A Vertically Connected Melt-Fluid-Rich Magma System Beneath the Tengchong Volcanic Field

        The growth of the Tibetan Plateau is resulted from the ongoing collision between the Indian and Eurasian plates since the Cenozoic. The western Yunnan region is located on the southeastern margin of the Tibetan Plateau and represents an important tectonic transition zone, characterized by outward material flow from the Tibetan Plateau, interactions among multiple tectonic blocks, and complex crust-mantle coupling processes. The Tengchong Volcanic Field (TCV), situated in western Yunnan, is the largest active volcanic field in China and is characterized by intense hydrothermal activity and frequent seismicity. Multiple geophysical observations indicate that low seismic velocities and low electrical resistivity at different depths in the crust beneath the TCV, which are commonly interpreted as the existence of magma chambers. However, how mantle-derived materials are transported upward and continuously supply the crustal magma system, as well as the specific pathways and dynamic mechanisms involved, remain poorly understood. Therefore, the crustal and uppermost mantle structure of the Tengchong magma system warrants further study.

          In this study, we develop a new joint inversion method that combines body-wave and surface-wave data. Using the chain rule, the sensitivity kernels of S-wave travel times and surface-wave dispersion with respect to Vs are transformed into sensitivity kernels for Vp and Vp/Vs, and S-P travel-time data are incorporated to further enhance constraints on the Vp/Vs structure. In addition, the variation of information constraint is introduced to strengthen the intrinsic coupling between the Vp and Vp/Vs models, and the objective function is efficiently solved using the L-BFGS optimization algorithm. Based on this approach, we obtain high-resolution three-dimensional Vp, Vs, and Vp/Vs models beneath the TCV. The results reveal a spatially continuous low-Vs and high-Vp/Vs anomaly in the lower crust beneath the TCV, which extends upward from the lower crust and closely corresponds to the shallow seismicity and the location of the volcanic centers, indicating the presence of partially molten or fluid-rich materials. An overlying high-velocity layer above the lower-crustal low-velocity anomaly reflects relatively dense and mechanically strong middle-crustal material, which exerts mechanical sealing and lateral confinement on the underlying partially molten or fluid-rich zone. This structure effectively controls the ascent pathways of magma and the spatial distribution of shallow seismicity. Our results suggest that the Tengchong volcanic system is not controlled by an isolated shallow magma chamber but is instead governed by a vertically connected magma system dominated by a deep-seated weak zone or a melt-fluid-rich conduit.