Crustal S-wave velocity structure of the Abaga area of Xing’an-Mongolia Orogenic Belt from ambient noise

Cenozoic intraplate volcanoes are widely developed in the central part of Inner Mongolia in the Xing’an-Mongolia Orogenic Belt. These volcanoes are mainly distributed in groups to the west of the Greater Xing’an-Taihang Mountain Gravity Lineament, forming a number of volcanic regions, such as Abaga, Beilike and Dalinor volcanic regions. And these three volcanic regions are connected with the Dariganga volcanic region in southern Mongolia. The origin and dynamic mechanism of these Cenozoic intraplate volcanoes is still a controversial topic. Previous studies have mainly proposed the following three mechanisms: lithosphere delamination; upwelling of mantle plume in mantle transition zone; westward subduction of the Pacific plate.

In this study, to further reveal the magma structure and origin of these intraplate volcanoes, a high-resolution 3-D S-wave velocity structure of the crust in the study area was established. Based on the continuous data of the Abaga Array deployed in the central part of Inner Mongolia in the Xing’an-Mongolia Orogenic Belt, the Rayleigh surface wave dispersion with period range of 5 s to 30 s was obtained. Then the high-resolution 3-D S-wave velocity structure of the crust was further obtained by using the Markov Chain Monte Carlo method. The results show the crustal velocity of the Abaga and Dalinor volcanic group is low, and there is an obvious low velocity zone at about 30 km depth in the lower crust, which may be the magma chamber in the lower crust of the volcanic groups. The results also imply the low velocity in the crust of Abaga volcanic group, Dalinor volcanic group, Dariganga volcanic group and Ulanhada volcanic group are interconnected. According previous results, 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, which invaded the crust along the suture zones or faults.