摘要
本文利用"中国地震科学台阵"探测项目在南北地震带北段布设的678个流动地震台站在2013年10月至2015年4月期间记录到的远震波形数据,经过波形互相关拾取到473个远震事件共130309条P波走时残差数据,通过远震层析成像研究获得了该区(30°N-44°N,96°E-110°E)下方0.5°×0.5°的P波速度扰动图像.结果显示,研究区下方P波速度结构显示强烈的不均一性和显著的分区、分块特征.岩石圈速度结构具有显著的东西差异:祁连、西秦岭和松潘甘孜地块组成的青藏东北缘地区显示明显的低速异常,而属于克拉通性质的鄂尔多斯地块和四川盆地则显示高速异常,表明东部克拉通块体对青藏高原物质的东向挤出起到了强烈的阻挡作用.阿拉善地块显示出弱高速和局部弱低速的异常并存的特征.阿拉善地块西部显示低速异常,而东部与鄂尔多斯相邻的地区显示高速异常,可能表明该地区的岩石圈的变形主要受到青藏高原东北缘的挤压作用.在鄂尔多斯和四川盆地之间的秦岭下方100~250 km深度上表现为明显的低速异常,表明该处可能存在软流圈物质的运移通道.鄂尔多斯北部的河套裂陷盆地下方在100~500 km深度内低速异常表现明显,说明该区有深部热物质上涌且至少来源于地幔过渡带.青藏东北缘上地幔显示低速异常且地幔过渡带中出现明显的高速异常,这种结构模式暗示了在青藏高原东北缘可能发生了岩石圈拆沉作用,而高速异常体可能是拆沉的岩石圈地幔.
The teleseismic waveform data used in this paper were recorded by 678 portable broadband seismic stations located on the northern South-North Seismic zone from October 2013 to April 2015. The 130309 P wave traveltime residuals were picked with the waveform correlation method. A three-dimensional upper mantle P wave velocity model beneath the northern SouthNorth Seismic Zone(30°N—44°N, 96°E—110°E) with ahorizontal grid space of 0. 50°×0. 5° was obtained by teleseismic tomography, which reveals significant inhomogenities and strong block characteristics. Velocity structures of the lithosphere beneath western and eastern part of the area show different characteristics: The northeastern margin of Tibetan Plateau, including Qilian block, western Qinling and Songpan-Ganzi blocks, show obvious low velocity anomaly; the Ordos block and the Sichuan Basin, which are cratons, show high velocity anomalies. This comparison indicates that the eastern stable cratonic blocks(the Ordos block and the Sichuan Basin) may obstruct eastward flow of the Tibetan Plateau material. Western Alashan shows low velocity anomaly, and Eastern Alashan, which is adjacent to the Ordos, shows high velocity anomalies,indicating the deformation of the lithosphere in the area may be mainly affected by the extrusion of the northeastern margin of the Tibetan Plateau. Low velocity anomalies are dominant at the depth of 100~250 km beneath the Qinling Mountains,between Ordos and Sichuan basin,which may implicate an asthenospheric channel. There is a low velocity anomaly beneath the Hetao basin in northern Ordos at the depth of 100~500 km, indicating that the deep hot mantle materials may be upwelling from the mantle transition zone. Upper mantle beneath northeastern margin of Tibetan Plateau shows low velocity anomaly and the mantle transition zone appears obvious high velocity anomalies, suggesting that lithosphere delamination might have taken place beneath the northeastern margin of the Tibetan Plateau.