青藏高原地壳厚度两倍于普通大陆，其岩石圈地幔略厚于地壳。羌塘地体下方，地壳发生部分熔融和地壳流，软流圈发生上涌，但二者关系不确定且有争议。岩石学和地球化学研究表明羌塘地体大量岩浆岩源于地壳和地幔。地震学指示羌塘下方Sn波传播低效，说明上地幔顶部存在明显负S波速度梯度。上地幔顶部是连接地壳和上地幔的关键区域，其P波、S波速度和波速比可有效反映温度、壳幔间热和物质组分变化。为厘清羌塘下方地壳流与软流圈上涌的关系，本项目拟开展青藏高原上地幔顶部三维P波、S波速度和Vp/Vs的研究。主要内容：1)建立青藏高原及周边上地幔顶部三维P波、S波速度和Vp/Vs波速比模型；2)利用数值模拟方法研究莫霍面起伏与速度横向变化对Sn波传播的影响；3)联合速度结构与波速比Vp/Vs确定青藏高原上地幔顶部部分熔融的空间分布尤其是深度方向；以揭示青藏高原地壳部分熔融和软流圈上涌的壳幔相互作用过程并提供地震学约束。

The Tibetan plateau has a crust that is nearly double the normal continental thickness, with a lithosphere little thicker than the crust. The structure of the crust is complex with possible partial melt and consequent crustal flow. Further asthenospheric upwelling may occur beneath the Qiangtang Terrane. But the relationships between the different features are still uncertain and controversial. A large amount of magmatic rocks have been found in the Qiangtang Terrane, whose petrologivcal and geochemical character indicate that they from both the crust and mantle. Seismological observations indicate a region of poor Sn propagation beneath Qiangtang, which suggests the presence of a significant negative S velocity gradient in the uppermost mantle. The uppermost mantle is a key region linking the crust and the mantle: its P wavespeed, S wavespeed and Vp/Vs ratio can help to reveal the temperature and chemical composition, and the transfer of heat and compositional variations between the crust and the mantle, particularly when combined with geochemical results. To constrain the relationships between crustal flow and the asthenosphere upwelling beneath Qiangtang, the project will characterize the 3-D P wavespeed, S wavespeed and Vp/Vs ratio for the crust and uppermost mantle beneath the Tibetan Plateau. The main components will be : 1) building a 3-D seismological reference model of uppermost mantle beneath the Tibetan Plateau and its surroundings; 2) using numerical simulation to investigate the influences of Moho topography and wavespeed structures on Sn propagation efficiency; 3) combining the wavespeed structures and Vp/Vs to determine the spatial distributions (especially in depth) of partial melt in the uppermost mantle beneath the Tibet Plateau. The project aims to constraint the interactions between the shallower crust and the mantle to impose seismological constraints on the mechanisms of rifting and shortening of the Tibetan Plateau.