青藏高原东北缘壳幔几何结构与物性研究

The northeastern Tibetan plateau (NETP) with a significant lateral variation on crustal thickening is the important region to understand the thickening and lateral growth of the Tibetan plateau. Diverse deformation modes have been put forward such as vertically coherent deformation, thrust imbrications in the upper crust, lower-crust injection, convective removal of mantle lithosphere, but these hypotheses need to be verified by the investigation on the structure of the crust and the upper mantle. The studies of geodynamics show that different modes are characterized by distinctive geometric structure and physical properties in the crust and the upper mantle. To acquire the structure of crust and upper mantle beneath this region is beneficial to assessment these modes of crustal thickening and deformation. In the NETP, there are large amounts of telesismic waveforms data and a sensible distribution of broadband seismic stations throughout the region. In this project, receiver function method is used to image the interfaces in the crust and upper mantle such as Moho and the lithosphere-asthenosphere boundary, to calculate the crustal thickness and the Poisson's ratio, to reconstruct the crustal S wave velocity (Vs) structure from modeling the waveform. The crustal thickness will be used to estimate the crustal shortening, and the upper and lower crust will be divided based on the Vs structure. We can study the deformation features of different layers by analysis the discrepancy of shortening between the upper and lower crust and the geometry of discontinuities in the crust and upper mantle. Integrating the physical properties such as the Vs structure, the Poisson's ratio, and the low velocity layer, we will try to understand comprehensively the crust deformation mode and the interaction between blocks.

青藏高原东北缘地壳增厚具有明显的横向变化，是理解高原增厚与侧向生长机制的关键帯域，人们提出了多种壳幔形变模式，例如：垂直连贯增厚、上地壳逆掩叠瓦、下地壳注入、岩石圈对流移除等，这些都有待深部结构探测的检验。地球动力学分析显示，不同模式具有特异的壳幔几何结构和物性组成，获取该地区壳幔结构有助于评价地壳增厚与变形模式。为此，本项研究拟利用东北缘宽频带远震波形数据丰富、台站分布较为合理的特点，使用接收函数方法进行壳、幔主要速度界面（Moho面、岩石圈底界面）成像，地壳泊松比和厚度计算，地壳S波速度结构波形拟合反演；利用地壳厚度估算地壳缩短量，依据S波速度结构进行地壳分层，通过分析上、下地壳缩短量的差异和地壳、上地幔中的速度界面的几何形态，研究不同圈层的形变特征；结合S波速度结构、地壳泊松比和低速层等物性信息，综合理解地壳形变模式及块体相互作用。

虽然地震学研究揭示青藏高原具有巨厚的地壳，但它是如何形成的，尚缺乏足够的证据和一致的认识。根据不同观测提供的线索，前人提出了诸如纯剪切缩短增厚、双层地壳叠置增厚、下地壳挤入增厚、地壳流注入增厚等模型。不同的增厚模型意味着不同的地表抬升历史过程和高原侧向生长演化过程。青藏高原东北缘最显著的抬升发生在中-晚中新世以来，目前仍处于高原地表抬升阶段，因此这里适于研究青高原侧向生长和地壳增厚模式。.本项目利用青藏高原东北缘地区固定地震台网宽频带远震数据，提取接收函数，通过Moho 面的P-to-S转换波和多次波振幅叠加扫描，获得了地震台下方地壳的厚度以及地壳平均速度比（P波与S波的速度之比，含义同泊松比）的横向变化，结合前人的人工源宽角反射折射地壳探测研究成果，通过地壳厚度估算东北缘地壳的缩短率。结果显示：地壳厚度与速度比的变化成反比，指示在地壳增厚过程中低速度比的地壳组成物质（如长英质岩石）在地壳中所占比例增加；海拔3千米以上地区地壳厚度小于Airy均衡模型的预测，指示在地壳增厚过程中低密度的地壳组成物质（如长英质岩石）在地壳中所占比例增加；沿N30°E方向，东北缘地壳缩短量为250-350公里， 远小于岩石圈板片所指示的缩短量（600-700公里），指示东北缘在缩短增厚过程中，地壳总体积在减小，可能是由于部分下地壳随地幔岩石圈向南俯冲，没有参与地壳的缩短增厚。.本项目的研究表明：青藏高原东北缘的地壳增厚主要来自于上地壳的贡献，如上地壳的纯剪切增厚或上地壳北向仰冲的简单剪切叠置增厚，而下地壳的底部与地幔岩石圈并未明显缩短。该研究成果为青藏高原的（早期）侧向生长和抬升机制研究提供了重要的参考。