Lithospheric Deformation Features In The Northeastern Edge Of The Tibetan Plateau From Gravity Data

The northeastern edge of the Tibetan Plateau lies in where the Tibetan block, Ordos block and Ala-Shan Block converge. The topography is very complicated, and the tectonic movement is very strong, with abnormally active earthquakes. It is quite significant to study the deep geophysical features of the northeastern Tibetan Plateau with the method of the lithosphere dynamics. And in this method, the effective elastic thickness (ab. Te) is a pivotal parameter, and it is a useful measure of the integrated strength of the lithosphere to resist the flexural deformation. The values of Te are related to and controlled by many factors, and it reflects a dynamic balance relationship among the topography, plate boundary forces and lithospheric structure. On the basis of lithosphere dynamics, it is possible to deduce whether the boundary fault cuts through the Moho into the upper mantle of the lithosphere or not, and it is also possible to get the distributions of the values of Te in the study area by gravity tomography. Therefore, it helps to further understand the deep lithospheric geodynamical features in this study area.The Qilian Shan fault is the northeastern boundary of the Tibetan Plateau, with strong modern tectonic activities and deformation. Therefore the Qilian Shan fault is a typical area for lithosphere dynamics study. Four profiles across the Qilian Shan Fault are projected, basically vertical to the strike of the fault. The 2D elastic plate theory mainly depends on the relationship of the topography to the Bouguer gravity anomalies. And on the basis of this theory, the size of the effective elastic thickness of the lithosphere is modified, and both one continuous plate model and two separate plates model with a broken point beneath the QilianShan Fault are tested. The Bouguer gravity anomalies predicted by the Airy model are shown for reference. And the effective elastic thickness is allowed to vary laterally. The result is that the Bouguer gravity anomalies predicted by the continuous plate can well explain the observed Bouguer gravity anomalies, while the Airy model and the broken plate cannot. The result indicates that the Qilian Shan Fault does not cut through the Moho into the upper mantle of the lithosphere, and it is an interior tectonic unit of the Tibetan Plateau. The values of the effective elastic thickness gradually increase from the southwest to the northeast in the study area, ranging from 23km to 49km. In the Tibetan block, the rigidity of the lithosphere is relatively weak, while in the other two blocks the rigidity is moderate. The conclusion should be relevant to the large differences in the thermal structure of the blocks, the lithospheric compositions and the block tectonics and so on between the Tibetan block and the Ala-Shan block or the Ordos block.