Multi-scale anisotropy in NE China: Implications for intra-plate volcanism

Northeast China is a very typical area for studying intra-plate volcanism in the back-arc setting. It is commonly proposed that the subduction of the Pacific plate has been responsible for widespread Holocene volcanoes in NE China. Yet, how this process drives volcanism remains a topic of vigorous debate. Investigation of seismic anisotropy can provide important evidence for the cause-and-effect relationship between mantle flow, lithospheric deformation and shallow structures. In this study, using seismic data from four networks across NE China and north Korea, we analyze shear wave splitting in converted P- to S-waves at the Moho (Pms), S-waves from the subducted slab interface (local S), and SKS phases. The Pms phases show a relatively weak crustal anisotropy (~0.25 s), with fast polarization directions aligned sub-parallel to major tectonic features. For the local S and SKS phases, fast polarization directions show significant lateral variations. We further perform a quantitative inversion to show that the depth of the anisotropy is ~150 km, thus driven by flow within the asthenosphere associated with Pacific subduction. However, the presence of many null SKS splitting phases, together with scattered local S-wave anisotropy suggests a localized region of vertical flow directly beneath Changbaishan volcano. Such patterns correspond well to regional upper-mantle seismic velocity structure, and suggest that a localized upwelling with a relatively deep origin drives volcanism in the Changbaishan region. Furthermore, we infer that mantle upwelling is deflected to the SW beneath Changbaishan and spreads asymmetrically at the base of the lithosphere, possibly because of the long history of volcanism in the region.