龙门山断裂带及其邻区电性结构特征

Electrical structure characteristics of Longmen fault zone and its adjacent areas

龙门山断裂带是中国大陆地壳中著名的造山带和地震带。为了研究龙门山逆冲推覆构造的深部结构及其与青藏高原物质东向逃逸之间的关系,布置了跨越西秦岭造山带、松潘—甘孜褶皱带、四川盆地的4条大地电磁测深剖面,每条剖面有宽频大地电磁测深点20个,共计80个宽频大地电磁测深数据以及16个长周期数据;进行了二维、三维反演,得到了电性模型。研究结果显示:在30 km以浅,龙门山断裂带基本上与电性梯度变化带相重合,说明龙门山断裂带是深达地壳中部的深大断裂带;松潘-—甘孜褶皱带在20 km以浅呈现以高阻为主的较为复杂的电性结构分布特征,这与其复杂的地表构造有关;在20 km以深,接近龙门山断裂带附近呈现为高阻,推测此高阻可能是对四川盆地的基底的反映,表明松潘—甘孜褶皱带以龙门山断裂为界推覆至四川盆地之上;推断青藏高原物质东向逃逸,有可能从松潘—甘孜褶皱带深部沿礼县—宕昌一带向东北方向逃逸,这一运移主要是发生在18 km以下、30 km以上的中下地壳范围内,物质逃逸的主要形式是部分熔融。

Longmen fault zone is one of the most famous orogenic belts and seismic zones in the continental crust of China. In order to study the deep structure of Longmen thrust nappe belt and its relationship with the eastward escaping of material in the Tibetan Plateau, the authors completed four magnetotellurics profiles across the West Qinling orogenic belt, Songpan-Garze fold belt and the Sichuan basin, which included 80 broad-band magnetotellurics stations and 16 long-period stations. The electrical model diagram was obtained through two-dimensional and three-dimensional inversion. The model indicates that the Longmen Mountain fault zone is basically coincided with the electrical gradient zone in the depth less than 30 km. It means that the Longmen fault zone is a fault zone which is deep in the middle of the crust. Above the depth of 20 km, the distribution characteristics of the electrical structure in Songpan-Garze fold belt are complex and of low-conductivity, which is related to the complex surface structure. Below 20 km, the low conductivity is similar to that of the Longmen fault zone. It is inferred that high resistivity may be a reflection of the basement of the Sichuan basin, indicating that the Songpan-Garze fold belt pushed over Sichuan basin bounded by Longmen fault zone. It is concluded that the material migration under the Tibetan Plateau happened under the Songpan-Garze fold belt. The material escaped northeastward from Lixian to Dangchang, at the depth of 18km to 30km with the partial-melting.