羌塘地块中部南北向断裂的构造特征及其动力学意义

青藏高原内部除大规模的东西向走滑断裂以外,另一个显著的地质特征就是在藏南及高原腹地广泛发育东西向的伸展构造,形成走向近南北的断裂构造,如亚东一谷露裂谷带及双湖断裂.伸展构造已经成为青藏高原地质研究的一个焦点问题.在羌塘地块89°E附近存在明显的低重力、负磁、深度达300 km的低速异常及连通壳幔的高导异常,且地表伴生大规模的新生代火山岩,这些特殊的地质及地球物理场特征的发生位置与地表双湖断裂的位置基本对应.本文通过卫星重力数据的多尺度小波分析结果发现,双湖断裂之下,存在一明显由上地壳一直向下延伸至地幔深部的低重力异常,说明双湖断裂向下延伸深度大,且上下连通性好.结合已有的地质和地球物理资料,认为由于双湖断裂的存在,使得深部幔源岩浆沿断裂构造薄弱带上涌,从而导致羌塘地块之下壳幔温度的升高及大规模部分熔融的发生.

Structural characteristics and their dynamic implications for the north-south trending rifts in central Qiangtang terrane,Tibetan plateau

In addition to large-scale east-west trending strike-slip faults, north-south trending rifts are widely distributed in the southern and central Tibetan plateau, such as the Yadong-Gulu rift at about 90°E in the Lhasa terrane, and the Shuanghu rift at about 89°E in the Qiangtang terrane. These north-south trending rifts are closely related to the formation mechanisms of the Tibetan plateau. Existing geophysical results show that the medium beneath the Shuanghu rift is of low-density, negative magnetism, high-conductivity from the middle crust to the upper mantle, and low-velocity extending from the crust down to about 300 km depth in the central Qiangtang terrane (89°E). Bouguer anomaly gravity data, analyzed by the multi-scale wavelet method, were used to invert the crustal structure for the Shuanghu rift in this study. The obvious low-density anomalies beneath the Shuanghu rift suggest that this north-south trending rift is a large-scale fault and acts as a tectonic weak zone associated with upwelling of mantle-derived magma. Such upwelling of mantle-derived magma is likely to result in the widespread partial melting in the mid-to-lower crust of the northern Qiangtang terrane, and further provides a reasonable explanation to the special geophysical features in this area. Combined the Bouguer gravity data and existing seismic data, it may be inferred that the Shuanghu rift was produced by the delamination of the Indian mantle and the upwelling of asthenosphere.