Segregation of a thermochemical anomaly and coalescence with a large low-velocity province

Thermochemical anomalies on the core–mantle boundary, including the Africa and Pacific large low-velocity provinces, exert a first-order control on mantle dynamics. However, due to limited observational constraints, their mobility and evolution remain poorly understood. Here we report an intermediate-scale thermochemical anomaly beneath the Northwest Pacific Ocean on the basis of existing tomographic models and use palaeogeographically constrained numerical models to study its evolution. Considering different plate configurations in the North Pacific, our models consistently show that this anomaly, named the Kamchatka anomaly, was separated from the intermediate-scale Perm thermochemical anomaly by the subduction of the Izanagi slab in the Cretaceous period. After the separation, it could have generated a mantle plume with the remaining thermochemical anomaly migrating towards the Pacific large low-velocity province. We propose that intermediate-scale low-velocity structures constantly undergo segregation and coalescing, and may be the sources of plumes that lie outside the two major large low-velocity provinces. Merging of the reported anomaly with the Pacific large low-velocity province suggests the latter is still under assembly. The intermediate-scale Kamchatka thermochemical anomaly segregated from the Perm anomaly and may have generated a mantle plume before merging with the Pacific large low-velocity province, according to a study of mantle tomographic and numerical models.