阿尔金断裂带东段地区深浅部构造综合分析

阿尔金断裂带东段地区的地质构造特征及其动力学机制一直是地学工作者关注的焦点。近年来小震资料越来越多应用到活动断裂空间展布、深浅构造分析及动力学机制研究领域。本文应用双差定位法获得研究区域2008~2017年间6013次地震事件的精确定位数据,通过多条小震深度剖面清晰刻画出断裂系统的空间展布形态。综合石油地震剖面、人工地震宽角反射/折射剖面、人工地震深反射剖面,充分利用小震精确定位信息以及浅表活动构造研究成果,建立研究区断裂系统的深浅部构造模型。研究区莫霍面由北往南逐渐加深,存在三处断错,呈阶梯状展布,地壳内存在一条厚约10km的低速层,在该层以上为地震多发区,断裂系统总体呈'Y'字型,上部为一系列叠瓦状逆冲断裂,造成祁连山的隆升,向下并入一条主干断层。最后探讨了青藏高原东北缘地区构造运动的动力学机制,亚洲板块俯冲至祁连山前,上地壳以逆冲推覆构造模式造成上地壳增厚现象,而中下地壳主要为亚洲岩石圈地幔下插,上地幔的拖曳作用下发生流动引起地壳增厚,上下地壳整体增厚。

Comprehensive analysis of deep and shallow structures in the eastern Altyn Tagh fault zone

The geological structural characteristics and kinetic mechanism of the eastern Altyn Tagh fault zone have been the focus of geologists for a long time. In recent years, small earthquake data has been applied to many research domains, such as active fault spatial distribution, deep and shallow structural analysis and dynamic mechanism. On the basis of previous studies, this paper collects earthquake data in the study area from 2008 to 2017, with a total 8025 earthquake events, and screens the final eligible earthquake events 6991. Using double differential positioning method repositioned, 6013 earthquake events were obtained. The focal depths of these earthquakes were generally shallow, basically within a depth of 26km, of which the earthquakes with a focal depth between 4km and 14km account for about 86% of the total. The spatial distribution of the faults and the current movement characteristics are delineated by vertical profiles of these small earthquakes. According to their spatial distribution characteristics, the dip angles of the faults of these small earthquakes were disclosed, which decreased gradually from the Altyn Tagh fault zone as represented by the near-vertical dip angle of Yemahe-Daxueshan fault, toward the direction of Qilian Mountain, which provides new evidence for the spatial distribution characteristics model of thrust faults. Based on petroleum seismic profiles, wide angle reflection/refraction profile, deep seismic reflection profile, small earthquake relocation data and research achievements of shallow active tectonics, the three-dimensional tectonic model of the study area is established. The Moho surface beneath the study area gradually deepens from north to south, and there are three faults distributed in a ladder shape. In the crust, there is a low velocity layer 10km thick, above which earthquakes occur. The fault system is generally Y-shaped, whose upper part is a series of imbricate thrust faults causing Qilian Mountain uplift and merge downward into a main fault. Finally, this paper discusses the dynamic mechanism of regional tectonic movements. The Asian plate subducted to the front of Qilian Mountains, and the upper crust was thickened by thrust tectonic model, while the middle and lower crusts thickening caused by flow under dragging of upper mantle. The upper and lower crusts are thickening as a whole.