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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 degrees and 100 degrees. 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 similar to 80 s with 0. 5 degrees X 0. 50 degrees horizontal resolution by applying a linear inversion strategy developed by Ditmar and Yanovskaya.

The phase velocity distributions at shorter periods (10 similar to 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 similar to 0. 2 km . s(-1) difference. For shorter periods, the difference is larger. At 20 similar to 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 similar to 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.