宁南弧形构造带甘肃大沟—内蒙古乱井中上地壳电性结构

The conductivity characteristics of Dagou-Luanjing middle and upper crust in Southern Ningxia arc structure

利用大地电磁探测方法在宁南弧形构造带内蒙古乱井—甘肃大沟段进行了中上地壳电性结构特征研究,通过二维反演获得了研究区的电性结构模型,模型清楚揭示出了该区典型的电性结构特征:(1)中上地壳电性结构整体表现为"碎块状"的构造特征,其中走廊构造带总体表现为相对低阻,北祁连碰撞造山带总体表现为相对高阻;(2)海原地震震源区的震源断裂所反映的低阻带构造样式既不像走滑断裂那样陡峭,也不像逆冲断裂那样平缓,而是兼具走滑与逆冲断裂带的双重特征;(3)研究区壳内低阻层呈现不连续分布,总体表现为南深北浅的构造样式,一般都终止于高阻地块边缘或与逆冲推覆构造的滑脱面相交汇处;(4)清水河裂褶带内发育的高阻地块可能为晚古生代或早中生代发育在元古宙基底上的古隆起,为该区燕山期构造运动提供了深部大地电磁资料证据。

In order to study the conductivity characteristics of Dagou-Luanjing middle and upper crust, the authors carried out magnetotelluric sounding work in the study area, and obtained the electrical structure model by the two-dimensional inversion method, which clearly reveals the typical electrical structure of Dagou- Luanjing： （1） The middle and upper crust shows the “chunky” structural style totally, and the authors infer that the conductivity of the corridor structure belt is relatively lower whereas the conductivity of the Qilian orogenic belt is relatively higher; （2） The low-resistivity zone caused by focal fracture of Haiyuan earthquake is neither steep like the strike-slip fault nor gentle like the thrust belt, but has both strike-slip and thrust characteristics; （3） The low-impedance and high-conductivity layers in the mid-upper crust of the study area intermittently exist, and show the structural style of deeper in the south and shallower in the north; it can also be inferred that these low-impedance and high- conductivity layers always reach their termination at the edge of the high impedance plots, or intersect the detachment layer under the thrust nappes; （4） A high resistance block exists in Qingshui River depression basin, which is likely to be an “island chain” paleouplift that was developed on the basin basement in Late Paleozoic and Early Mesozoic period,thus providing evidence for the tectonic movement of Yanshanian.