青藏高原上地幔顶部三维P波、S波速度及地震波速比Vp/Vs研究

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.

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。

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；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波速度梯度或者部分熔融；但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仍不清楚。.本项目收集青藏高原及周边地区固定台阵和流动台阵记录的地震事件震相和波形等数据，开展如下研究：1)青藏高原及周边地区地壳和上地幔顶部P波速度结构；2）建立S波速度模型和精细限定了Sn波不发育区(低效传播区域)的水平和深度范围；3) 发展了基于共反射点偏移叠加地震成像新方法，可用于岩石圈间断面精细结构探测。.不同频率段的Sn波不发育区域分布差异揭示了青藏高原羌塘地体下方存在明显的部分熔融程度差异，部分熔融程度在羌塘地体下方最高，其他地区上地幔顶部的部分熔融程度较低。P波速度层析成像结果显示在羌塘下方印度岩石圈发生撕裂，其上方的低速结构可能指示了岩石圈撕裂后的热物质上涌。这一结果厘清了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，即印度岩石圈在羌塘地体下方发生撕裂，导致软流圈热物质上涌至上地幔顶部以及地壳甚至是地表。这为印度岩石圈俯冲到青藏高原下方的动力过程提供了新的认识。