Convective coupling between lower crust and upper mantle

为了研究岛弧表面地质现象与下地壳和上地幔动力学之间的关系，本文对日本东北部岛弧上地幔密度非均质引起的地壳和上地幔响应与地震层析成像预测震级进行了评价。对于低黏度下地壳和低黏度上地幔的流变模型，低密度下地壳物质在地幔底流上升流上受到软流圈剪应力的机械牵引，由弧后角向弧前角流动。在5 ~ 50 Ma的地质时间尺度上，该过程的地表沉降约为300m，在地幔底辟之上可形成与地幔底辟横向规模大致相同的沉积盆地（“S”期）。岛弧内的俯冲板块将对下地壳物质的横向运动起到屏障作用。随着这一过程的进行，在覆岩板块的边缘可能会出现更多低密度下地壳物质的堆积。在弧前区地幔楔压力的关键阶段，高温下地壳物质可能沿构造线暴露（“E”阶段）。由于下地壳物质沿构造线排出，下地壳压力将降低，沉积盆地将加速沉降并伴有火山活动。日本西南部白垩纪地质现象可以用这一物理机制及其与地表地壳运动和下地壳物质流动的联系来解释。在甘门沉积盆地观测到的140 ~ 120 Ma的相对缓慢的沉积可以解释为“S”阶段的地质事件。约110Ma以来，甘门沉积盆地补给了大量的火山沉积物，同时经历了扩张与突然沉降的过程。与此同时，甘蒙群以南地区出现了低压-高温变质作用和花岗岩活动（Ryoke带）。这些沿中构造线（MTL）的地质现象可以解释为与“E”阶段相对应的地表地质事件。然而，强调这种耦合的时间和空间尺度是由下地壳和上地幔的流变结构和地幔底辟的尺度决定的。少

In order to examine the relationship between surface geological phenomena in island-arcs and lower crustal and upper mantle dynamics, crustal and upper mantle responses induced by the upper mantle density heterogeneities with the magnitude expected from the seismic tomography beneath northeastern Japan arc have been evaluated. For the rheological model with a low-viscosity lower crust and low-viscosity uppermost mantle, low-density lower crustal material above mantle diapiric upwelling is mechanically dragged along by the asthenospheric shear stresses, and then lower crustal material flows from the back-arc corner to fore-arc corner. The surface subsidence for this process is about 300m on the geological time scale of 5-50 Ma, and formation of sedimentary basin with about the same lateral scale as mantle diapir can be expected in the area above the mantle diapir (stage "S"). The subducted slab in island-arc will operate as a barrier to the lateral movement of lower crustal material. Th … More en accumulation of low-density lower crustal material may occur at the edge of the overriding plate as the progress of this process. At a critical stage of the pressure in the mantle wedge of fore-arc region, high temperature lower crustal material may be therefore exposed along the tectonic line (stage "E"). As the result of discharging lower crustal material along the tectonic line, the pressure in the lower crust will be decreased and the accelerated subsidence accompanied by volcanism is expected in the sedimentary basin. The Cretaceous geological phenomena in Southwest Japan may be explained by this physical mechanism and its linked surface crustal movement and the flow of lower crustal material. A relatively slow deposition in the period of about 140-120 Ma observed in the Kanmon sedimentary basin may be interpreted as the geological event of stage "S". From about 110Ma, the Kanmon sedimentary basin was supplied with a large quantity of volcanic sediments, and simultaneously expanded and suddenly subsided. At about the same time, low pressure-high temperature metamorphism and granitic activity (Ryoke Belt) occurred along the areas to the south of the Kanmon Group. It may be possible to interpret these geological phenomena along the Median Tectonic Line (MTL) as the surface geological events corresponding to stage "E". It is, however, stressed that the time and spatial scale for this coupling is determined by the rheological structure of the lower crust and upper mantle and by the scale of mantle diapir. Less