额尔古纳地块东缘和兴安地块西缘电性结构研究

Deep electrical structure of the eastern margin of the Erguna massif and the western margin of the Xing'an massif

中国东北地区位于中亚造山带东段,夹持在西伯利亚板块、华北板块和太平洋板块之间,是解决东亚大陆构造演化的关键区域,其中额尔古纳地块和兴安地块位于东北地区西部,是两个十分重要的地质构造单元.横过额尔古纳地块东缘和兴安地块西缘的大地电磁测深剖面揭示了两块体结合带附近的深部电性结构,进而为该区构造演化提供新的电性依据.本文通过对剖面测点数据做标准化处理,并对二维偏离度、构造走向等进行计算与分析,采用非线性共轭梯度(NLCG)算法对TE+TM模式的数据做了二维反演,获得了该剖面的地壳和上地幔电性结构模型,划分出三个典型构造单元:额尔古纳地块东缘、碰撞拼合带和兴安地块西缘.研究结果表明,研究区上地壳基本呈高阻特征,可能为岩浆岩,代表其经历了多期次岩浆作用,而额尔古纳地块东缘和兴安地块西缘中下地壳的高导体反映其地壳非刚性的特点,可能形成于后期伸展环境;拼合带中下地壳存在大范围高导体并与上地幔高导体相连,指示出地幔物质上升的通道,反映出地幔物质的上升作用可能是后期构造伸展的重要动力.

The Northeast China region geologically lies to the east of the Central Asian Orogenic Belt （CAOB）, where is approximately in the centre of the ancient Siberia Plate, North China Plate and the Pacific Plate. Therefore, it is a critical area for solving the tectonic evolution of the East-Asian continent and has drawn much attention. The Erguna massif and the Xing＇an massif are two important tectonic units in the west part of the Northeast China,a 300 km long magnetotelluric sounding profile of 63 broadband MT sites across the eastern margin of the Erguna massif and the western margin of the Xing＇an massif has been completed. Data process and analysis include calculation of the 2D skewness and electric strike of the MT profile, and the nonlinear conjugate gradient （NLCG） 2D inversion is adopted to invert the data with combined TE and TM modes. An electrical structure model of crust and upper mantle was obtained,which can be divided into three tectonic units：the Erguna massif,the collision belt and the Xing＇an massif, which is consistent with the regional geology. The research shows that the upper crust of the study area may be igneous rocks representing multi-periodic magmatic activities,the conductive bodies in the middle-lower crust of the Erguna massif and Xing＇an massif imply that the crust is non-rigid and may be formed in the post orogenic extensional environment.The Derbugan Fault may be a crustal fault. The conductive bodies in the middle-lower crust of the collision belt connected with the below conductive bodies may indicate the upwelling material channel of the mantle reflecting the rising of the mantle material is an important impetus of the extension, and the formation of the Hailar Basin may be related to the thermal uplifting.