西准噶尔深部地球物理结构及其动力学意义

Many researchers have understood the main part of the Western Junggar is consisted of ancient ocean crust by studies of tectonics, petrology, and geochemistry. However, there are some different geodynamic models proposed to characterize its form, including double-directional subduction of ocean ridge, forearc ocean crust subduction, etc. The geophysical data especially which can be used to image the structure of crust and top mantle is extremely rare in this area. Based on the geological mapping data, we design a dense MT profile crossing the main structural belts and a small seismic array in the Dalabute area. The electrical structure will be given by two-dimensional inversion of MT data; and the S-wave velocity model will be derived by tomography of ambient noise based Rayleigh waves dispersive data. Using the regional relation between the physical parameters and the lithology, we propose a new sequence inversion scheme to construct a geological structure from resistivity and S-wave velocity models. Incorporating the knowledge from the investigation of modern ocean ridge and subduction zone, the geodynamic setting of the Western Junggar will be well constrained by the constructed model. The proposed project concerns the fore-cutting topic of the mechanism and the fate of an ocean crust landing. The study will strongly support the activity of mineral exploration in the Western Junggar by the fresh geophysical data.

前人对西准噶尔进行了大量构造地质学、岩石学及地球化学研究，基本认可其地壳主体为洋壳物质，但对其形成的动力学背景却有洋中脊双向俯冲和弧前单向俯冲等不同的认识。西准噶尔地球物理研究程度较低，根据现有的地球物理资料难以构建其深部岩性结构和构造模型。在前期地质调查工作的基础上，本项目拟对西准噶尔达拉布特带，开展横穿主要构造线的2-3 km点距的宽频大地电磁测深剖面观测和10-20 km台站间距的密集小台阵地震背景噪声观测，利用先进的成像方法获取高分辨的电阻率和S波速度模型。然后，通过大地电磁测深和面波频散两种数据的联合顺序反演，构建西准噶尔达拉布特带地壳上地幔有几何结构的物性-岩性模型。综合前人在现代海洋洋中脊和俯冲带研究中所获得的认识，限定西准噶尔形成的动力学环境。研究目标是为研究洋壳登陆机制和洋壳登陆的命运等前沿问题提供地球物理研究范例，并为在西准噶尔地区实现找矿突破提供重要的深部信息。

西准噶尔地处中亚造山带的西南缘，是全球最大的显生宙岩石圈增生区，包含前寒武纪微陆块、显生宙大洋残片和诸多岛弧带等。达尔布特带以发育规模宏伟的达尔布特断层和多条蛇绿混杂岩带为标志，其西为塔城盆地，东为准噶尔盆地，北为阿尔泰山，南为北天山，整体为北东走向。带内分布有萨尔托海铬铁矿、哈图金矿、包谷图斑岩型铜矿等一批重要矿床，是解析中亚造山带定型期（晚古生代）动力学背景和成矿的关键区带。.项目组已经获得研究区高质量的大地电磁和地震观测数据。查明了达尔布特带地壳上地幔物性分布和构造样式。结合相关资料确定了西准噶尔形成于晚古生代，为一夭折的洋内俯冲系统：扎伊尔山地壳主体为残留洋内岛弧，但受到碰撞后伸展期岩浆作用的改造。克乌断裂以东的西准噶尔盆地基底为准噶尔洋（古亚洲洋的组成部分）岩石圈，隐伏的克乌断裂主体继承了洋内俯冲带上方增生棱内部逆冲断裂,并推测其下部延伸到现今岩石圈地幔。达尔布特断层不具有缝合带意义，沿其两侧分布的蛇绿混杂岩是后期构造楔入形成的。萨尔托海-哈图一带是弧前向弧后的转换地带，是俯冲期地幔楔内部分熔融物质多期次上升就位的主要场所，因而形成重要的金属成矿带。