祁连山东端冷龙岭隆起及邻区深部电性结构与孕震构造背景

The deep electrical structure and seismogenic background of Lenglongling uplift and its adjacent areas in the eastern end of Qilian Mountains

祁连山东端冷龙岭隆起及其附近地区是青藏高原东北缘与阿拉善地块强烈相互挤压碰撞区域,也是历史地震活动极为强烈区域.为了揭示冷龙岭隆起及其附近区域的断裂深部延伸状况、强震孕育构造背景以及区域动力学特征等,我们在已有大地电磁数据的基础上,新近在冷龙岭隆起附近以及西南侧区域进行了数据采集,获得了一条自西南向北东穿过西秦岭地块、陇西盆地、祁连山冷龙岭隆起和阿拉善地块的长约460 km的大地电磁剖面(LJS-N)数据,并利用三维电磁反演成像技术对全剖面数据进行了反演,同时也对位于该剖面西侧约80 km外的一条大地电磁剖面(DKLB-M)数据进行了三维反演成像.2条电磁探测剖面结果均揭示了祁连—西海原断裂带展现为略向西南倾斜的大型超壳电性边界带,该断裂是祁连山东端冷龙岭隆起区域最重要的主边界断裂,其北东侧和西南侧地块的深部电性结构呈现出截然不同电阻率分布特征,其西南侧的南祁连地块、陇西盆地以及西秦岭地块在地壳尺度展示为埋深深浅不一的高-低-次高阻结构特点,而其北东侧古浪推覆体表现为西南深、东北浅“鼻烟壶”状较完整的高阻结构特征,再往北到阿拉善地块则呈现为高-低-次高水平三层结构样式.1927年M 8.0古浪、1954年M 7.0民勤和2016年M 6.4门源地震的震源都处于统一的高阻古浪推覆体之中.在青藏高原北东向挤压作用的控制下,祁连山东端冷龙岭隆起区域的祁连—西海原断裂、祁连山北缘断裂和红崖山—四道山断裂以叠瓦状向北北东向顺序推覆拓展到阿拉善地块,这种拓展作用是该区中强地震的动力来源.

The Lenglongling uplift and its adjacent areas in the eastern end of Qilian Mountains are the areas where the northeastern margin of the Tibetan Plateau and the Alxa Block(ALB)are strongly squeezed and collided with each other.Historic earthquakes in this area are extremely strong.In order to reveal the deep extension of faults near Lenglongling uplift and the dynamic background of strong earthquakes in this area,on the basis of the existing magnetotelluric profiles,we have made intensive measurements of the areas near the Lenglongling uplift and the desert-covered areas on its northern side and extended the profiles southward.A magnetotelluric profile(LJS-N)with the length of about 460 km across the West Qinling Block(WQL),Longxi Basin(LXB),Lenglongling uplift of Qilian Mountains and ALB from southwest to northeast has been formed.In this paper,three-dimensional magnetotelluric inversion imaging of LJS-N and DKLB-M(about 80 km west of the LJS-N)profiles are studied.The results of two profiles reveal that the Qilian-West Haiyuan fault(QHF)zone is a large vertical super crustal electrical boundary zone with a slight tilt to the southwest,which is the most important boundary fault in Lenglongling uplift area.The deep electrical structure of the blocks on the northeast and southwest sides of the fault zone presents different resistivity distribution characteristics.The Southern Qilian Block(SQL),LXB and WQL on the southwest side of the fault zone show three-layer resistivity distribution characteristics of high-low-middle high.The Gulang Nappe Transition(GLT)on the northeast side is characterized by a relatively complete high resistivity structure with deep southwest and shallow northeast snuff bottles,while the three-layer resistivity distribution characteristics of high-low-sub-high level is presented from north to ALB.The earthquake source of the 1927 M 8.0 Gulang,1954 M 7.0 Minqin and 2016 M 6.4 Menyuan earthquakes are all in a unified high resistivity structure GLT.Controlled by NE-trending compression of the Tibetan Plateau,the QHF,Northern Qilian fault(NQLF)and the Hongyashan-Sidaoshan fault(SHF)in the Lenglongling uplift area in the eastern end of the Qilian Mountains extend to the ALB in an imbricate NNE-trending sequence,which is the dynamic source of moderately strong earthquakes in the area.