关键词: 大同火山群, 背景噪声, 面波成像, 直接反演, S波速度结构

Abstract:

The Datong volcanic group is located in the central part of the North China Craton, and it has attracted widespread attention due to its complex geological tectonic environment with a high level of seismicity. In this study, we collect continuous seismic waveforms from January to December 2020 recorded at 56 provincial permanent seismic stations of the China Earthquake Administration. In data processing, we first conduct rigorous preprocessing of the raw waveform data, including mean removal, detrending, and bandpass filtering to ensure data quality. After performing cross-correlation of ambient noise, we manually extract Rayleigh wave dispersions in the periods of 5-30s, which effectively reflect the velocity structural characteristics of the crustal and uppermost mantle. Based on the extracted dispersion data, we apply the direct surface wave tomographic method to construct a three-dimensional S-wave velocity structure model extending to a depth of 40km with a spatial resolution of 0.75°×0.75° in the horizontal directions.

Our imaging results show that the distribution of S-wave velocities corresponds well with geological structural features in the upper crust. The Shanxi rift zone generally exhibits low-velocity anomalies, reflecting the structural characteristics of the Taiyuan Basin, Xinding Basin, and Datong Basin, which are speculated to be related to the Cenozoic sedimentary layers covering the shallow subsurface in this area. In contrast, the Lüliang Mountains and Taihang Mountains exhibit high-velocity anomalies due to their exposed bedrock. In the middle to lower crust, the low-velocity anomaly beneath the Datong volcanic group extends across the northern part of the Shanxi rift zone to the west of the zone, possibly caused by extensive magma activity in the crust due to the upwelling of hot mantle materials under the region. A discontinuous low-velocity anomaly body exists in the crust beneath the Datong volcanic area, potentially serving as a conduit for magma upwelling, but with a possibly discontinuous magma supply. Combining previous deep mantle imaging studies, we speculate that the crustal low-velocity anomalies reflecting hot materials beneath the Datong volcanoes could be jointly caused by the westward deep subduction of the Pacific slab, the extrusion of asthenospheric mantle materials by the Indo-Eurasian collision, and mantle plume activities. Our findings not only deepen our understanding of the deep structure and dynamics of Datong volcano, but also provide new insights into understanding the tectonic evolution of the North China Craton.

Key words: Datong volcanic group, ambient noise, surface wave, direct tomography, S-wave velocity structure