青藏高原地壳地震纵波速度的层析成像

Crustal P-wave Seismic Tomography of the Qinghai—Xizang(Tibetan) Plateau

本次研究利用地方地震台站的数据开展青藏高原地壳地震波速度的三维层析成像研究,得到分辨率达到1°×1°×20 km的地壳纵波三维速度结构,揭示了青藏高原地壳内部地壳波速结构特征。结果表明,青藏高原P波波速随深度产生巨大变化,说明地壳内部发生了大规模的层间拆离和水平剪切,用传统的地块运动不能准确地描述地壳物质运动。从P波波速扰动图上看到,青藏高原上地壳和上地幔的P波波速扰动为大范围正异常区,可以认为青藏高原在同碰撞和后碰撞期频繁的岩浆活动和结晶作用,造成了现今相对比较坚固的上地壳和岩石圈地幔,使青藏高原保持一个整体。分布在可可西里和羌塘北部的高钾质和钾质火山岩带,反映为青藏高原地壳的P波波速扰动负异常带,从上地壳到下地壳都有分布。说明由于大陆碰撞使三叠纪的东昆仑缝合带重新破裂,造成大量地幔流体物质上涌和火山爆发,对高原的形成和隆升都有一定的贡献。通过地震层析成像取得的三维地壳波速图像,进一步证实了由密度扰动三维成像指出的存在青藏高原下地壳流和新生代裂谷深部到达了中地壳底部的结论。

Objectives: 3D seismic tomography has been carried out for imaging the mantle under the Qinghai-Xizang( Tibetan) Plateau,but has not been performed for imaging the crust beneath the Qinghai-Xizang Plateau. This paper presents results of 3D P-wave tomography of the Qinghai-Xizang Plateau for studying continental dynamics.Methods: We perform P-wave seismic tomography using seismic data collected from both the national and local stations for studying the crust beneath the Qinghai-Xizang( Tibet) Plateau,and obtain 3D crustal velocity disturbance images of scale of 1^° × 1^° × 20 km. We use more than 260000 travel-time picks for the inversion of Pwave velocity,producing accurate velocity images of the upper,middle and lower crust.Results: Our 3D P-wave tomography results show that patterns of the velocity disturbance change greatly with depth,indicating that the crust occurred inter-layers detachment and near horizontal shearing,and therefore,the crustal movement cannot be described by the so-called "block-moving". The upper-crust and uppermost mantle of the Qinghai-Xizang Plateau are occupied by large areas of high velocity,indicating that rather solid upper-crust and uppermost mantle have already formed by frequent magmatic intrusions and crystallization after the subduction and collision processes. Therefore,the Plateau maintains as a whole. The volcanic zones located along Cocoxili and northern Qiangtang show low P-wave velocity anomalies from upper to lower crust,indicating that the collision caused re-rupture of the East-Kunlun suture and a great amount of mantle magma uplift caused by volcanic eruptions,making contribution to the formation of the thick crust of the plateau. Our tomographic images also improve the conclusion drawn from crustal density imaging,and show that the Cenozoic rifts located in the southern plateau extend to bottom of the middle crust.Conclusions: Our results of the crustal P-wave seismic tomography support the following new discoveries: the lower-crust flows,the contribution of northernvolcanic zones and the southern rifting process.