关键词: 阿尔山火山群, 岩石圈拆沉, 火山活动, 面波层析成像, S波速度结构

Abstract:

Since the Cenozoic era, a series of intraplate volcanic groups have developed along the east and west sides of the Songliao Basin in the eastern part of the Central Asian orogenic belt. The A'ershan volcanic group is one of the Cenozoic intraplate volcanoes in the eastern section of the Central Asian orogenic belt. Further study of this volcanic group is of great significance for exploring and understanding the genesis of intraplate volcanoes in the eastern section of the Central Asian orogenic belt. In the past, the distribution of mobile seismic stations and some fixed stations used for imaging research on the A'ershan volcanic group was relatively sparse and did not fully cover the A'ershan volcanic group. The resolution of the crust-mantle structure obtained in the past was also slightly insufficient for exploring the genesis mechanism of the A'ershan volcanic group. This article utilizes the vertical teleseismic waveforms of 29 broadband mobile seismic stations near the A'ershan volcanic group and 8 fixed stations around them from May 2019 to December 2021. Through frequency-time analysis technology, 11 775 Rayleigh wave phase velocity dispersion between two stations is extracted. After excluding non-monotonic rise and phase velocity dispersion curves that differ significantly from most dispersion distributions, 11 010 high-quality Rayleigh wave fundamental phase velocity dispersion curves were ultimately obtained. Then, based on classical ray theory, the two-dimensional phase velocity distribution with a period of 10-80s and a grid size of 0.5°×0.5° is inverted by using the traditional dual station method. Except for areas not covered by radiation in the edge zone, the lateral spatial resolution of phase velocity in the study area is basically within 50km. The checkerboard test also showed that dividing the grid size of the study area into 0.5°×0.5° is feasible, and anomalies with a central area scale less than 0.5°×0.5° can also be identified. Afterward, the CRUST1.0 model was used as the initial crustal model, and the PREM model was used as the initial mantle model. The crustal thickness results obtained from the receiver function were used to constrain the thickness of each layer in the initial crustal model, further reconstructing the three-dimensional S-wave velocity structure of the crust and upper mantle in the study area. The results show that: within the range of the middle and lower crust, the S-wave velocity in the A'ershan volcanic area exhibits apparent low-velocity anomalies. Based on the characteristics of the high wave velocity ratio in the area, it is speculated that there may be a crustal magma chamber in the A'ershan volcanic group. There are multiple high-velocity anomaly structures within a depth range of 40-150km in the A'ershan volcanic group. The difference in the depth of high-velocity anomalies indicates the heterogeneity of the lithosphere thickness, and it is speculated that the thickness of the lithosphere in the A'ershan volcanic area does not exceed 100km. The deeper distribution of high-velocity anomalies may represent the dismantled lithosphere, while the shallower distribution of high-velocity anomalies may represent the undeveloped lithosphere or residual lithosphere after dismantling, reflecting the possibility of lithospheric detachment and subsidence in the region. There are S-wave low-velocity anomalies in the upper mantle on the north and south sides of the A'ershan volcanic group, connecting the asthenosphere and the exposed positions of the A'ershan volcanic group on the surface. The low-velocity anomalies on the north and south sides merge at a depth of 150km. Based on the high heat flux value, high V_P/V_S, and crustal thinning characteristics of the surface near the distribution area of the A'ershan volcanic group, as well as the previous conclusion based on remote seismic P-wave and S-wave travel time tomography results that there is a clear connection between the low-velocity anomaly below the A'ershan volcanic group and the southern edge of the Songliao Basin in the deep mantle, it is speculated that this low-velocity anomaly may be caused by the upwelling of asthenosphere material caused by the detachment of the lithosphere in the Songliao Basin.

Key words: the A'ershan volcanic group, lithospheric delamination, volcanic activity, surface-wave tomography, Shear wave velocity structure