滇西牟定-香格里拉电性剖面及深部构造

Deep structure derived from the Mouding-Shangri-La magnetotelluric profile in western Yunnan Province

通过对滇西NW—SE向牟定—香格里拉大地电磁测深剖面的反演并结合地质构造、小震精定位资料,综合分析了剖面经过地区的深部构造.在滇西北地区深部发育一规模巨大的近水平产出地壳高导层,此高导层从香格里拉一直延伸到永胜盆地以东,沿剖面水平延伸超过200km.地壳高导层又可分为两段,两段高导层在剖面上形成两个香蕉型相连的复合形态,高导层之上的高阻体则形成两个碗型相连的复合结构,碗型内部有向东倾斜的相对低阻带存在.滇西北地区整体构造格架可以解译为一套两个主推覆面构成的叠瓦式推覆系统,两个主推覆面高低起伏形成总长度超过250km的底部滑脱带.第一个主推覆面的长度超过150km,并在丽江东部上翘到接近地表,小金河断裂是其延伸到地表的主要破裂带.第二个主推覆面长度超过120km,过程海断裂后上翘并在永胜以东地区出露地表.根据反向低阻带影像,可以推断推覆体反冲构造发育,在玉龙山至丽江以东和永胜盆地西缘至平川盆地东缘形成两处冲起构造.推覆面在剖面上呈现出平缓-陡峭-再平缓-再陡峭的断坪和断坡相间的阶梯状结构.

The electrical structure of western Yunnan is revealed by a NW—SE trending Mouding-Shangri-La magnetotelluric（MT）sounding profile.It shows a huge sub-horizontal high conductivity layer（HCL）in crust beneath northwestern Yunnan.This high conductivity layer extends from Shangri-La to east of the Yongsheng basin for at least 200 km with thickness more than 40 to 50km and a depth of the bottom about 80 to 90km.The HCL can be divided into 2sections.The west section extends from Shangri-La to the east of the Lijiang basin,steeping upward in east Lijiang with depth less than 10 km from the earth surface.The east section extends from the east Lijiang basin to the Yongsheng basin and steepens upward near the Yongsheng basin.The two HCL sections form a composite shape in the profile like two bananas linked each other.The two high resistivity bodies（HRB）on the top of the HCL look like two HRB bowls.A comprehensive analysis of the tectonics is made based on 2-D inversion of MT data in combination with geology and relocation of small earthquakes.The tectonic pattern of the study area is characterized by a system of imbricate thrust nappes,which slip up and down stretching at least a total length more than 250 km.The first main nappe stretches more than 150 km,and rises up in the east of Lijiang close to the surface.Of this nappe,the Xiaojinhe fault is the main fault which extends to the surface.The second main nappe is more than 120 km long,extending nearly to the earth surface in the east of Yongsheng after crossing the Chenghai fault.According to the reverse low resistance images,we infer the existence of back thrusting structure that forms pop-ups in two regions.One lies at the east margin of Jade Dragon Mountain to the east of Lijiang,while the other is at the western margin of the Yongsheng basin to the Pingchuan basin.Listric faults in the profile reveal a gentle-steep-gentle-steep shaped ladder-like structure.