青藏高原东南部地壳导电性结构与断裂构造特征——下察隅—昌都剖面大地电磁探测结果

根据2004年在青藏高原东南部完成的下察隅—昌都(1000线)宽频带大地电磁探测剖面数据研究高原东南部地壳导电性结构及断裂构造特征,这有助于推进印度与亚洲岩石圈碰撞、俯冲构造模式的研究。研究结果表明,沿剖面上地壳大范围分布的是规模不等的高阻体,电阻率大约在90～3000Ω.m,厚度由南向北增加,底界面的深度大约在5～30km变化。高阻层之下发现由不连续高导体构成的中地壳低阻层,其电阻率小于10Ω.m;其结构与青藏高原中、西部的壳幔高导体相似,但规模小得多,底面埋深也浅得多。沿剖面的上地壳存在多组规模不等、产状不同的横向电性梯度带或畸变带,它们反映了沿剖面地区地壳的断裂分布。通过与该区高精度重力资料对比,在重要的电性梯度带上,均存在布格重力低异常和负重力均衡异常。结合区域地质资料分析推断了嘉黎—然乌、班公—怒江和甲桑卡—赤布张错等主要断裂构造带的空间格局。

The electrical structure and fault feature of crust of south-eastern Tibetan plateau-the result of magnetotelluric prospecting on profile from Xiachayu-Changdu

From broad band magnetotelluric data along a profile from Xiachayu to Changdu south-eastern Tibetan plateau,the electrical structure and the fault feature of crust of south-eastern Tibet plateau have been inferred,The results show that there are many high resistive bodies of varied sizes widely distributed in the upper crust along the profile,with resistivity arranging from 90 to 3 000 Ω·m;the lower surface of this high resistive layer is to the depth of 5-30 km;and its thickness increases from South to North.Beneath the high resistive layer,there exits a conductive layer with resistivity less than 10 Ω·m,which is composed of individual conductive bodies.Compared to the conductive bodies existing in the middle and northern Tibetan plateau,the scale of these conductive bodies is smaller,and the depth to the lower surface is shallower.Along the profile,there are a series of lateral electrical gradient zones or distortion zones of different scale and different attitude,indicating the distribution of faults in the upper crust.A comparison of the magnetotelluric data with gravity data shows that both gravity lows and negative isostasy gravity anomalies correspond to all the main electrical gradient zones.An integration of the magnetotelluric data and regional geology infers the locations and structures of the main zones of faults including the Jiali-Ranwu,Bangong-Nujiang and Jasangka-Chibuzhangcuo fractures.The study is of significance for better understanding collision and thrusting between the Indian and Asian plate.