中亚造山带南部—鄂尔多斯盆地北部的壳幔结构特征及其构造意义

Crustal and upper mantle structure along a profile from the southern Central Asian Orogenic Belt to the northern Ordos Basin and its tectonic implications

鄂尔多斯盆地北部至中亚造山带的广大地区,经历了早前寒武纪华北克拉通的形成、古生代古亚洲洋闭合、中生代陆内造山、新生代裂陷等系列重大地质事件,且该区赋存大规模能源和矿产资源、发育河套地震带,因此该区壳幔结构特征对其地质演化过程、资源富集机制、地震孕育机理均具有重要指示意义.本文基于内蒙古满都拉—陕北延川宽频带线性地震台阵资料,开展同剖面人工源地震测深结果约束下的多频接收函数和面波频散联合反演、接收函数共转换点叠加成像研究,构建了纵跨中亚造山带南部—阴山造山带—河套地堑—鄂尔多斯盆地北部一线的地壳上地幔精细结构.研究结果显示:沿剖面Moho面存在较大起伏,深度变化范围为35~48 km,整体上呈现以河套地堑为中心,相向汇聚、分段加深的近似对称图案;河套地堑地段,Moho面以穹窿形上隆的方式完成过渡,Moho面上隆、上地幔低速区的分布范围与孔兹岩带分布之间具有较好的一致性;大青山、乌拉山、色尔腾山交汇部位的地震活动分布,在空间上呈现速度结构迥异、相对独立的花状构造特征.以上特征指示研究区地壳在古元古代鄂尔多斯—阴山地块碰撞拼合、古生代中亚造山带造山的构造格架基础上,主要记录了中生代燕山运动B幕的强烈南北向挤压变形,并叠加了新生代以来,特别是新近纪至今鄂尔多斯北缘的强烈伸展断陷作用;渐新世河套地堑的初始发育是基于古元古代鄂尔多斯—阴山地块的碰撞拼合带启动的,意味着克拉通陆核的早期结合带在后期演化中依然保持着构造薄弱带的属性;大青山山前断裂、色尔腾山山前断裂是交汇区的主干断裂,它们均具备正断和走滑性质,而乌拉山山前断裂、北缘断裂分别在两者之间起到调节作用,推测1996年包头6.4级地震的发震断层对应北部色尔腾山山前断裂东段沿南倾方向在地壳深部的延伸.

In the regions from the northern Ordos Basin to the Central Asian Orogenic Belt (CAOB), many major geologic events have successively happened, such as the craton formation in the early Pre-Cambrian, the closure of the Paleo-Asian Ocean in the Paleozoic, the intra-continental orogeny in the Mesozoic, and the rifting and sagging since the Cenozoic. There are large scales of energy and mineral resources, and the Hetao Seismic Belt, and thus the characteristics of the crustal and upper mantle structure would be helpful to reveal the geological evolution and mechanisms of resource enrichment and seismogenesis. In this paper, constrained by the P-wave velocity model constructed by a Deep Seismic Sounding (DSS) investigation along the same south-northward profile from Mandula Town in the Inner Mongolia Autonomous Region to Yanchuan County in the northern Shaanxi Province, China, we carried out the joint inversion of multi-frequency P-wave receiver functions (RFs) and surface wave dispersions to obtain the S-wave velocity model beneath the profile. And then, we conducted the Common Conversion Point (CCP) stacking of RFs to construct the geometric structure of the crust and upper mantle constrained by both models derived from the DSS and joint inversion mentioned above. Some characteristics of the crustal and upper mantle structure along this profile are revealed and listed as follows: (1) the Moho topography fluctuates within the range of 35~48 km, and shows a nearly symmetric pattern centered by the Hetao Graben System and featured by step-wise converging and deepening Moho discontinuity;(2) the Moho discontinuity beneath the Hetao Graben System is uplifted with a dome shape, and the domal area coupled with upper mantle low-velocity zone is just beneath the Khondalite Belt;(3) the seismicity in the transition zone of the Daqingshan, Wulashan, and Sertengshan mountains, show two spatially isolated patterns of the flower structure with distinct velocity architectures. These features indicate (1) the crustal pattern beneath the north part of the profile mainly reflects the strong north-south compression and shortening deformations induced by B-episode events of the Yanshanian Movement within a tectonic framework of the amalgamation of the Ordos and Yinshan blocks in the Paleo-Proterozoic and the orogeny of the CAOB in the Paleozoic, and also impose the subsequently extensional deformation since the Cenozoic, especially from the strong rifting and sagging since the Neogene;(2) At the collision zone between the Ordos and Yinshan blocks, was the rifting of the Hetao Graben System initiated in the Oligocene, which implying the junction zone between the continental nuclei of the craton is still tectonically weak up to today;and (3) the Daqingshan and Sertengshan piedmont faults are the major faults with normal and striking senses, while the Wulashan piedmont fault and the northern marginal fault both play the adjusting roles between them. The seismogenic fault of the 1996 Baotou MS6.4 earthquake is likely the deeply extending part of the eastern part of the south-dipping Sertengshan piedmont fault.