南盘江坳陷右江断凹区大地电磁测深区域构造研究

Regional Tectonic Characteristics Magnetotelluric Prospecting in Youjiang Faulted Sag,Nanpanjiang Depression

利用2001年度西林—潞城地区26条MT测线924个测点资料,建立了岩石电性特征表,构筑了合理的地电模型,对南盘江坳陷右江断凹区,包括隆林凸起、右江断凹及乐业断阶中的凌云凸起等构造单元的主要构造层位和断裂进行了追踪判别,揭示了三叠系底、石炭系—二叠系底和泥盆系底三个主要地质界面的高程形态,给出了主要断裂和向斜的构造资料。研究结果显示,利用大地电磁测深资料进行区域及局部构造分析是行之有效的。对构造单元具有划界作用的一级断裂共计11条,均为逆断层,大部分NW—NWW向,多数延伸达10～100km,南倾,倾角50°～60°,浅部较陡,深部较缓,具同沉积断裂性质。主要向斜包括小河口、八桂、央边及西平等四个,全落在右江断凹,构造幅度在三叠系底面上为850～1400m,石炭系—二叠系底为1000～1750m,泥盆系底为1500～2500m。区内还分布有十多个出露或隐伏的背斜和凸起构造,全区尤其是右江断凹,构造形态比较复杂。

Electrical characteristics of rock and a geoelectric model were set up based on the total 24 sites of 26 magnetotelluric （MT） surveying lines that were laid in Youjiang Faulted Sag of Nanpanjiang depression in 2001. Tracking a major structural layers and faults in this region reveals the 3-D elevation configuration of the Triassic, Carboniferous-Permian and Devonian bottoms. The result shows that MT is an effective way to study regional or local structure. There are 11 main faults with relevance to geologic structures that are all thrust faults, which most are NW-NWW-trending,10-100km extensional, 50°-60° southward-dipping structures with synsedimentary feature. Main MT synclines, including Xiaohekou, Bagui, Yangbian and Xiping synclines, are all congruous with geophysical results in the sag. The structural amplitudes are at 850-1 400m for the Triassic bottom,1 000-1 750m for Carboniferous-Permian bottom and 1 500-2 500m for Devonian bottom. Distribution of the region also has more than 10 exposed or concealed anticlines and heaves. This result indicates that the geologic structures are complex in the whole region, especially in Youjiang Faulted Sag.