蒙古中南部地区面波相速度层析成像

本文采用小波变换频时分析技术提取了1893条蒙古中南部地区双台间基阶Rayleigh波相速度频散曲线,通过对提取到的相速度频散进行二维反演,首次重构了蒙古高原中南部10~80s周期内分辨率为0.5°×0.5°的相速度分布图.结果表明,短周期相速度频散(10~20s)明显受地表地形控制,表现为杭爱—肯特山盆地为相对高速异常,而南部的戈壁带表现为相对低速异常;在30s到60s周期内,南部的戈壁带和北部的杭爱—肯特山盆地都显示出稳定的高速异常,而中部戈壁带则表现为低速异常.该低速异常区与新生代火山岩出露位置的一致性,暗示该低速异常可能与新生代火山活动有关.穿过蒙古高原中部的两条主要断裂带与研究区内的地震波速度分布具有很好的一致性,可能暗示两条断裂带一直延伸到整个岩石圈.此外,蒙古中南部地区的壳幔速度低于全球平均大陆值,且其相速度频散曲线与遭受破坏的克拉通,尤其大陆裂谷很相似,可能暗示着研究区具有薄的、活跃的岩石圈构造.

Rayleigh wave tomography of the phase velocity in the central and southern Mongolia

The Central Asian Orogenic Belt (CAOB) is one of the World's largest Paleozoic orogeny zones, with more than 50% juvenile crust and provides a unique place to study the continental growth mechanism, as a matter of debate. Geological structure is influenced by the complicated orogeny. In order to study the structure of the crust and upper mantle in the middle of CAOB, we collected the vertical components of seismograms recorded at 69 broadband seismic stations deployed in the south-central Mongolia from August 2011 to August 2013. The fundamental mode Rayleigh-wave phase velocity dispersion along 1893 inter-station paths in the central and southern Mongolia were estimated by the continuous wavelet transformation method. We selected 208 events with M_w between 5.5 and 7.5, focal depths<70 km, and epicentral distances between 15°and 100°. The vertical-component seismograms of these events were decimated to 1 Hz and removed the average and linear trends. The dense distribution of seismic stations provided a good spatial and azimuthal coverage across the region. After determining the inter-station phase velocities, these dispersion data were utilized to construct the first 2-D phase velocity maps at 10~80 s with 0.5°×0.5° horizontal resolution by applying a linear inversion strategy developed by Ditmar and Yanovskaya.The phase velocity distributions at shorter periods (10~20 s) are controlled by geologic units on the surface. It shows that Hangay-Hentiy Basin is associated with high velocity anomaly and the Gobi desert in the south low velocity with 0.1~0.2 km·s^-1 difference. For shorter periods, the difference is larger. At 20~40 s, phase velocities are strongly influenced by the velocity structure in the middle, lower crust and even upper mantle as well as by crustal thickness. According to the receiver function, the crustal thickness in the northwest is about 46km and is reduced to 38 km in the southeast. It is interesting that the northwest still shows high phase velocities. At 40~80 s, the distributions in the Southern Gobi and Hangay-Hentiy Basin show high velocity while the Middle Gobi shows low velocity. All phase velocity dispersion curves from this study is averaged and then compared with dispersion in some typical regions. The phase velocity across the region is obviously lower than that of AK135 model. Compared to eastern North China Craton, the difference becomes less with period longer than 40 s. Our averaged dispersion curve is strikingly similar with that of the Western branch of the East African Rift. Based on the distributions of Cenozoic volcanic rock in the Middle Gobi, it is inferred that the low velocity anomaly is related to the Cenozoic volcanism. The locations of the two main faults in the middle Mongolia is consistent with the features of phase velocity distributions at almost all periods. This indicates that the depth of faults extends to entire lithosphere. In addition, the velocity in the crust and upper mantle is lower than AK135 and the dispersion shows the similar characteristic with that of destructed craton, especially the continental rift. It indicates that the lithosphere in the central and southern Mongolia is thin and active.