A Crustal Magma Chamber beneath the Tengchong Volcanic Field Revealed by Ambient Noise Adjoint Tomography

The Tengchong Volcanic Field (TVF) is one of the largest active intraplate volcanoes in China, situated in the west of the southeastern Tibetan Plateau. The TVF is characterized by large-volume magmatic gases (CO ₂ and ³ He/ ⁴ He) emission, high temperature hydrothermal activities, and intense volcanic activities during the Holocene. The most recent eruption occurred at AD1609. A recent magnetotelluric survey detected three isolated magma chambers at depths between 10 and 20 km (Ye et al., 2018). However, it still remains enigmatic that how the shallow magma chambers are feed by deep sources and if there exists a deep crustal magma reservoir beneath the TVF. Imaging of the whole volcanic plumbing system is essential for the quantitative assessments of long-term volcanic eruption hazards of the TVF.

In this study, we construct a high-resolution 3D crustal model of the TVA using full-waveform ambient noise adjoint tomography with ambient noise cross-correlations from selected seismic stations of Himalaya I project. Our preliminary 3D model reveals a significant low shear velocity body (LVB) at a depth range between 20 and 30 km. This LVB may represent a deep, basaltic magma reservoir in the mid-to lower crust with a volume of ~30,000 km ³ , bounded by the Gaoligong Fault (GLF) and located to the northwest of TVA. The greatest shear wave velocity reduction of this LVB is ~11% corresponding to ~1.4% partial melting. In the uppermost mantle (40-50 km), we also observe a large-scale low velocity body right beneath the GLF to the east of TVA.

We suggest that our full-wave tomographic model provides seismic evidence of a multi-level volcanic plumbing system of the TVA, similar to those observed in Yellowstone (Huang et al., 2015) and Long Valley (Flinders et al., 2018). The uppermost mantle low velocity zone beneath the GLF can be interpreted as the source of magma where partial melting generates mafic melts and feeds the volcano. The GLF acts as a pathway that transports basaltic melts from the uppermost mantle to the deep crustal magma reservoir, and the final melt fractionation occurs at the upper crustal silicic magma chambers.