天山造山带地区瑞利面波相速度与方位各向异性

天山造山带作为世界上陆内最大的造山带之一，现今地震活动频繁，造山运动强烈，是开展陆内造山和内陆地震活动研究的天然试验场.本文利用整个天山造山带地区国内及国际台网的108个地震台站连续三年的背景噪声资料，提取了8~50 s周期的瑞利面波相速度频散曲线，并构建了整个天山造山带地区的二维瑞利面波相速度与方位各向异性分布图像.结果表明：浅部结构与地表的地质构造单元具有较大的相关性.低波速异常主要分布于沉积层厚度较大的盆地地区，而高波速异常主要分布于构造活动比较活跃的山脉地区.东天山地区中下地壳存在比较弱的低波速异常，而塔里木盆地和准噶尔盆地汇聚边缘的上地幔区域则表现为明显的高波速异常，各向异性快波方向呈现近NS向的特征，暗示着塔里木盆地和准噶尔盆地的岩石圈已经俯冲至东天山的下方.中天山地区的中下地壳至上地幔区域均呈现为明显的低波速异常，且各向异性快波方向变化比较复杂，表明中天山地区的整个岩石圈结构已经弱化，热物质上涌可能对介质的方位各向异性有一定的影响.西天山及帕米尔高原的上地幔区域存在低波速异常，各向异性表现为NW-SE方向，可能与欧亚板块的大陆岩石圈南向俯冲有关.塔里木盆地内部存在相对弱的低波速异常，推测塔里木盆地可能已经受到上涌的地幔热物质的侵蚀和破坏.

Rayleigh wave phase velocity and azimuthal anisotropy of Tien Shan orogenic belt from ambient noise tomography

The Tien Shan orogenic belt is one of the largest intercontinental orogenic belts in the world with intensive seismic and tectonic activities. It is an ideal site to study the formation and evolution mechanism of the intercontinental orogenic belts. In this study, we collect 108 seismic stations from the Tajikistan National Seismic Network, Kyrgyz Seismic telemetry Network, Kazakhstan Network, Kyrgyz Digital Network and China National Seismic Network during the period of January 2012 to December 2014 throughout the entire Tien Shan orogenic belt and its surroundings. The empirical Green's functions of Rayleigh wave are extracted using the cross-correlation technique, and the dispersion curves of Rayleigh wave phase velocity from 8 to 50 s periods are measured from the empirical Green's functions of each station pair. Furthermore, we determine the structure of Rayleigh wave phase velocity and azimuthal anisotropy with the surface wave tomography technique.
Our new tomographic model reveals prominent lateral heterogeneities beneath the entire Tien Shan orogenic belt. The shallow structures in the study area show good spatial correlation with surface tectonics. Low velocity anomalies and high velocity anomalies are imaged beneath basins and surrounding mountain ranges, respectively. The Eastern Tien Shan exhibits distinct low velocity anomalies from the middle crust to the lower crust, and high velocity anomalies with the N-S fast wave directions are observed on the margin between the Tarim Basin and Junggar Basin, which may indicate the lithosphere of the Tarim Basin and Junggar Basin has underthrusted into the upper mantle beneath the Eastern Tien Shan. Prominent low velocity anomalies beneath the Central Tien Shan extend from the middle crust to the upper mantle, and the directions of fast wave are complicated, thus we can infer that the lithospheric structure of the Central Tien Shan is weak. The upwelling of hot mantle materials may influence the pattern of azimuthal anisotropy beneath the Central Tien Shan. The low velocity anomalies are observed beneath the Western Tien Shan and the directions of fast wave are NW-SE trending, which may represent the continental crust of the subducting Eurasia lithosphere that has been pulled down into the mantle beneath the Western Tien Shan. The lower velocity anomalies beneath the Tarim Basin suggest that the Tarim basin is not a stable cratonic lithosphere but has been eroded and destructed by the upwelling hot low velocity materials.