摘要
在青藏高原东北缘祁连山造山带至阿拉善地块之间完成了一条372km的大地电磁剖面,通过二维反演计算,获得了沿剖面180km深的壳幔电性结构模型,结合研究区地质和地球物理资料开展综合分析,研究结果表明:(1)剖面自南向北所经过的祁连山造山带、走廊过渡带和阿拉善地块对应3种壳幔电性结构模型:东祁连壳幔高-低-高阻似层状电性结构、河西走廊壳幔低阻带状电性结构和阿拉善南缘壳幔高-低-高阻层状电性结构.(2)剖面所经过的主要断裂带在电性结构上表现为低阻异常带或电性梯度带,并且止于中上地壳或消失于下地壳低阻层中.除这些分布于中上地壳的断裂系统以外,在下地壳至上地幔顶部还存在两条切割莫霍面的壳幔韧性剪切带:西华山北缘壳幔韧性剪切带和阿拉善南缘壳幔韧性剪切带.其中,西华山北缘壳幔韧性剪切带可能是1920年海原8.6级地震发生的深部背景之一;而阿拉善南缘壳幔剪切带可能是卫宁北山燕山晚期和喜山期幔源岩浆上升到地壳浅部或喷出到地表的通道,为在该区域寻找晚中生代至新生代含矿隐伏岩体提供了深部电性结构依据.(3)由若干形状不规则、彼此不相连的"碎块状"极高阻块体组成的中上地壳与"似层状"的中下地壳低阻层共同构成的地壳电性结构,是引起青藏高原东北缘强烈破坏性地震最佳的地壳电性结构组合之一.印度板块向欧亚板块俯冲碰撞楔入引起青藏高原块体向北东方向运移与阿拉善地块向南的俯冲碰撞楔入,是青藏高原东北缘强震活动带产生的动力学背景.
A 372 km long magnetotelluric sounding profile was completed from the east part of Qilianshan orogenic belt to the south margin of Alax block in the northeastern margin of the Tibetan Plateau.The crust and uppermost mantle electrical structural model along this profilewas obtained by two dimensional NLCG inversion calculation.Integrating the geological and geophysical data of the study area,the results revealed that(1)The Qilianshan orogenic belt,the corridor transition zone and the Alax block correspond to three types of crust and uppermost mantle electrical structural models which are the eastern part of Qilianshan high-low-high resistivity stratoid electrical structure,Hexi Corridor low resistivity belt electrical structure and the southern margin of Alxa block high-low-high resistivity layered structure,respectively.(2)The major faults that are crossed by the profile appear as apparent low resistivity anomaly belts or electrical gradient zones on electrical structure image,and most of them terminate in mid or upper crust,or disappear in low resistivity layer distributed in low crust.In addition to these mid and upper crust faults system mentioned above,there are two crust-mantle ductile shear zones located in low crust and uppermost mantle,which cut the Moho.They are the northern margin of Xihuashan crust-mantle ductile shear zone and the southern margin of Alax crust-mantle ductile shear zone,the former may be one of the deep backgrounds of the Haiyuan MS8.6 earthquake,and the latter may be the channel for mantle-derived magma upwelling to shallow crust or erupting to surface during late Yanshan and Himalaya in Weiningbeishan,which provide deep electrical structure evidence for searching the late Mesozoic to Cenozoic ore-bearing concealed rocks in this area.(3)The crustal electrical structure consisting of a number of irregularly shaped,unconnected"fragmented"extremely high resistivity blocks in mid-upper crust and stratoid low resistivity layer in mid-low crust is one of the optimal crustal structures for strong and destructive earthquakes occurrence in the northeast margin of Tibet Plateau.The northeast direction lateral expansion of the Tibetan Plateau caused by Indian plate subduction-collision-wedge towards the Eurasian plate and the southward subduction-collision-wedge of the Alax block together make up the dynamic background of the strong earthquake active zones in the northeastern margin of the Tibetan Plateau.