Assembly of the basal mantle structure beneath Africa

Sluggish thermochemical basal mantle structures support their long-lived stability

Large low shear-wave velocity provinces (LLSVPs) in the lowermost mantle are the largest geological structures on Earth, but their origin and age remain highly enigmatic. Geological constraints suggest the stability of the LLSVPs since at least 200 million years ago. Here, we conduct numerical modeling of mantle convection with plate-like behavior that yields a Pacific-like girdle of mantle downwelling which successfully forms two antipodal basal mantle structures similar to the LLSVPs. Our parameterized results optimized to reflect LLSVP features exhibit velocities for the basal mantle structures that are ~ 4 times slower than the ambient mantle if they are thermochemical, while the velocity is similar to the ambient mantle if purely thermal. The sluggish motion of the thermochemical basal mantle structures in our models permits the notion that geological data from hundreds of millions of years ago are related to modern LLSVPs as they are essentially stationary over such time scales.

Full-waveform inversion reveals diverse origins of lower mantle positive wave speed anomalies

Determining Earth's structure is paramount to unravel its interior dynamics. Seismic tomography reveals positive wave speed anomalies throughout the mantle that spatially correlate with the expected locations of subducted slabs. This correlation has been widely applied in plate reconstructions and geodynamic modelling. However, global travel-time tomography typically incorporates only a limited number of easily identifiable body wave phases and is therefore strongly dependent on the source-receiver geometry. Here, we show how global full-waveform inversion is less sensitive to source-receiver geometry and reveals numerous previously undetected positive wave speed anomalies in the lower mantle. Many of these previously undetected anomalies are situated below major oceans and continental interiors, with no geologic record of subduction, such as beneath the western Pacific Ocean. Moreover, we find no statistically significant correlation positive anomalies as imaged using full-waveform inversion and past subduction. These findings suggest more diverse origins for these anomalies in Earth's lower mantle, unlocking full-waveform inversion as an indispensable tool for mantle exploration.

Globally distributed subducted materials along the Earth's core-mantle boundary: Implications for ultralow velocity zones

Ultralow velocity zones (ULVZs) are the most anomalous structures within the Earth's interior; however, given the wide range of associated characteristics (thickness and composition) reported by previous studies, the origins of ULVZs have been debated for decades. Using a recently developed seismic analysis approach, we find widespread, variable ULVZs along the core-mantle boundary (CMB) beneath a largely unsampled portion of the Southern Hemisphere. Our study region is not beneath current or recent subduction zones, but our mantle convection simulations demonstrate how heterogeneous accumulations of previously subducted materials could form on the CMB and explain our seismic observations. We further show that subducted materials can be globally distributed throughout the lowermost mantle with variable concentrations. These subducted materials, advected along the CMB, can provide an explanation for the distribution and range of reported ULVZ properties.