木里-盐源地区深部电性结构及构造意义

Deep electrical structure and tectonic implications beneath the Muli-Yanyuan area

木里—盐源地区地处青藏高原东南缘,属于古特提斯洋构造域,是松潘—甘孜地块及扬子地块的交接地带,是研究青藏高原东南缘构造演化过程的重要区域.本文介绍的是横穿木里—盐源地区的大地电磁剖面,自北西向南东依次跨越锦屏山断裂、木里弧形构造区、丽江—小金河断裂、盐源盆地、金河—箐河断裂等构造.维性分析表明木里弧形构造区和金河—箐河断裂都表现为较强的三维性,因此本文采用大地电磁三维反演技术,获得了木里—盐源地区的精细电性结构.电性模型显示,沿剖面可以划分为4个主要的电性构造单元.锦屏山断裂以北的川西北次级地块下方10~20 km处,发育北西向低阻体,推断是古老的义敦岛弧区残留的物质;锦屏山断裂以南至丽江—小金河断裂为高阻体,可能是锦屏山山根;丽江—小金河断裂下方~10 km处发育北东向的低阻体,与龙门山—锦屏山构造带走向一致,结合剖面附近表现为张性的震源机制解特点,推测该低阻体很可能是北部的塑性物质受阻后一部分往西南沿着丽江—小金河断裂缝隙挤入的结果;盐源盆地下方在3~7 km发育厚度约5 km、长度达40 km的低阻层,电性主轴方向为北西向,与盐源断裂走向一致,解释为盐岩层,尤其是南段低阻体表现为延伸至地表的特征,与地表盐泉对应,为在盐源地区开展深部找钾盐矿提供了电磁方面的证据.

The Muli-Yanyuan area is essential for studying the tectonic evolution process in the southeastern margin of the Tibet Plateau.It belongs to the Paleo-Tethys Ocean structural domain,the junction zone of Songpan-Garzêterrane and Yangtze Block.We obtain one magnetotelluric(MT)profile crossing the Muli-Yanyuan area.From NW to SE,the MT profile across the Jinpingshan fault,Muli arc structural area,Lijiang-Xiaojinhe fault,Yanyuan basin,Jinhe-Qinghe fault,and others.Three-dimensional characteristics exist at the Muli arc structural area and the Jinhe-Qinghe fault based on dimensionality analysis.We used the three-dimensional MT inversion to obtain the Muli-Yanyuan area′s electrical structure.The resulting resistivity model reveals four primary electrical structure units.To the north of the Jinpingshan fault,an NW-trending low resistivity layer develops at a depth of 10~20 km beneath the western Sichuan sub-block,which was inferred as the ancient residual material of the Yidun arc.From the south of Jinpingshan fault to the Lijiang-Xiaojinhe fault,a high resistivity body may be the root of Jinpingshan.Beneath the Lijiang-Xiaojinhe fault,a low resistivity body develops at a depth of 10~20 km.The electrical strike is NE-trending and consistent with the trending of the Longmenshan-Jinpingshan thrust belt.Combined with the extensional characteristics of the focal mechanisms,we conclude that this low resistivity body is probably part of the northern ductile materials blocked and squeezed into the southwestern along with the Lijiang-Xiaojinhe fault.There is a continuous low resistivity layer beneath the Yanyuan basin at a depth of 3~7 km.The length and thickness of the low resistivity layer are about 40 km and 5 km.The electrical strike is NW-trending and consistent with the trending of the Yanyuan fault.We interpret it as a halite layer.In particular,the southern part of the low resistivity body that extends to the surface corresponds to the salt spring.We provide electromagnetic evidence to find sylvine ore in the Yanyuan region.