【摘要】 地震层析成像是研究地球深部结构和动力过程的重要手段。文章简单介绍了岩石圈地震层析成像的几种基本方法。相对于人工震源地震勘探,基于天然震源的地震层析成像是提供盆地基底和周边深部背景的有效和极其经济的手段。笔者最近得到了中国大陆岩石圈高分辨率表面波层析成像的三维S波速度模型,该模型结合了地震和噪声互相关的数据,大大提高了射线覆盖,结果显示了中国西部与青藏高原接壤的三大盆地(塔里木、柴达木和四川盆地)共同特征:(1)盆地上地壳速度很低,反映了盆地沉积层很厚;(2)相对于周边山系,盆地中、下地壳的S波速度较快,上地幔顶部尤其明显;(3)盆地的地壳厚度比相邻的山脉区薄,同时莫霍深度在盆山结合带变化大。盆地内地壳和岩石圈地幔存在明显的横向结构,尤其是塔里木盆地和四川盆地。塔里木盆基底东西向中央古缝合带在地壳和岩石圈地幔的速度、莫霍面深度图中有清楚显示。笔者推测区域构造挤压的影响很可能涉及盆地的整个岩石圈,进而提出了一个简单的盆地形成的机械模型,即挤压隆升沉降模型,认为在挤压环境下,较弱的周边山系的岩石圈增厚和隆升,高强度的盆地块体在重力均衡下整体沉降,形成陆内叠合盆地。挤压应力可能导致巨大的塔里木和四川盆地产生岩石圈范围的褶皱变形。在西部盆地漫长的地质历史中,新生代的印藏碰撞和新元古代以来的多次构造运动产生的岩石圈挤压对西部盆地的形成和演化可能起了决定性的作用。更多还原

【Abstract】 Seismic tomography is an important tool for studying the structure and dynamics of the Earth’s interior.We gave a brief introduction to the main methods used for seismic tomography of the lithosphere.Relative to seismic exploration with controlled sources,seismic tomography with passive natural sources is an effective and economic method for imaging the deep structure of the basement and the surrounding of a sedimentary basin.We’ve recently obtained a high-resolution S-wave velocity model of the lithosphere in China from surface-wave tomography,which combines data from earthquakes and ambient noise correlations with much improved ray coverage.Our results show major common features of the three basins(Tarim,Qaidam,and Sichuan)surrounding the Tibetan Plateau in the western China:(1)the upper crust of the basins shows slow velocities corresponding to the thick sedimentary layers;(2)the mid-crust,lower crust,and in particular,the uppermost mantle show faster speeds than the surrounding mountains;(3)the basins have thinner crust than the surrounding mountains and the Moho depth change is rapid.Significant lateral structure also exists within the basins,in particular,the Tarim and Sichuan Basins.The east-west paleo-suture in the central part of the Tarim is clearly visible in the crustal and mantle lithosphere velocities and in Moho depth variation.These results suggest that regional compression may involve the entire lithosphere.We propose a mechanical model(called compressional uplift-subsidence model)for the formation of the basins:under the regional compressional stress,the weaker lithosphere blocks surrounding the basins are thickened and uplifted,and the stronger basin blocks subside relative to the surrounding mountains under the lithosphere isostacy,forming an intra-orogenic basin.The compressional stress may have also deformed the giant Tarim and Sichuan Basins and created folds in a lithosphere scale.In the long geologic history of the formation of the major basins in the Western China,the lithosphere-scale compression from the Cenozoic India-Eurasian collision and several orogenic episodes since the Neo-Proterozoic may have played a determining role in the formation and evolution of the basins.更多还原