Abstract:

Located in the Tengchong-Baoshan region, the Tengchong volcanic group represents one of the youngest intraplate volcanic systems in mainland China. The area is characterized by frequent seismic activity, numerous hot springs, hydrothermal eruptions, and the potential for future volcanic events. Owing to its unique geographical location and complex geological setting, the Tengchong-Baoshan region has long been a hot point of scientific research. However, controversies remain regarding the distribution and sources of magma in the volcanic area, largely due to differences in datasets and the non-uniqueness of inversion methods. Imaging the S-wave velocity structure of the crust and uppermost mantle is therefore essential for advancing our understanding of magmatic and seismic processes in the Tengchong volcanic field. Yet, the relatively low spatial resolution of large-scale models has limited the ability to resolve fine-scale structural features. Previous studies in this region were further constrained by the sparse and uneven distribution of seismic stations.

To address these limitations, we analyzed teleseismic waveform data recorded by 76 seismological stations across the Tengchong-Baoshan region. P-wave receiver functions were extracted using a time-domain iterative deconvolution technique. Employing a two-step joint inversion approach that combines receiver functions with Rayleigh wave phase velocity, supplemented by a bootstrap resampling procedure, we derived a three-dimensional S-wave velocity model of the crust and uppermost mantle down to ~150km depth. A comparative analysis with prior models was conducted, followed by an integrated interpretation using results from geothermal, electromagnetic, helium isotope, and seismic velocity ratio studies, in order to investigate the distribution and source of magma in the Tengchong volcanic area.

Our results show that, while the overall velocity structure is broadly consistent with previous studies, the low-velocity regions in our model exhibit the lower absolute velocities and more detail. Three prominent low-velocity zones (V_S<3.4km/s) are identified in the crust along the Tengchong volcanic belt from north to south, interpreted as partially molten magma chambers. These zones, designated LV1, LV2, and LV3, occur at depths of 10~30km, 10~30km, and 10~26km, respectively. LV1 and LV2 are situated along the Tengchong Fault with the Nujiang Fault forming the eastern boundary, whereas LV3 lies between the Nujiang and Longling-Ruili faults. These fault systems-the Tengchong, Longchuanjiang, Nujiang, and Longling-Ruili faults-play a key role in controlling magmatic activity. In addition, a low-velocity layer is observed at 10~30km depth within the Baoshan block to the east of the Nujiang Fault, though its velocities are higher than those beneath the Tengchong volcanic area. We infer that this anomaly may reflect the influence of high-temperature, volatile-rich magma migrating from the deeper magma reservoir beneath Tengchong, as well as contributions from fluids and fault-related fissures.

Since 1900, earthquakes of M_S≥5.0 have predominantly occurred in transitional zones between high- and low-velocity regions. Notably, the epicenters of the 1976 M_S7.4 and M_S7.3 Longling earthquakes are underlain by a low-velocity layer, suggesting that magma and fluids may have contributed to rupture initiation in fault-fractured regions. In the upper mantle(60~120km depth), widespread low-velocity anomalies are observed beneath the Tengchong-Baoshan area, which extend upward and connect to the crustal low-velocity zones beneath Tengchong. This large-scale low-velocity mantle anomaly likely serves as a magma source feeding the crustal magma chambers. Furthermore, asthenospheric upwelling and associated lithospheric thinning provide the geodynamic mechanism driving magmatism and volcanism in the Tengchong region.

Key words: Tengchong volcano, receiver function, surface wave dispersion, two-step joint inversion technique, S-wave velocity structure, magma chamber

摘要：

腾冲-保山地区集地震、 火山和地热活动于一体, 一直是研究的热点地区, 但关于腾冲火山的岩浆分布情况和岩浆来源一直存在争议。文中利用腾冲-保山地区分布的76个地震观测台站记录的远震波形数据, 采用接收函数与瑞利波相速度的两步联合反演方法和Bootstrap重采样技术, 获取了腾冲-保山地区地壳上地幔150km深度范围内的三维S波速度结构。与前人的研究成果进行了对比分析, 讨论了腾冲火山区的壳内岩浆分布情况和腾冲火山的起源, 得到以下几点结论: 与前人结果相比, 文中S波速度结构的整体特征大致相同, 在一些细节上存在差异。沿腾冲火山区从北向南, 地壳中存在LV1、 LV2、 LV3 3个岩浆囊, 深度范围分别为10~30km、 10~30km及10~26km; LV1和LV2主要沿着腾冲断裂分布, 以怒江断裂为东边界, LV3位于怒江断裂与龙陵-瑞丽断裂之间, 这些断层控制着火山区的岩浆活动。1976年龙陵 M_S7.3 和 M_S7.4 地震的震源区下方存在低速层, 震源区断层裂隙发育, 岩浆、 流体等可能参与了地震成核过程。腾冲-保山地区的上地幔60~120km深度内普遍存在低速异常, 并且在腾冲下方向上延伸与地壳中的低速区相连。上地幔中的大型低速层可能为地壳中岩浆存储层提供岩浆物质, 软流圈上涌导致岩石圈拉张与减薄, 导致腾冲火山的形成。

关键词: 腾冲火山, 接收函数, 面波频散, 两步联合反演, S波速度结构, 岩浆囊