天山造山带地壳速度、密度结构与物性研究

As one of the most spectacular and active intercontinental mountain ranges in the world, the Tien Shan has been regarded as a classical lab of intercontinental orogeny used to understand the geodynamics of mountain-building processes. In central Tien Shan, the style of the present-day surface deformation is dominated by north-south compression and is characterized by the presence of large-scale, NEE-SWW trending thrust faults and associated earthquakes. Global positioning system (GPS) measurement reveal that present-day shortening rates between the Tarim craton and stable Eurasia reaching a maximum of ~20 mm/yr. All these evidences indicate that crustal deformation and shortening in the Tian Shan occurs due to northward push of the India plate. However, the mechanism of crustal shorting and thickening are still topics of debate, mainly resulted from poor knowledge of lithospheric (especially the middle to lower crust) structure and composition beneath the Tien Shan region. In this project, we plan to construct 3D crustal velocity and density models based on integrative analysis of seismic (chiefly, receiver function and seismic surface wave studies) and gravity data, which are recorded by broadband seismic stations located in the Tien Shan region and extracted from the global gravity model (EGM2008), respectively. The knowledge of the crustal velocity and density heterogeneity is also important for determining of physical properties and composition of the lithosphere and providing evidences of intracontiental-belt dynamic, such as magmatic underplating and crustal flow in the crust beneath the central Tienshan and underthrusting Tarim block beneath the southern Tienshan. All these results would provide new insights into geological structure and tectonic process of this region.

天山是中亚最令人瞩目的一条由陆-陆汇聚而成的陆内造山带，被认为是研究陆内造山及其动力研究的天然实验室。地质与地球物理调查研究表明天山地区地壳正在经历强烈的南北向缩短与增厚。然而，关于该造山带变形过程中地壳缩短与增厚的机制仍有不同的看法。其关键原因在于，现有的研究结果没有全面地刻画大陆碰撞过程中岩石圈（尤其是中、下地壳）的状态与物质组成。本研究拟通过多种地球物理手段（接收函数、面波和重力方法）联合约束，获取该区地壳P、S波速度、密度等深部信息，综合判断地壳不同深度的物质状态与组成。如，天山造山带是否存在高速、高密度的玄武质下地壳？是否存在低速、低密度的壳内部分熔融体？塔里木块体是否逆冲到天山之下？等，进而为天山造山带动力学模式的确定提供依据。

天山是研究陆内造山及其动力研究的天然实验室。关于该造山带变形过程中地壳缩短与增厚的机制仍有不同的看法。项目利用MANAS计划横跨中天山造山带布设的40个流动地震台站记录的远震资料，通过开展接收函数、面波联合反演研究，构建了中天山地壳S波速度结构，并通过解释重力布格异常资料给出了密度模型。结果显示：研究区南部的Kokshaal range和北部的Kyrgyz range地壳厚度深达~60-70km；而夹在南、北天山之间的Naryn 盆地、塔里木盆地北缘及哈萨克地盾下方地壳厚度厚~50-58km。中部天山中下地壳内显著的低速异常可能与上地幔热物质上涌导致的地壳部分熔融有关。天山台站下方铁镁质下地壳厚度与地壳厚度具有一定的正相关性，暗示岩浆底侵可能是造成该区地壳增厚的主要原因之一。利用Chinarray项目2期流动观测资料，采用双台法开展了面波成像研究，得到了青藏东北缘及邻区的壳幔速度结构。结果显示，西北祁连造山带和松潘-甘孜地块下方存在两个明显的壳内低速异常，推测松潘-甘孜地块下方壳内低速异常可能与软流圈上涌导致的地壳部分熔融有关；而西北祁连造山带中下地壳的低速异常则可能与深地壳流体有关。河套地堑地壳上地幔表现为低速异常，且该低速异常至少可以追踪到200km深处。鄂尔多斯和阿拉善块体东北部岩石圈表现为高速异常，而阿拉善西南和祁连山造山带岩石圈厚度明显减薄，表明阿拉善块体并没有俯冲到青藏东北缘下方。利用中国科学台阵2期近40个流动地震台站观测资料，通过接收函数H-k分析及接收函数、面波联合反演研究，获得了台站下方的地壳厚度、波速比及地壳S波速度结构。结果显示青藏高原东北部上地壳存在明显的S波低速异常，可能与壳内部分熔融有关，也可能是地质构造上的软弱层。青藏高原地壳厚达45-59km，其下地壳速度明显偏低，且没有Vs>4.0km/s的高速壳幔过度带，推测其地壳增厚可以用上地壳增厚/或者均匀增厚+拆沉来解释。鄂尔多斯块体西南部地壳厚度~47km，地壳波速比和全球均值接近，其下地壳平均速度~3.8-4.0km/s，推测鄂尔多斯块体西南部不存在厚的、高速的铁镁质下地壳。