岩石圈地幔结构及其对中国大型盆地的演化意义

Structure of Lithospheric Mantle and Its Implications for the Evolution of Major Basins in China

Pn 波是通过莫霍面下方的上地幔顶部的地震波 .由于Pn 波的速度随温度和物质成分而变化 ,以及Pn 波各向异性可以反映地幔形变的历史 .因此Pn 波的速度以及各向异性成为探索岩石圈结构的重要工具 .中国岩石圈地幔的Pn 速度的特征是很高速的异常区和很低速的异常区呈镶嵌状出现 ,反映了地质结构的不均匀性 .西部大型盆地 (塔里木、准噶尔、吐哈、柴达木和四川盆地 )具有较高的Pn 速度和较弱的各向异性 ,反映出这些盆地的岩石圈是冷的和坚硬的 ,其变形较小 .大面积的华北地区 ,在太古代的基底下具有明显的Pn 波低速度 .研究结果表明与这些地区裂谷、岩石圈减薄和地幔上涌区相一致 .Pn 波各向异性与在最新 (和目前正在进行 )的大规模变形期间 ,岩石圈地幔沿NNE向右旋简单剪切相一致 .华北的金矿藏以及华北和松辽盆地的石油储藏的位置明显地与该区的低Pn 波速度区相吻合 ,表明该区金属成矿和油储的形成与中、新生代以来在岩石圈地幔中的热活动 。

Seismic Pn waves sample the top of the upper mantle right beneath the Moho. Because Pn velocity varies with changes in temperature and material composition and Pn anisotropy may indicate the history of mantle deformation, Pn velocity and anisotropy have become an important tool to probe lithospheric structure. The lithosphere mantle Pn velocities of China are characterized by a mosaic of very fast and very slow anomalies, mirroring the heterogeneity of geology at surface. The major basins in the west (Tarim, Junggar, Tuha, Qaidam, and Sichuan basins) have high Pn velocities and small P anisotropy, which suggests that the lithosphere of these basins is cold and strong with little deformation. A large area of North China shows prominent low Pn velocities beneath the Archean basement. Our observations are consistent with rifting, lithospheric thinning, and mantle upwelling in the region. The Pn anisotropy is consistent with a dextral simple shear in the NNE direction in the lithosphere mantle during the last (and ongoing) major deformation period. The locations of gold ore deposits in North China and oil deposits in North China and Songliao basins correlate remarkably well with low Pn velocities of the region, suggesting the metallogenesis and oil formation of the region may be closely related to magma and thermal activity in lithospheric mantle and crust-mantle interaction since Mesozoic and Cenozoic.