Abstract:

The Longgang volcano group, located about 150km west of the Tianchi volcano in Changbaishan, is one of the typical monogenic volcanoes formed in China since the Quaternary. The volcano group has the characteristics of high-density distribution and multi-center explosive eruption. At present, more than 160 low-level craters, volcanic cones and caldera lakes have been discovered. The eruption of Longgang volcano group is characterized by multi-cycle, multi-period and multi-stage eruption. In recent years, a large number of studies have shown that Jinlongdingzi volcano in the northwest of Longgang volcanic group underwent a large-scale eruption about 1600 years ago, and this volcanic group now has potential eruption risk. By exploring the electrical structure of the crust and upper mantle in the volcanic area, the structure of the underground magma system can be imaged, which provides key data for volcanic eruptive hazard modeling and further enriches our understanding of the formation mechanism of continental monogenetic volcano in Northeast China. In this paper, the data of a magnetotelluric profile with broadband dense measuring points with a length of more than 160km from Meihekou city in the west to the Changbaishan in the east, passing through the core area of Longgang volcano and Jinlongdingzi volcano, are used for phase tensor decomposition and two-dimensional inversion to obtain the deep electrical structure characteristics along the profile. Whether there are high-level magma chambers in the crust in Longgang volcanic area is discussed. The analysis shows that high-resistivity structures are distributed at different depths in the crust beneath the Longgang volcanic group and its adjacent area, and the high-resistivity structures are deeper under the early volcanic group, which are speculated to be related to the consolidation of magma. There are some obvious large-scale low-resistivity structures under the high-resistivity structures. These low-resistivity structures correspond to the distribution depth of high-resistivity structures in the upper crust of the region and have various depths from west to east. On the whole, these low-resistivity structures may be interconnected at the lower crust and mantle scales and show a trend of continuing to extend to the east and west sides of the study area. It is supposed that these low-resistivity structures are the magmatic system of the middle and lower crust, and the crustal uplift and seismic activity in the study area may be related to the magmatic activity. There may be a magma channel beneath the newly erupted Jinlongdingzi volcano(below 10km), connecting the magma system of the middle and lower crust, and the magma above 10km may have been consolidated. C3 area with a wide range of magma occurrence at a depth of about 30km is located in the east of Longgang volcanic area, which relatively corresponds to the depth and location of magma occurrence obtained from the inversion of previous deformation data. The deformation data reveal that the crustal uplift rate above the region is large, and the seismic data reveal that the region is seismically active, which is a region worthy of keeping an eye on the magmatic activity. The low-resistivity structures of the middle and lower crust found in the eastern part of the section show that they continue to extend to the eastern Changbaishan Tianchi volcanic area. Combined with previous magnetotelluric and seismological research results, it is speculated that the Longgang volcanic group and the Changbaishan volcano may share one magmatic system in the middle and deep parts. The results obtained can provide geophysical basis for volcanic eruption risk prediction and disaster evaluation in the Longgang volcano group.

Key words: magnetotelluric, electrical structure, magmatic system, Longgang volcano

摘要：

龙岗火山群是中国主要的活火山之一, 其中的金龙顶子火山在距今约1600a前发生过大规模喷发活动, 具有潜在喷发危险。文中利用一条西起梅河口市、 东至长白山景区西门, 穿过龙岗火山核心区及金龙顶子火山, 长度超过160km的宽频带密集测点的大地电磁剖面数据进行相位张量分解和二维反演计算, 获得沿剖面的深部电性结构特征。分析表明, 龙岗火山群及邻区地壳范围内分布深浅不一的高阻结构, 且在早期形成的火山群下方分布更深, 推测与岩浆的固结作用有关。高阻体下方存在明显的大规模低阻结构, 推测为中下地壳岩浆系统, 研究区的地壳隆升及地震活动等可能与岩浆活动有关。最新喷发的金龙顶子火山下方(10km以深)可能存在着岩浆通道, 且与中下地壳岩浆系统相连接, 10km以浅的岩浆可能已经固结。在剖面东部发现的中下地壳低阻结构显示出继续向东部的长白山天池火山区延伸, 结合前人的大地电磁及地震学等研究结果推测, 龙岗火山群可能与长白山天池火山在中深部共用岩浆系统。

关键词: 大地电磁, 电性结构, 岩浆系统, 龙岗火山