黑龙江佳木斯地块南端及邻区深部三维电性结构及其构造意义

Lithospheric electrical structure of the southern segment of the Jiamusi massif and its adjacent areas revealed by 3D magnetotelluric inversion

佳木斯地块及邻区位于中亚造山带(CAOB)的东部,由于受到了古太平洋板块俯冲和牡丹江洋闭合的影响,该地区形成了复杂的构造.为了揭示该区深部电性结构,探讨其动力学机制等科学问题,我们采用基于非线性共轭梯度算法的三维大地电磁反演,获得了一个长约400 km的大地电磁剖面电性结构模型,该剖面反映了松嫩地块东北缘、佳木斯地块南端和完达山地体的深部岩石圈电性结构特征.并结合研究区地质资料和其他地球物理资料,我们对剖面所经过的主要地质构造单元展开了综合分析.结果表明:(1)松嫩地块东北缘的小兴安岭岩石圈电性结构以巨厚的高阻体为主,可能与多期次形成的花岗岩叠置作用有关;(2)三个地块之间的缝合带下方均存在高导异常,这些高导异常可能与软流圈物质上涌有关;(3)牡丹江断裂、跃进山断裂和敦化—密山断裂可能为地壳尺度的深大断裂,这些断裂可能作为微板块的边界,与块体之间的古缝合带重合;(4)佳木斯地块南端存在大规模高阻异常体,表明古老的佳木斯地块南端具有较冷的刚性块体特征,我们推测可能是冈瓦纳大陆或塔里木克拉通的一部分.

The Central Asian Orogenic Belt(CAOB)is the largest accretionary orogenic belt in the world.The Jiamusi Massif and adjacent areas,which are located along the eastern segment of the CAOB,form a complex structure influenced by the subduction of the paleo-pacific plate and the closure of the Mudanjiang Ocean.Understanding the complexity of the deep structure of the study area is vitally important for identifying the evolution and dynamics of the study area.Here we report a high-resolution 3-D electrical resistivity model from the inversion of magnetotelluric data collected in 80 sites of study area.Given sufficient consideration to recent geological and other geophysical attempts,the following issues will be addressed:(1)The overall electrical structure of the Xiaoxinganling at the northeastern edge of the Songnen massif is the nature of a huge high resistance,which can be extrapolated to the superimposition of granites formed in multiple phases;(2)There are high conductivity anomalies below the suture zone between the three massifs,and these findings have wider relevance with the upwelling of asthenosphere material;(3)The Mudanjiang Fault,the Yuejinshan Fault and the Dunhua-Mishan Fault are recognised as crust-scale fractures.Taken together,these results suggest that there is an association between the crust-scale fault and micro-plate boundary;(4)The property of high-resistance anomalies in the southern segment of the Jiamusi Massif provides some tentative initial evidence that the southern segment of the ancient Jiamusi Massif could be attributed to colder and more rigid blocks,which can be explained in part by the proximity of the Gondwana continent or the Tarim craton.