西秦岭造山带(中段)及其两侧地块深部电性结构特征

Deep electrical structure of the central West Qinling orogenic belt and blocks on its either side

本文对跨过西秦岭造山带(中段)的阿坝—若尔盖—临潭—兰州大地电磁剖面(WQL-L1)所采集到的数据进行了精细化处理分析和二维反演研究,结合跨过2013年岷县漳县地震区的WQL-L6剖面大地电磁探测结果和以往的地质与地球物理资料,对西秦岭造山带(中段)的深部电性结构、主要断裂带延伸状况以及与南北两侧地块的接触关系等进行了分析研究,结果表明:东昆仑断裂带塔藏段、迭部—白龙江断裂和光盖山—迭山断裂带共同组成了东昆仑断裂系统,分隔了松潘—甘孜地块和西秦岭造山带(中段);西秦岭北缘断裂带为主要的高角度南倾大型电性边界带,延伸深度穿过莫霍面;临潭—宕昌断裂带具有电性边界带特征,其延伸情况具有东、西差异.西秦岭造山带(中段)自地表到深度约20km范围表现为东北和西南浅、中部深的倒"梯形"高阻层,在高阻层之下广泛发育低阻层,低阻层与高阻层相互契合,呈现相互挤压堆积的式样,其西南侧的松潘—甘孜地块中下地壳存在西南深、东北浅低阻层,其东北侧的陇西盆地具有稳定的成层性结构,显示出西秦岭造山带(中段)正处于松潘—甘孜地块向北挤压和陇西盆地向南的阻挡挤压作用中.松潘—甘孜地块从西南向东北推挤、东北侧陇西盆地相对阻挡的相互作用是2013年岷县漳县6.6级地震发生的外部动力学机制,同时地震震源区特殊介质属性是该次地震发生的内部因素.西秦岭造山带(中段)中上地壳倒"梯形"高阻体埋深西薄、东厚的分段差异与该段内部中强地震分布差异有关.东昆仑断裂玛沁段和塔藏段内部的深部电性结构差异和延伸状况与东昆仑断裂自西向东走滑速率减小有内在联系.

The West Qinling orogenic belt is an important portion of the east-west trending orogenic system in China's Mainland.In order to study the east-west differences and tectonic setting of major earthquakes in this area,a magnetotelluric（MT）sounding（WQL-L1）has been carried out across the central part of this orogenic belt,named the Aba-Zoigê-Lintan-Lanzhou profile.MT data were collected and two-dimensional inversion and interpretation were made to thesedata.Combining the MT results of WQL-L6 across the Minxian-Zhangxian earthquake zone in2013 and geological and geophysical data available,we study the deep electrical structure,the major faults and their contact relationship with the north and south sides.The results show that the East Kunlun fault system is composed of the Diebu-Bailongjiang fault and Guanggaishan-Dieshan.This fault system is the boundary between the Songpan-Garzêblock and West Qinling orogenic belt.The West Qinling fault is a significant electrical boundary zone that extends through the Moho.The Lintan-Dangchang fault is also an electrical boundary,extending toward east and west in different manners.Along the central West Qinling orogenic belt the overall electrical structure presents a high-resistivity characteristic from the earth′s surface to a depth of about 20 km.It is shallow in northeast and southwest while deep in the middle,like a inverted＂trapezoid＂.Under the high-resistivity layer of the West Qinling orogenic belt is low-resistivity layer.They coupled each other,showing a compression and accumulation style.The low-resistivity layer in the north of the Songpan-Garzêblock is deep in southwest and shallow in northeast,implying northeastward movement of the Tibetan plateau.The Longxi basin has a stable layered structure.The central West Qinling orogenic belt now is under the extrusion and squeezing from the Songpan-Garzêblock and Longxi basin.The seismic source of the Minxian-Zhangxian MS6.6earthquake is located in the core of inverted＂trapezoid＂of the lowresistivity layer of the central West Qinling orogenic belt,which is the contact between the upper crust high-resistivity layer and lower crust low-resistivity layer.It is also located near the LintanDangchang fault that shows a low-resistivity zone cutting the crust.The Songpan-Garzêblock pushes from southwest to northeast and the Longxi basin block resists from northeast to southwest,which is the external dynamic mechanism of the MinxianZhangxian MS6.6 earthquake,as well as the internal factor of material property.Such an environment of compression and its internal special structure might have caused the crust of the central West Qinling to be shortened in the horizontal direction and to rotate clockwise as well as to uplift strongly.The inverted ＂trapezoid＂body of the central West Qinling orogenic is buried shallow in the west and deep in the east,which may be related with less and more historical seismicity in western and eastern sections of the Lintan-Dangchang fault,respectively.This system accommodates the slip rates of the east-trending Kunlun fault,which may be the deep reason why the strike slip rate along the East Kunlun fault reduces from west to east.