynamic tomography and upper mantle rheology

动态断层扫描和上地幔流变学

基本信息

批准号：
03640373
负责人：
NAKADA Masao
金额：
$ 0.77万
依托单位：
Kumamoto University
依托单位国家：
日本
项目类别：
Grant-in-Aid for General Scientific Research (C)
财政年份：
1991
资助国家：
日本
起止时间：
1991 至 1992
项目状态：
已结题

项目摘要

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

通过数值模拟，研究了地壳和上地幔对与热地幔底辟上涌有关的软流圈对流的动力学响应，以探讨其对裂谷作用、弧后扩张、沉积盆地和造山运动等地质现象的意义。对于岛弧区（如日本列岛）和脊俯冲区（如盆岭区），下地壳和上地幔在某些演化阶段都表现为韧性层，两层对流的力学耦合对于研究这些地质现象的物理机制是非常重要的。地幔底辟上涌的地表动力学响应是活跃的，直到整个地壳有效地表现为弹性岩石圈。在这种流变状态下，地表抬升是可以预测的，但其幅度取决于岩石圈厚度和地幔柱的空间尺度。然而，低聚物的有效粘度 ...更多信息随着热地幔底辟上涌持续时间的延长，地壳厚度可能减小。如果这两层的有效粘度为10^<19>-10 ^Pa·s，则在日本列岛上地幔中观测到的半径为50-100 km的地幔底辟上涌所引起的软流圈对流<21>，在1-1000 m的地质时间尺度内，可能侧向挤出下地壳物质，地表沉降500-1000 m，莫霍面向上迁移数公里至10公里。十马。因此，下地壳和上地幔对流之间的耦合是机械的，下地壳物质被作用在下地壳底部的剪应力拖曳。在这一过程中，原下地壳物质占据的空间被地幔物质所取代，导致下地壳减薄。这种两层对流耦合主要是由于莫霍面处约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，由于下地壳与上地幔对流耦合状态的改变，可以预期主动裂谷作用向被动裂谷作用的转变。少