青藏高原中部INDEPTH-Ⅲ剖面上地幔远震S波层析成像

印度板块向欧亚俯冲前缘位于班公—怒江缝合带附近,但是印度岩石圈地幔的俯冲形态和形变过程仍然缺乏共识,在不同地区使用不同方法获得的结果之间存在明显差异.本文使用青藏高原中部INDEPTH-Ⅲ剖面远震S波波形数据,提取走时信息,通过层析成像方法获得剖面下方S波速度扰动图像.结果显示:在班公—怒江缝合带下方100至300km深度范围内存在一个高角度(约65°)北倾的S波高速体,推测可能是回退的印度岩石圈板片或/和小规模对流引起的岩石圈拆沉后残留的印度大陆岩石圈板片.

Upper mantle structure beneath central Tibet derived from teleseismic S wave tomography along the INDEPTH-Ⅲ profile

The Tibetan Plateau is produced by the continental-continental collision between the India and Eurasian plates as well as the subsequent convergence. It has become a consensus that the crustal thickness beneath the plateau is twice of the normal continental crust, while the deformation of its mantle lithosphere, as an important part of the developing plateau, remains controversial. This ambiguity about the upper mantle structure is resulted from uneven distributed seismologic observations in Tibet, and upper mantle structure is possibly heterogeneous along the east-west direction. Another reason is that the seismic wave speed contrast between the continental mantle lithosphere and asthenosphere might not be significant to detect by current seismic methods. Because the geodynamic of mantle lithosphere is closely related to the processes of plateau uplift and the presence of volcanic rocks, revealing the geometry of Indian and Eurasian mantle lithosphere can help understanding the dynamic processes of mantle lithosphere as well as the evolution of the plateau.The INDEPTH-Ⅲ profile located in the center of Tibet is about 400 km long with totally 60 broad-band seismometers. We calculated the residuals of travel times by multichannel cross-correlation of the teleseismic S waveforms recorded by the INDEPTH-Ⅲ broad-band seismic array. In order to improve the reliability, we choose the events recorded by at least 8 stations, and choose the events of which back azimuth is less than 45° to the profile. We totally select 622 rays. The damping coefficient is fixed at 10. Then we calculate the perturbation of S wave velocity beneath the profile. The tomography of slowness anomalies is obtained beneath the Bangong-Nujiang suture after four iterations. Our results show that a north-dipping S wave high velocity anomaly is present in a depth range of 100 to 300 km beneath the Bangong-Nujiang suture, with a dipping angle about 65°. The resolutions are 0.5° in horizontal and 80 km in vertical directions, respectively.The high velocity anomaly found in this study is almost same as the P wave tomography of previous study (Tilmann et al. 2003), which is interpreted as the roll-backed Indian lithosphere or/and remnant of Indian mantle lithosphere from convective removal. S waves are more sensitive than P waves to temperature anomalies, while P waves are more sensitive to material composition. It results in some slight differences between our S wave tomography and the P wave tomography of previous study.