用重力资料研究青藏高原东北缘深部岩石圈形变特征

Lithospheric Deformation Features in the NortheasternEdge of the Tibetan Plateau Obtained from Gravity Data

青藏高原东北缘位于青藏、鄂尔多斯和阿拉善3个块体的结合部位,地形地貌复杂、构造活动强烈、地震异常活跃。祁连山断裂带是青藏高原的东北边界,现代构造活动和变形比较集中,是研究岩石圈动力学最典型的地区。作者在研究区切了穿过祁连山断层并与其走向相垂直的4条测线。对于每条测线,利用二维弹性板理论,通过调试岩石圈有效弹性厚度Te,试验了横向不均匀弹性刚度的单一弹性板模型和有断点的2个独立弹性板模型,得到的结论是变刚度单一弹性板模型与我们的观测布格重力异常值吻合得很好。说明祁连山断层并没有穿过莫霍面进入岩石圈上地幔,是地壳内大断裂,即青藏高原内部构造单元。Te值由西南向东北呈现出逐渐增大的趋势,这应当与3个块体在板块热构造、岩石圈物质组成、构造活动性等方面的差异有关。

The northeastern edge of the Qinghai-Tibetan Plateau lies in the convergence zone of the Tibetan block,Ordos block and Ala-Shan Block.The topography is quite complicated,and the tectonic movement is active,with abnormally active earthquakes.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 lithoshphere dynamics study.We project four profiles across the Qilian Shan Fault,basically vertical to the strike of the fault.For each profile,on the basis of 2D elastic plate theory, by modifying the effective elastic thickness of the lithosphere,we test both one continuous plate and two separate plates with a broken point beneath the study area,the effective elastic thickness being allowed to vary laterally.Our conclusion is that the continuous plate model can well explain the observed Bouguer gravity anomalies,while the broken plate model cannot.The result indicates that the Qilian Shan Fault does not cut through the Moho discontinuity into the upper mantle of the lithosphere,and is an interior tectonic unit of the Tibetan Plateau.The values of the effective elastic thickness gradually increase from southwest to northeast,which should be relevant to the large differences in the thermal structure of the blocks,the lithospheric compositions and the block tectonics etc.