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1. Mantle Transition Zone‐Penetrating Upwellings Beneath the Eastern North American Margin and Beyond

   https://doi.org/10.1029/2024JB030005  

   Luo, Yantao; Long, Maureen D; Rondenay, Stéphane; King, Scott D; Mazza, Sarah E; Wolf, Jonathan (April 2025, Journal of Geophysical Research: Solid Earth)

   Abstract Low‐velocity anomalies in the upper mantle beneath eastern North America, including the Northern Appalachian Anomaly (NAA), the Central Appalachian Anomaly (CAA), and the weaker Southern Coastal Anomaly (SCA), have been characterized by many continent‐scale and regional seismic studies. Different models have been proposed to explain their existence beneath the passive margin of eastern North America, variously invoking the past passage of hot spot tracks, modern upwelling due to edge‐driven convection, or other processes. Depending on the nature and origin of these anomalies, they may influence, and/or be influenced by, the mantle transition zone (MTZ) structure beneath them. Previous receiver function studies have identified an overall thinner MTZ beneath the eastern margin of the US than beneath the continental interior. In this study, we resolve the MTZ geometry beneath these low‐velocity anomalies in unprecedented detail using the scattered wavefield migration technique. We find substantially thinned MTZ beneath the NAA and the CAA, and a moderately thinned MTZ beneath the SCA. In all cases, the thinning is achieved via a minor depression of the 410‐km discontinuity and a major uplift of the 660‐km discontinuity, which suggests the presence of a series of MTZ‐penetrating deep upwellings beneath eastern North America. The upwellings beneath eastern North America and a similar style upwelling beneath Bermuda may initiate from ponded thermally buoyant materials below the MTZ fed by hot return flows from the descending Farallon slab in the deep mantle. 

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2. Lateral Motion of Mantle Plumes in 3‐D Geodynamic Models

   https://doi.org/10.1029/2018GL081404  

   Li, Mingming; Zhong, Shijie (May 2019, Geophysical Research Letters)

   Abstract It has been proposed that hot spot tracks are caused by moving rigid plates above relatively stationary hot spots. However, the fixity of hot spots remains under debate. Here, we perform 3‐D very high resolution (<25 km laterally) global mantle convection models with realistic convection vigor to investigate the lateral motion of mantle plumes. We find that the lateral motion of plumes beneath the Pacific plate is statistically similar to that beneath the Indo‐Atlantic plates. In the past 80 Ma, the majority (>90%) of plumes move laterally with an average speed of 0–20 mm/year under the no‐net‐rotation reference frame, and there are a small portion (~10–20%) of plumes whose lateral motion is less than 5 mm/year. The geodynamic modeling results are statistically in a good agreement with the hot spot motions in the last 5 Ma estimated from observation‐based kinematic models. Our results suggest a small‐to‐moderate (0–20 mm/year) lateral motion of most plume‐induced hot spots. 

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3. Lateral Variations of Shear‐Wave Velocity in the D″ Layer Beneath the Indian‐Eurasian Plate Collision Zone

   https://doi.org/10.1029/2019GL086856  

   Li, Guohui; Bai, Ling; Ritsema, Jeroen (March 2020, Geophysical Research Letters)

   Abstract Seismic tomography has demonstrated that the shear‐wave velocity is relatively high over a 3,000‐km wide region in the lowermost mantle beneath southern and eastern Asia. This seismic anomaly demarcates the current position of slab remnants that may have subducted in the Cretaceous. To further characterize the seismic structure at smaller scales, we measure 929 residual travel time differences (δt) between the phasesScSandSusing recordings of eight earthquakes beneath the Indian Ocean at stations from the Chinese Digital Seismic Network. We interpret variations of δtup to 10 s as due to horizontal shear‐velocity variations in D″ beneath northern India, Nepal, and southwestern China. The shear velocity can vary by as much as 7% over distances shorter than 300 km. Our observations provide additional observational evidence that compositional heterogeneity and possibly melt contribute to the seismic structure of the lower mantle characterized by long‐term subduction and mantle downwelling. 

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4. Upper Mantle Earth Structure in Africa From Full‐Wave Ambient Noise Tomography

   https://doi.org/10.1029/2018GC007804  

   Emry, E. L.; Shen, Y.; Nyblade, A. A.; Flinders, A.; Bao, X. (January 2019, Geochemistry, Geophysics, Geosystems)

   Abstract Our understanding of the tectonic development of the African continent and the interplay between its geological provinces is hindered by unevenly distributed seismic instrumentation. In order to better understand the continent, we used long‐period ambient noise full‐waveform tomography on data collected from 186 broadband seismic stations throughout Africa and surrounding regions to better image the upper mantle structure. We extracted empirical Green's functions from ambient seismic noise using a frequency‐time normalization method and retrieved coherent signal at periods of 7–340 s. We simulated wave propagation through a heterogeneous Earth using a spherical finite‐difference approach to obtain synthetic waveforms, measured the misfit as phase delay between the data and synthetics, calculated numerical sensitivity kernels using the scattering integral approach, and iteratively inverted for structure. The resulting images of isotropic, shear wave speed for the continent reveal segmented, low‐velocity upper mantle beneath the highly magmatic northern and eastern sections of the East African Rift System (EARS). In the southern and western sections, high‐velocity upper mantle dominates, and distinct, low‐velocity anomalies are restricted to regions of current volcanism. At deeper depths, the southern and western EARS transition to low velocities. In addition to the EARS, several low‐velocity anomalies are scattered through the shallow upper mantle beneath Angola and North Africa, and some of these low‐velocity anomalies may be connected to a deeper feature. Distinct upper mantle high‐velocity anomalies are imaged throughout the continent and suggest multiple cratonic roots within the Congo region and possible cratonic roots within the Sahara Metacraton. 

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5. Seismic Full‐Waveform Inversion Reveals Radially Anisotropic Upper Mantle Structures Beneath the Australian Plate

   https://doi.org/10.1029/2024JB029260  

   Bodur, Ömer; Li, Xueyan; Lumley, David; Zhu, Hejun (December 2024, Journal of Geophysical Research: Solid Earth)

   Abstract To explore seismic structures beneath the Australian continents and subduction zone geometry around the Australian plate, we introduce a new radially‐anisotropic shear‐wavespeed model, AU21. By employing full‐waveform inversion on data from 248 regional earthquakes and 1,102 seismographic stations, we iteratively refine AU21, resulting in 32,655 body‐wave and 35,897 surface wave measurements. AU21 reveals distinct shear‐wavespeed contrasts between the Phanerozoic eastern continental margin and the Precambrian western and central Australia, with the lithosphere‐asthenosphere boundary estimated at 250–300 km beneath central and western Australia. Notably, a unique weak radial anisotropy layer at 80–150 km is identified beneath the western Australian craton, possibly due to alignments of dipping layers or tilted symmetry axes of anisotropic minerals. Furthermore, slow anomalies extending to the uppermost lower mantle beneath the east of New Guinea, Tasmania, and the Tasman Sea indicate deep thermal activities, likely contributing to the formation of a low wavespeed band along the eastern Australian margin. In addition, our findings demonstrate the stagnant Tonga slab within the mantle transition zone and the Kermadec slab's penetration through the 660‐km discontinuity into the lower mantle. 

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