滇西龙陵地区地壳电性结构及其对大瑞铁路地质选线影响研究

高黎贡山地处印度板块与欧亚板块碰撞缝合带附近的横断山脉南段,是大理一瑞丽铁路(大瑞线)的必经之地,地形起伏大、构造复杂、活动性强,高黎贡山隧道作为全线控制性工程之一,其地质选线的最大困难就是对隧道深部构造环境的了解,特别是缺少对与地热、地震等联系紧密的深部地质构造的认识.为此,本文以大地电磁方法为手段,以高黎贡山隧道为主要研究对象,通过对滇西龙陵地区高黎贡山隧道越岭段两条大地电磁剖面数据的处理解释对研究区的地壳电性结构特征进行了勘探研究.结合区域地质构造特征与主要工程地质问题之间关系的分析,根据隧道主要断层地质条件设计了三维垂直断层模型,利用三维有限元开展正演模拟研究发现,测点点距、位置与横向分辨率密切相关,点距越密,分辨率越高,测点位于断层在地表投影位置能有效提高分辨率.采用大地电磁阻抗张量分解技术对两条剖面上各测点的二维偏离度和电性走向进行了计算和分析,对剖面视电阻率和阻抗相位数据进行了二维NLCG联合反演研究,揭示了沿剖面的腾冲地块、龙陵一瑞丽断裂带及保山地块10 km深度的电性结构特征及相互关系.结果表明:剖面CD电性结构呈现区域构造的三分性,腾冲地块电性结构成层性较好,保山地块成层性较差,两者均以中高阻电性特征为主,中间夹龙陵一瑞丽断裂带,电性结构反映从3 km深度以下存在几乎近于直立延伸的低阻带,推测为班公湖一怒江缝合带滇西段丁青一怒江缝合带的反映;剖面AB共划分了6条与工程密切相关的深部隐伏断裂,结合地震地质、地表地质及龙陵地震深部背景研究,推测F7-3断裂为1975龙陵7.3级地震断裂;从地表黄草坝断裂开始向下延伸,有一条发育最大深度约为4 km的低阻通道,推测为地热断裂深循环通道,其与黄草坝断裂共同控制研究区地下热水的补给、径流和排泄条件,在高黎贡山隧道线位位置形成了一个相对低温通道,为隧道方案成立的关键工程地质条件.勘探结果表明:滇西龙陵地区地壳电性结构有效的反映了高黎贡山隧道深部隐伏断裂和地热断裂深循环通道等深部构造特征,为大瑞线隧道工程地质选线提供了深部地质背景依据.

Electrical structure of the Longling area in western Yunnan and its effect on route selection of the Dali-Ruili railway

The Gaoligong Mountains, where the Dali-Ruili railway will pass through, is located in the southern section of the Hengduan Range near the collision suture zone between the India and Eurasian plates, featured by large terrain relief, complex structure and strong activity. The Gaoligongshan tunnel is one of the key projects, for which the biggest difficulty in geological route selection is how to understand the deep tectonic environment. Especially it lacks knowledge on the deep geological structure related to the geothermics, earthquakes and so on. This paper takes the Gaoligongshan tunnel as the main research object, by processing, interpreting and inverting the magnetotelluric (MT) data along the Gaoligong Mountain zone, to probe the deep electrical structure below the Gaoligongshan tunnel. A 3D vertical fault model is constructed according to the geological conditions around the tunnel, which is also based on the analysis of the relationship between the regional geological structure and the main engineering geological problems. The 3D finite element forward modeling results show that the lateral resolution is related to the distance and position of survey points, the closer distance leading to higher resolution. And the resolution can be enhanced when the survey points coincide with the projection locations of the fault on the surface. Data processing and analysis include calculation of the electric strike and 2D skewness of the area using MT phase vector decomposition and NICG 2D inversion of apparent resistivity and phase data, which reveals the electrical structure above 10 km depth of the Tengchong block, Longling-Ruili structure zone and Baoshan block and the relationship between them. The results show that the electrical structure of the profile CD can be divided into three parts. The electrical structure of the Tengchong block is bedded better than Baoshan, both of which are characterized by high resistivity. Below 3 km depth of the Longling-Ruili structural zone between the two blocks is a low resistivity belt nearly vertically extending below 3km, likely reflecting the Dingqing-Nujiang collision suture zone in western Yunnan. The six deep buried faults closely related to the engineering are explained by the profile AB, and it is inferred that the fault F7-3 is responsible for the 1975 Longling M_S7.3 earthquake according to the seismic geology, surface geology and deep background. There is a low resistance channel with maximum depth of about 4 km which begins from the Huangcaoba fault on the surface and extends downward, probably controlling the deep-circulation channel of geothermal together with the Huangcaoba fault, which is considered to be the important engineering geological condition for the Gaoligongshan tunnel scheme, forming a relatively low-temperature channel at the position of the line. In sum, the exploration results show that the electrical structure of the Longling area in western Yunnan reveals the characteristics of deep buried faults and the fractured deep-circulation channel of geothermal of the Gaoligongshan tunnel, which provides the evidence of deep geological background for the route selection of the engineering project.