Evidence for an Upper Crustal Melt Lens Beneath Tianchi (Changbaishan) Volcano

Changbaishan volcanic field (CBVF) located on the border of China and the Democratic People's Republic of Korea (DPRK) is famous for intense volcanism in the Cenozoic. Many studies show evidence for partial melt beneath the volcano, but details on the structure of the magmatic system are lacking, due to a lack of data in the region. In this study, we obtained a high-resolution crust and upper mantle shear wave velocity (Vs) model beneath the CBVF by ambient noise tomography and receiver functions using a new dense seismic array. The absence of velocity anomalies beneath Wangtian'e and Namphothe volcanoes suggests a lack of magma within the crust. However, our models reveal two low Vs anomalies beneath Tianchi associated with magma reservoirs. The shallow low velocity anomaly (<4 km) overlaps with petrological estimates of the assembly depth of erupted rhyolite magma reservoir and the depth inferred for a hydrothermal reservoir from a recent MT study. The deeper one is located between 7 and 14 km depth with a lateral extent of ∼30 km, with a melt fraction of ∼6%–12%. Underlying the deeper low velocity zone in the lower crust is a region of faster velocity compared to the surrounding region. We interpret this as a low melt fraction crystal mush. This structure is similar to that seen at other large caldera volcanoes worldwide, suggesting a possible common mechanism underlying magmatism at these volcanoes.