中国大陆现今构造应变率场及其动力学成因研究

通过分析中国大陆地壳运动GPS速度场得到现今构造应变率场。结果显示在印度板块北向推挤作用下 ,青藏高原内部及其邻域形变场并不局限于少数大型走滑断裂 ,而是在大范围内广泛分布 ,各地区构造运动驱动机制也可能各有不同。藏南地区主应变率场呈均衡的约 2× 10 -8a-1南北向挤压和东西向拉张 ,显示印度板块下插造成的地壳增厚和岩石圈拆离可能形成上地壳与上地幔间形变解耦 ,地壳内部在南北向挤压及重力场作用下产生东向塑性流驱使上地壳产生东西向拉张。西藏中部羌塘地区主应变率场显示均衡的约 2× 10 -8a-1北北东向挤压和北西西向拉张 ,反映本地区一系列走向北东和北西的共轭剪切断裂的活动 ,可能源于南北向挤压和软流层内东向塑性流的驱动。柴达木盆地及周边地区主应变率场呈约 2× 10 -8a-1北东向压缩和约 (0 1)× 10 -8a-1北西向拉张 ,表明地壳增厚造成的地壳温度上升可能还不足以造成上下地壳的充分解耦 ,南北向的消减还未能有效地转换成东西向的拉张 ,形变以褶皱和逆冲断裂运动为主。当今青藏高原形变场的形成应是构造运动从南到北阶段性发展过程中地壳与上地幔介质性质差异造成驱动机制不同的结果。

CONTEMPORARY TECTONIC STRAIN RATE FIELD OF CHINESE CONTINENT AND ITS GEODYNAMIC IMPLICATIONS

We obtain contemporary tectonic strain rates of Chinese continent from analyzing crustal motion inferred from GPS velocity field. Our result shows that driven by the northward indentation of the Indian plate, the Tibetan plateau and its vicinity areas undergo extensive deformation, rather than concentrated along narrow deformation zones of a few large-scale strike-slip faults delineating the plateau boundaries; this deformation field is broadly distributed within the plateau, driven by possibly different mechanisms from region to region. The strain rate field in southern Tibet appears to be north-south compression and east-west extension, at the rate of about 2×10 -8 a -1 for both components. This could be the result of crustal thickening and lithospheric delamination, resulting in decoupling of the upper crust from the upper mantle.This could produce eastward ductile flow in the lower crust responding to the northward compression from the Indian plate, causing east-west tension in the upper crust.The strain rate field in the Qiangtang region, central Tibet, also shows nearly equal amount of NNE compression and NWW extension at about 2×10 -8 a -1 , reflecting active faulting along a group of conjugate strike-slip faults oriented northeasterly and northwesterly, possibly resulted from the north-south compression and eastward ductile flow in the asthenosphere. The Qaidam basin and its vicinity have a strain rate field of about 2×10 -8 a -1 compression in northeast direction and (0~1)×10 -8 a -1 extension in northwest direction.This pattern suggests that a temperature increase in the crust resulted from crustal thickening is not enough to cause full decoupling between the upper and lower crust, thus it is less efficient to accommodate the north-south shortening through east-west extension (i.e. ductile flow in the lower crust) than through folding and thrusting along faults (i.e. deformation along narrow weak zones). We conclude that contemporary deformation field of the Tibetan plateau has been the result of progressive tectonic development from south to north, causing systematic differences in regional lithospheric structures,which affect the driving mechanisms of crustal deformation consequently.