The Formation of Hot Thermal Anomalies in Cold Subduction-Influenced Regions of Earth's Lowermost Mantle

The Earth's lowermost mantle is characterized by two large low shear velocity provinces (LLSVPs). The regions outside the LLSVPs have been suggested to be strongly influenced by subducted slabs and, therefore, much colder than the LLSVPs. However, localized low-velocity seismic anomalies have been detected in the subduction-influenced regions, whose origin remains unclear. Here, three-dimensional geodynamic calculations are performed, and they show that linear, ridge-like hot thermal anomalies, or thermal ridges, form in the relatively cold, downwelling regions of the lowermost mantle. Like the formation of Richter rolls due to sublithosphere small-scale convection (SSC), the thermal ridges form as a result of SSC from the basal thermal boundary layer and they extend in directions parallel to the surrounding mantle flow. The formation of thermal ridges in subduction regions of the lowermost mantle is very sensitive to the thermal structures of the subducted materials, and thermal heterogeneities brought to the bottom of the mantle by subducting slabs greatly promote the formation of thermal ridges. The formation of thermal ridges is also facilitated by the increase of core-mantle boundary heat flux and vigor of lowermost mantle convection. The thermal ridges may explain the low-velocity seismic anomalies outside of the LLSVPs in the lowermost mantle. The results suggest that the relatively cold, subduction-influenced regions of the Earth's lowermost mantle may contain localized hot anomalies. Plain Language Summary Scientists have found that the Earth's lowermost mantle contains two large, continental-size blobs, one beneath Africa and the other beneath the central Pacific Ocean, whose temperatures are expected to be at least several hundred degrees higher than the surrounding mantle. It is suggested that the two blobs are surrounded by regions that have been strongly influenced by ancient, relatively cold subducted slabs. However, some localized regions outside the two blobs are found to have abnormal physical properties which may be caused by increase of temperature, but the origin of the smaller-scale thermal heterogeneities outside the two blobs remains unclear. In this study, I use computer simulations to investigate the dynamics and thermal structures in the lowermost mantle. I find that hot, ridge-like thermal anomalies, or thermal ridges, readily form near the subducted slabs in the lowermost mantle. The formation of thermal ridges is promoted as the core heat flux and/or the vigor of the lowermost mantle convection increases and when thermal heterogeneities are brought to the bottom of the mantle by subducted slabs.