新疆准噶尔北缘覆盖区深部地质结构及其地质意义——来自综合地球物理探测的证据

Deep structure beneath the overburdened area of the northern margin of Junggar Basin and its geologic implications:Evidence from 3D inversion of integrated geophysical data

本文利用准噶尔北缘覆盖区的高精度地面重磁数据开展三维约束反演,揭示出准噶尔盆地北缘浅覆盖区内三维密度结构具有横向非均匀性,纵向有向深部延伸的趋势;深部广泛分布石炭纪、泥盆纪火山岩地层等高磁性体,零散分布热液蚀变导致的热退磁的火山岩;北西向分布的低密度低磁性物质,对应无磁性的断裂破碎带.结合多尺度边缘检测方法推断得到研究区隐伏断裂体系以及火山岩地层的空间展布.对贯穿研究区的综合地球物理探测剖面进行人机交互联合反演,结果显示控制扎河坝蛇绿岩带分布的南北界深大断裂在深部构成开口向下的对称扇形构造样式,说明此蛇绿岩带所代表的古洋盆具有双向俯冲的特征;结合区域地质资料,以及岩石地球化学研究成果,认为剖面穿过的3个酸性岩体均为形成于晚石炭世后造山拉张构造环境的A型花岗岩,可能是准噶尔东北缘造山作用结束的标志.最后结合大量地质、地球化学和地球物理资料,对准噶尔东北缘区域构造演化进行了讨论,认为扎河坝—阿尔曼太古洋盆于中奥陶世开始汇聚,古洋盆向南北两侧双向俯冲,在晚志留纪-早泥盆纪洋盆闭合.

This work carried attempted to image deep structures beneath the overburden area in the northern margin of Junggar Basin based on 3D inversion of integrated geophysical data.It built on 1∶50000 high-precision ground gravity and magnetic data from surveys in this area.The inversion results show that the density structure within the shallow cover of the study area has a horizontal non-uniformity and a vertical tendency to extend down to the deep subsurface.Magnetic bodies such as Carboniferous and Devonian volcanic strata are widely distributed at depth.Thermally demagnetized volcanic rocks caused by hydrothermal alteration are scattered in the study area.The low-density,low-magnetic materials distributed in the north-west direction may be attributed to fractured rocks within fault fragmentation zones.Combined with multi-scale edge detection results,we inferred the spatial spreading of hidden fault systems and volcanic stratigraphy in the study area.We conducted a joint human-machine inversion of the integrated geophysical profiles through the study area.The results show that the deep major faults controlling the distribution of the Zaheba ophiolite belt form a symmetrical fan-shaped tectonic pattern with downward openings at depth.It indicates that the oceanic basin represented by this ophiolite belt is characterized by two-way subduction.Combined with regional geology and petrogeochemistry research,we infer that the three acidic rock bodies crossed by the section are all A-type granites formed in a post-Late Carboniferous orogenic extension environment,which may be a sign of the end of orogeny in the northeastern margin of Junggar.Finally,we discuss the regional tectonic evolution of the northeastern margin of Junggar and suggest that the Zaheba-Almentai ocean basin began to converge in the Middle Ordovician;the paleo-ocean basin subducted in both directions to the north and south,and closed in the Late Silurian-Early Devonian.