Crustal Structure Of The Middle Segment Of Central Asian Orogenic Belt

The Central Asian Orogenic Belt is the largest phanerozoic accretionary orogenic belt in the world,located in the south of the Siberian Craton and the Baltic Craton,and the north of the Tarim Craton and the North China Craton.It is influenced by the three dynamic systems of the ancient Asian Ocean,the Tethys Ocean and the Pacific Rim.It is a complex giant orogenic patchwork composed of microlandmasses,ophiolite hybrid belts,collision belts and multi-cycle composite orogenic belts.In the Paleozoic and Mesozoic,it was mainly controlled by the multi-stage tectonic evolution of the Paleo-Asian Ocean and the Mongol-Okhotsk Ocean.Since Cenozoic,under the influence of the far field effect of Indo-Asian plate collision and Pacific plate westward subduction,the Central Asian Orogenic Belt is in extensional tectonic background,with extensive extensional basins and intra-continental volcanoes.The time and location of the closure of the ancient Asian Ocean and the Mongol-Okhotsk Ocean,the crustal extensional structure and its mechanism,and the genetic mechanism of intra-plate volcanoes have been the hot research issues in this area.The high-resolution structure of the crust including sedimentary layers helps to answer these questions,but due to the limitation of station,the imaging results of the middle Central Asian Orogenic Belt are few and low resolution.Funded by the International Cooperation Program of the Ministry of Science and Technology,the Institute of Geophysics of the China Earthquake Administration and the Institute of Astronomy and Geophysics of the Mongolian Academy of Sciences have worked together to set up 69 wide-band seismographs in central and southern Mongolia from August 2011 to August 2013.Supported by the National Natural Science Foundation of China,the Institute of Geophysics of the China Earthquake Administration has set up 36 mobile seismic observation stations from January 2013 to July 2015 in Abaga area of Inner Mongolia,which is adjacent to the central and southern Mongolia.In this study,the high resolution three-dimensional shear velocity structure of the crust in the middle of the Central Asian Orogenic Belt was established by the joint inversion of Rayleigh wave phase velocity,ellipticity and teleseismic P-wave waveform in the above two regions.On the one hand,this study provides the seismic evidence of the closure location of the ancient Asian Ocean and the Mongol-Okhotsk Ocean;on the other hand,it provides the distribution characteristics and internal structure of the Mesozoic extensional basin in the middle of the Central Asian Orogenic Belt,which is of significance to the evolution of the extensional basin and the regional tectonic stress environment,and also provides the constraint of the genetic mechanism of intra-plate volcanism in the region.The Rayleigh wave phase velocity,crustal S-wave velocity,Rayleigh wave ellipticity,sedimentary basement depth,Moho depth,crustal wave velocity ratio,and Lg Q value in south-central Mongolia show significant variation along the Mongol-Okhotsk suture and the Main Mongolian Lineament,indicating that these two sutures are at least crustal tectonic boundaries.The sedimentary thickness increases gradually from northwest to southeast in general,and the Hangay-Hentay Mountain is almost negligible,the average sedimentary thickness is 1.0 km in the Middle Gobi Blet,and the maximum is 2 km in the southeast of the East Gobi Desert.The crust thickness gradually decreases from northwest to southeast,ranging from 40-47 km,which is thicker than the global average continental crust thickness.In the Middle Gobi Belt,Middle Gobi volcano and Bus-Obo volcano are interconnected in the crust,and there is a large low-velocity layer in the lower crust,which implies the magma chamber of the two intraplate volcanoes exists there.Furtherly it is speculated that the magma chamber is related with underplating by mantle convection beneath Hangay Dome.The Rayleigh wave ellipticity,sedimentary basement depth,Moho depth and crustal wave velocity ratio in Abaga area of Inner Mongolia show significant variation along the Erenhot-Hegenshan suture and the Solonker-Xar Moron suture,indicating that these two sutures are at least crustal tectonic boundaries.The sedimentary thickness is generally divided by the Erenhot-Hegenshan suture and the Solonker-Xar Moron suture.The sedimentary thickness of the Bayanbaogeli uplift north of the Erenhot-Hegenshan suture and the Wenduermiao uplift south of the Solonker-Xar Moron suture is almost negligible.The sedimentary thickness of most areas in Wulanchabu Rift is 1.0 km and the maximum is 1.4 km,the average sedimentary thickness of Manit Rift and Tenggar Rift is 1.5 km,and the average edimentary thickness of other areas is 0.8 km.The crustal thickness is between 35-44 km,which is thinner than the average crustal thickness of the global orogenic belt on the whole,and it gradually decreases from southwest to northeast.The low crustal velocities of Abaga,Dalinor,Dariganga and Wulanhada volcanic group are interconnected.No magma pockets are found in the crust of the Abaga volcanic group,but there is a magma pocket in the crust of the Dalinor volcanic group.The magma sources of the Abaga volcanic group and the Dalinor volcanic group are in the uppermost layer of the upper mantle.Furtherly it is speculated that the formation of the Abaga and Dalinor volcanic group is due to the upwelling of hot material in the upper mantle and the intrusion into the crust along the suture zones or faults.The structure of the middle segment of the Central Asian Orogenic Belt is not only related to the paleostructure formed during the Paleozoic and Mesozoic,but also influenced by the neotectonic activities caused by the Cenozoic Indo-Eurasian collision and the westward subduction of the Pacific Ocean.The interaction of these two mechanisms formed the present tectonic pattern of the middle segment of the Central Asian Orogenic Belt.