Crust And Upper Mantle Structure Of Mongolia

Mongolia is the interior of Central Asian Orogenic Belt (CAOB), which is one of the largest Paleozoic orogenic belts in the world. It is suggested that more than 50% of the crust is juvenile crust accreted during Phanerozoic time. CAOB is an important natural laboratory for the analysis of the accretionary tectonics and crustal evolution. This paper briefly summarized the open questions of CAOB, the geological structure of Mongolia, as well as the results of previous studies.Through adopting the data from the seismic array deployed in the South central Mongolia, we apply the receiver function, seismic wave attenuation and Pn velocity tomography to obtain the crust-mantle structures of South central Mongolia. Due to the lack of seismic data of densely deployed broadband seismostations, the crustal structures beneath Mongolia is still poor understood to date. This study is not only making up the blank of the crust-mantle structure research, but also significant for understanding the evolution and tectonic history of Mongolia.This study utilizes receiver functions from 69 portable broadband seismic stations deployed in south central Mongolia. The primary results reveal that the local crustal thickness varies from 38 to 46 km with an average value of 43 km. Thicker crust is found beneath the western Hentey Mountains, while thinner crust is located in the southern area of the Zuunbayan fault zone which is near the Chinese border area. The crustal thickness is gradually thinner from northwest to southeast. The Moho has a step change about 5 km around the Main Mongolian Lineament. The Bouguer gravity anomalies exhibit a strong correlation with the overall crustal thickness pattern throughout most of our regime. The mean crustal Vp/Vs ratio ranges from 1.68 to 1.83. The average value is 1.74, which is lower that the global continental average value of 1.78. Low Vp/Vs ratio is found beneath the western Hentey Mountains and high Vp/Vs ratios occur in the Middle Gobi volcanic regions and the Mesozoic-Cenozoic Southern Gobi Basin. Low Vp/Vs ratios in the western Hentey Mountains correlate well with regions of quartz rich crust and high heat flow, which may be the result of granitic magma and mantle activities related to the broken-off Mongol-Okhotsk oceanic subducted slab. Moreover, a new approach which intergrates the Bouguer anomaly gradient and the receiver function-derived crustal thickness is adopted to calcilate the density of the lower crust. Fairly dense lower crust of approximately 3000 kg/m3 is found in the Middle Gobi Desert, which correlate with the high Vp/Vs ratios, implying a mafic granulite composite. This mafic granulite composite may be the results of the residual of partial melting being related with the Holocene volcanoes.Through utilizing complete records of 69 broadband seismic stations, we manually pick 228 local earthquakes to map the tomographic model of Lg Qo for Southcentral Mongolia by using reverse two-station method. The reverse two-station method is capable of obtaining more reliable Q results because it eliminates the effects of site response, instrument response and source function. With the 17,015 reverse two-station ray paths for Lg wave, we have further tomographically mapped Lg Qo value of the south central Mongolia in order to understand the spatial variations in crustal attenuation. Taking the Mongol-Okhotsk suture as a boundary, high Qo values are found in the north and low Qo values occurre in the southern. The Lg attenuation models correlate well with the surface tectonic structure, the corresponding lateral variation of velocity and major tectonic units across south central Mongolia. The western Hentey Mountains with stable crust are characterized by the low crustal attenuation. High attenuation regions are found in the volcanic regions of the Middle Gobi Desert and tectonically active regions of South Gobi. High attenuation regions seem to be associated with low velocities, Quaternary volcanoes and Cenozoic tectonic activities.Travel times of Pn rays from CENC and ISC catalogs as well as hand-picked arrivals are inverted to map the velocity structure of the lithospheric mantle in Mongolia and adjacent regions. Compared to the previous studies, it has a better resolution in central and eastern Mongolia. Significant Pn velocity variations are associated with the regional tectonic activities. The average Pn velocity is 8.1 km/s in Mongolia and adjacent regions, which is similar to 8.09 km/s for the global average and slightly higher than 8.05 km/s for China. High Pn velocities represent colder mantle lid or a stable and rigid lithosphere like the Eastern Mongolia, Junggar Basin, Tarim Basin. Low Pn velocities are found in tectonically active regions like extension and rifting, or hot and partially melting lithosphere. The low Pn velocities in Sayan region, Baikal Rift and Hangay Plateau are thought as a result of the Cenozoic volcanoes or asthenosphere upwelling. Furthermore, low Pn velocities are found in the Hentey Mountains. But low Vp/Vs ratios and low attenuation of Lg wave in the Hentey Mountains show that the crust of Hentey Mountian is stable. It implies that the crust of Hentey Mountains seem not to be influenced by the hot material in the upper mantle.