青藏高原东部的Pn波层析成像研究

Pn wave tomography of eastern Tibetan plateau

利用INDEPTH/ASCENT台阵和其它布设在青藏高原的流动宽频带地震仪数据,反演了青藏高原东部和周边区域的上地幔顶层Pn波速度以及台站延迟.研究区域的平均Pn波速度是8.1km/s,略高于中国大陆的平均Pn波速度.低速区主要分布在羌塘地块的西部和松潘—甘孜地块,高温异常的岩石圈上地幔很可能是导致这一低速区的原因.班公-怒江缝合带东端区域的Pn波速度达到8.35km/s,这一高速区可能与向北俯冲的印度板块(东端)有关.另一Pn波高速区分布在祁连山和昆仑山之间,主要由柴达木盆地和共和盆地及其周边地区,两个并不完全连续的高速异常区组成,它可能对应于特提斯洋闭合时北部增生的克拉通地体;在后来的欧亚板块与印度板块的碰撞中,这一地体有可能阻挡了青藏高原向北的生长.相对密集的台站提供了高分辨率的速度结构横向分布和地壳厚度变化.台站延迟显示青藏高原北部和东部的地壳存在显著的减薄——松潘—甘孜地块东北缘的地壳厚度仅为约50km,而羌塘地块东部唐古拉山地壳最厚,达到75km,这可能是由于印度—欧亚板块碰撞引起的羌塘地块内部变形增厚所致.

Data from recent INDEPTH IV and other broadband deployments in Tibet are used to invert for crustal delays and Pn velocities beneath the eastern Tibetan Plateau and surrounding regions. The average Pn velocity for the region is 8. 1 km/s but varies from 7.8 to over 8. 3 km/s.Generally low Pn velocities are found in the northeastern Tibet in the Qiangtang and Songpan- Garzfi terrains. This includes a zone of very low Pn velocity northwest of the Longmen Shan thrust, along the eastern Kunlun fault, and beneath the eastern Qilian Shan. A region of high Pn velocity underlies the eastern end of the Bangong-Nujiang Suture （bounded by 31--33~N~ 90-- 98~E）. It could represent part of the underthrusting Indian shield. The region between the Qilian Shan and Kunlun Shan is also characterized by high Pn velocity with several zones of extremely high velocity. This includes two high velocity features beneath Qaidam basin and Gonghe basin. These features may correspond to cratonic fragments that accreted during the closure of the Tethys Ocean and have impeded, but not stopped, the northward growth of the plateau. The station delays show the thickest crust is beneath Tanggula Shan in central Tibet and should be the result of the internal deformation within Qiangtang terrain due to the collision of Indian and Eurasia plates. There is a significant decrease of crustal thickness in the northeast plateau and lateral variation also exists within the region.