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

摘要：

大同火山群位于华北克拉通中部, 因地质构造复杂和地震频发而引起广泛关注。文中基于地震科学国际数据中心数据服务平台^①收集的2020年1—12月期间中国地震局省级宽频带(60s~50Hz、 120s~50Hz、 360s~20Hz)固定地震台站记录的垂直分量连续波形资料, 拾取5~30s背景噪声面波频散曲线, 利用面波直接反演方法获得了研究区深至40km、 空间分辨率为0.75°×0.75° 的三维S波速度结构模型。成像结果显示, 上地壳S波速度分布与地表地质构造特征吻合较好, 山西断陷带普遍表现出低速异常, 主要反映了太原盆地、 忻定盆地和大同盆地的结构特征, 推测与盆地浅部覆盖的新生代沉积层有关, 而吕梁山脉和太行山脉因位于基岩区而表现为高速异常。在中下地壳, 大同火山群下方的低速异常范围扩展至整个山西断陷带北部及以西地区, 可能与地幔热物质上涌至地壳后在大同火山及邻区造成大面积热物质作用有关。大同火山区下方在地壳内存在一个不连续的低速异常体, 说明火山下方存在岩浆上涌通道, 但岩浆补给可能存在着不连续性。结合前人的地幔深部成像结果, 文中推测大同火山区下方反映的热物质低速异常, 可能与太平洋板块西向深俯冲、 印度-欧亚板块碰撞软流圈物质挤出及地幔柱活动共同作用有关。

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