青藏高原地壳的三维密度结构和物质运动

应用区域重力场小波多尺度分析和反演于青藏区后,得到6个地壳等效层密度扰动图件,刻划了地壳三维密度结构,为研究地壳构造和物质运动提供了重要佐证.研究表明在青藏高原地壳内密度变化有以下三个规律.1)从上地壳到下地壳,平面分布上低密度区的分布范围逐渐扩大;在下地壳只有刚性克拉通地体才有显示高密度.2)从上地壳到下地壳,平面分布上密度扰动区的尺度逐渐扩大;到下地壳高或低的密度区不仅数量大为减少,而且边界更加清晰.3)从上地壳到下地壳,青藏高原南部的低密度带不断向北移动,反映印度陆块向欧亚大陆的向北俯冲.青藏高原下地壳密度高的克拉通地体有羌塘、柴达木和巴颜喀拉三个;而昆仑山、阿尔金山、祁连山和冈底斯地块都属于低密度的中新生代构造活动单元.拉萨地块也是低密度地块,在中下地壳它与冈底斯地块相连,应归属于中新生代构造活动单元.松潘甘孜地块在下地壳为低密度,但在上中地壳逐步变为高密度,并与巴颜喀拉克拉通地体连接.这种情况可能反映巴颜喀拉地体的上地壳随印澳板块俯冲向东南方向挤出.青藏高原低密度的物质也由下地壳向上挤出,在中上地壳体积迅速减小.由于下地壳低密度的物质向上挤出,中地壳密度高的克拉通地体会相应发生裂解,使克拉通地块的数目增加.高原北缘的下地壳低密度物质侧向挤出的枝杈有三支;其中一支从西昆仑到天山,另一支从龙门山西秦岭到银川盆地.第三支从高原南缘理塘到大理.它们可能反映下地壳管道流,宽度约180~300km.7级以上地震震中都位于下地壳低密度物质侧向挤出枝杈周围,可能与下地壳管道流位置吻合.

Three-dimensional density structure of the Tibetan plateau and crustal mass movement

The crust of the Tibetan plateau is the product of collision between the Eurasia and India plates. Though some geophysical profiles have been performed to reveal the lithospheric structure, but very few work on deep mapping of the whole plateau have been done. We study the density disturbance images of the plateau for the upper, middle and lower crust, respectively.#br#This paper uses the method of multi-scale wavelet analysis for regional gravity data processing with its application to the Tibetan plateau for the 3D crustal density structure. The method has been developed by our group, combining the multi-scale wavelet analysis, spectral analysis of potential fields and geophysical inversion of potential fields, producing density disturbance maps on planes of different depths.#br#The method produces six maps of density disturbance corresponding to different depths in the crust, providing important evidence for study of crustal structure and mass movement. The research results show three characteristics of the density disturbance in the study area as follows. (a) The distribution area of low-density expands gradually from the upper crust to lower crust, only rigid craton terranes in the lower crust are of high-density. (b) On the planes of fixed depths, the scale of density disturbances increases gradually from the upper crust to the lower crust, so the number of the high-or-low density anomalies decrease from the upper to the lower crust. (c) The southern low-density zone of the plateau moves gradually from the south to the north as depth increases, indicating the northward subduction of the India plate beneath Eurasia. In the Tibetan plateau, low-density mass squeezes upward from the lower crust like diapirs, resulting in terrane splitting in the upper crust. Low-density mass in the lower crust also squeezes laterally out in different directions, mainly containing three branches: one from Kunlunshan to Tianshan, another from Longmenshan to Yinchuan Basin, and the third in the southeast from Litang to Dali. They have widths about 180~300 km and may be corresponding to the so-called channel flows. Comparison between the possible channel flows and earthquakes shows that the Low-density branches coincide with earthquakes, meaning that the low-density zones in the lower crustal indicates areas of mass rheological motion, which may cause earthquakes.#br#The method of multi-scale wavelet analysis has been successfully applied to the Tibetan plateau, providing some new evidence for understanding the deep structure in the plateau.