Asthenospheric basal drag primarily drives the Pacific Plate

Introduction: Previous geodynamic studies have indicated the dynamic importance of positive buoyancy in the mantle beneath the eastern Pacific in driving the Pacific Plate. However, it is not well understood whether the shallower or deeper low-seismic-velocity regions of the mantle are involved in the motion and the intraplate stress of the Pacific Plate. Materials and methods: In this study, instantaneous global mantle flow calculations were performed to investigate the origin of the driving forces of the Pacific Plate and the relationship between plate motion and intraplate stress, which cannot be directly constrained by geophysical observations. Density and temperature anomalies in the mantle were derived from seismic velocity anomalies obtained from a whole mantle seismic tomography model. Lateral variations in viscosity were incorporated into the mantle and lithosphere, and plate boundaries were defined based on a global strain rate model. Results: When both negative and positive density anomalies were included throughout the mantle, the predicted motion of the Pacific and the Nazca Plates agreed well with the observed motion, with variance reductions (VR) of 0.96 and 0.90, respectively. On the other hand, removing negative density anomalies below a depth of 300 km further improved the agreement, increasing the corresponding VR to 0.98 and 0.98. Negative density anomalies associated with the shallow, buoyant mantle promote a reverse-faulting regime within the stable interior of the Pacific Plate. Conclusions: The present results suggest that mantle drag associated with negative density anomalies in the shallow, buoyant mantle plays a primary role in reproducing the observed motion of the Pacific and Nazca Plates, and slab pull due to positive density anomalies of the subducting slabs plays a secondary role. In contrast, negative density anomalies in the deeper mantle are not necessary to reproduce the observed plate motions. This may suggest that the large low-velocity provinces in the deep mantle are not simply thermally buoyant structures, but may instead have a thermochemical origin and be approximately neutrally buoyant. In addition, the present results concluded that the patterns of intraplate stress and the dominant faulting regimes of the Pacific Plate are sensitive to the depth range over which negative density anomalies are distributed in the mantle beneath the eastern Pacific.