基于P波三重震相的华南地区上地幔速度结构研究

华南块体是研究太平洋板块俯冲和岩石圈减薄机制等问题的最佳场所之一.本文基于中国地震观测台网和大型流动台阵记录到的震中距10°~30°之间的两个中深源地震P波记录,利用三重震相波形拟合技术,获得了中扬子克拉通和华夏地块上地幔高精度P波速度结构.研究结果表明:(1)中扬子克拉通过渡带底部存在高速异常,系太平洋俯冲板块的滞留体.俯冲的板块并没有进入下地幔,660-km间断面下沉约11 km,与后尖晶石相变的克拉伯龙斜率为负有关.而华夏地块过渡带底部并无明显高速异常,接近全球平均模型;(2)整个华南块体,410-km间断面上方普遍存在低速层,主要与上地幔部分熔融有关,与IASP91相比P波速度减小了1.38%~2.29%;(3)在研究区域内,中扬子克拉通和华夏地块都存在岩石圈减薄(80 km),推测可能与太平洋板块俯冲和快速回撤导致的岩石圈拆沉有关.且华夏地块减薄程度较明显,下伏软流圈速度较低,说明其上地幔强度较弱、温度较高.另外,中扬子克拉通过渡带中存在一个较宽的速度梯度带,可能与520-km间断面有关,其具体成因有待进一步研究.

Upper mantle velocity structure beneath South China derived from triplicated seismic P waveforms

The South China block is one of the best places to study lithospheric thinning mechanisms associated with Pacific plate subduction. Triplication waveforms of two moderate earthquakes ranging from 10° to 30° recorded by CDSN (Chinese Digital Seismic Network) and temporary seismic stations deployed in the study area are used to image high resolution upper mantle structure beneath the Middle Yangtze craton and Cathaysia block. The results show that (1) a high-velocity anomaly is present in the lower mantle transition zone beneath the Middle Yangtze craton, which is interpreted as the stagnant Pacific slab. The subducted slab didn't penetrate into the lower mantle but depressed the 660-km discontinuity by 11 km as a result of negative Clapeyron slope of post-spinel phase transition. Without an obvious high velocity anomaly, the mantle transition zone beneath the Cathaysia block is similar to the IASP91 model. (2) A low-velocity layer atop the 410-km discontinuity with the P velocity reduced by 1.38%~2.29% compared with IASP91 model in the region is presumably caused by partial melting of upper mantle materials. (3) Within the study area, the seismic lithospheric thickness is less than 80 km on average for both the Middle Yangtze craton and Cathaysia block, indicating that the lithospheric thinning is likely due to delamination related to the subduction and rapid retreat of the Pacific plate. The thinner lithosphere and somewhat lower asthenosphere velocity in the Cathaysia block suggest a slightly weaker and perhaps warmer uppermost mantle than that in the Middle Yangtze craton. Moreover, there exists a broad velocity gradient belt probably corresponding to the 520-km discontinuity beneath the Middle Yangtze craton, and its genesis needs further study.