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Two large, seismically slow regions in the lower mantle beneath Africa and the Pacific Ocean are sometimes referred to as "superplumes". This name evokes images of large-scale active upwellings. However, it remains unclear whether these features are real or represent collections of multiple regular mantle plumes. Here, we investigate the implications of these upwellings for dynamic topography. We combine detailed measurements of oceanic residual topography from Hoggard et al. (2016) with continental constraints derived from CRUST1.0 to produce a global model expanded in spherical harmonics. Observed dynamic topography is subsequently compared to predictions derived from mantle flow following Steinberger (2016) using tomographic density models. Results yield relatively good overall agreement and amplitude spectra with similar slopes, except for degree two (i.e. > 10,000 km wavelengths) where predicted amplitude is more than two times as large and is dominated by contributions from the lower mantle. Predictive models suggest two large-scale uplifted regions above the "superplumes" that are barely seen in the observed topography. We suggest that this mismatch can only partly be reconciled by altering the seismic velocity to density conversion factor or by including the effects of lower mantle chemical heterogeneity. In addition, it may be important to consider more significant revisions to the lower mantle flow patterns, such as those possibly induced by different radial viscosity profiles and laterally-varying or anisotropic lower mantle viscosity. • Lower mantle dominates degree-two dynamic topography. • Upper mantle dominates all other degrees. • Predicted dynamic topography amplitude at degree two exceeds observations by more than a factor 2. [ABSTRACT FROM AUTHOR]

Age-progressive seamount tracks generated by lithospheric motion over a stationary mantle plume have long been used to reconstruct absolute plate motion (APM) models. However, the basis of these models requires the plumes to move significantly slower than the overriding lithosphere. When a plume interacts with a convergent or divergent plate boundary, it is often deflected within the strong local mantle flow fields associated with such regimes. Here, we examined the age progression and geometry of the Samoa hotspot track, focusing on lava flow samples dredged from the deep flanks of seamounts in order to best reconstruct when a given seamount was overlying the mantle plume (i.e., during the shield-building stage). The Samoan seamounts display an apparent local plate velocity of 7.8 cm/yr from 0 to 9 Ma, 11.1 cm/yr from 9 to 14 Ma, and 5.6 cm/yr from 14 to 24 Ma. Current fixed and mobile hotspot Pacific APM models cannot reproduce the geometry of the Samoa seamount track if a long-term fixed hotspot location, currently beneath the active Vailulu’u Seamount, is assumed. Rather, reconstruction of the eruptive locations of the Samoan seamounts using APM models indicates that the surface expression of the plume migrated ∼2° northward in the Pliocene. Large-scale mantle flow beneath the Pacific Ocean Basin cannot explain this plume migration. Instead, the best explanation is that toroidal flow fields-generated by westward migration of the Tonga Trench and associated slab rollback-have deflected the conduit northward over the past 2-3 m.y. These observations provide novel constraints on the ways in which plume-trench interactions can alter hotspot track geometries. [ABSTRACT FROM AUTHOR]

Mantle plumes upwelling beneath moving tectonic plates generate age-progressive chains of volcanos (hotspot chains) used to reconstruct plate motion. However, these hotspots appear to move relative to each other, implying that plumes are not laterally fixed. The lack of age constraints on long-lived, coeval hotspot chains hinders attempts to reconstruct plate motion and quantify relative plume motions. Here we provide s40Ar/39Ar ages for a newly identified long-lived mantle plume, which formed the Rurutu hotspot chain. By comparing the interhotspot distances between three Pacific hotspots, we show that Hawaii is unique in its strong, rapid southward motion from 60 to 50 Myrs ago, consistent with paleomagnetic observations. Conversely, the Rurutu and Louisville chains show little motion. Current geodynamic plume motion models can reproduce the first-order motions for these plumes, but only when each plume is rooted in the lowermost mantle. [ABSTRACT FROM AUTHOR]

Kevin Burke's original and thought-provoking contributions have been published steadily for the past 60 years, and more than a decade ago he set out to resolve how plate tectonics and mantle plumes interact by proposing a simple conceptual model, which we will refer to as the Burkian Earth. On the Burkian Earth, mantle plumes take us from the deepest mantle to sub-lithospheric depths, where partial melting occurs, and to the surface, where hotspot lavas erupt today, and where large igneous provinces and kimberlites have erupted episodically in the past. The arrival of a plume head contributes to continental break-up and punctuates plate tectonics by creating and modifying plate boundaries. Conversely, plate tectonics makes an essential contribution to the mantle through subduction. Slabs restore mass to the lowermost mantle and are the triggering mechanism for plumes that rise from the margins of the two large-scale low shear-wave velocity structures in the lowermost mantle, which Burke christened TUZO and JASON. Situated just above the core-mantle boundary, beneath Africa and the Pacific, these are stable and antipodal thermochemical piles, which Burke reasons represent the immediate after-effect of the moon-forming event and the final magma ocean crystallization. [ABSTRACT FROM AUTHOR]

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The magmatic activity (0-16 Ma) in Iceland is linked to a deep mantle plume that has been active for the past 62 My. Icelandic and northeast Atlantic basalts contain variable proportions of two enriched components, interpreted as recycled oceanic crust supplied by the plume, and subcontinental lithospheric mantle derived from the nearby continental margins. A restricted area in southeast Iceland- and especially the Öræfajökull volcano-is characterized by a unique enriched-mantle component (EM2-like) with elevated 87Sr/86Sr and 207Pb/204Pb. Here, we demonstrate through modeling of Sr-Nd-Pb abundances and isotope ratios that the primitive Öræfajökull melts could have assimilated 2-6% of underlying continental crust before differentiating to more evolved melts. From inversion of gravity anomaly data (crustal thickness), analysis of regional magnetic data, and plate reconstructions, we propose that continental crust beneath southeast Iceland is part of ~350-km-long and 70-km-wide extension of the Jan Mayen Microcontinent (JMM). The extended JMM was marginal to East Greenland but detached in the Early Eocene (between 52 and 47 Mya); by the Oligocene (27 Mya), all parts of the JMM permanently became part of the Eurasian plate following a westward ridge jump in the direction of the Iceland plume. [ABSTRACT FROM AUTHOR]

Abstract: Large Igneous Province (LIP) eruption sites of the past 300 My lie vertically above 1% slow shear wave velocity (V s) contours bounding the African and Pacific Large Low Shear Velocity Provinces (LLSVPs) at the core–mantle boundary (CMB), or in the cases of the Siberian and Columbia River LIPs, bounding one or other of two smaller, Low Shear Velocity Provinces (LSVPs). Steep gradients in V s at the CMB coincide with those 1% slow contours. The sites of 24 active hotspot volcanoes project down to the same narrowly defined borders of the LLSVPs at the CMB. Plumes that have generated LIPs and major hotspot volcanoes have risen only from the immediate neighbourhoods of the 1% slow V s contours at the CMB which thus define Plume Generation Zones (PGZs). PGZs projected vertically upward approximately match the +10 m elevation contour of the geoid showing that the LLSVPs are a dominant control on the positively elevated geoid. Minima in the frequency distribution of shear wave velocities in the lowermost mantle near V s =−1% indicate that regions with more negative velocities, forming ∼2% of total mantle mass, are likely to be of material compositionally different from the rest of the mantle. Because all LIP eruption sites with ages younger than 300 Ma lie above the borders of LLSVPs or LSVPs at the CMB, PGZ footprints are inferred to have remained in the same places for the past 300 My. Because no plumes have risen from the interior of the LLSVPs and because no lithospheric slabs have penetrated those bodies the volumes of the LLSVPs are inferred to have also remained unchanged for the past 300 My. Because the LLSVPs are the dominant control on the positively elevated areas of the geoid those too must have remained as they now are since 300 Ma. The LLSVPs are not rising buoyant objects but stable features of the deep mantle. LIPs have been erupted throughout the past 2.5 Gy indicating that PGZs comparable to those of the past 0.3 Gy and LLSVPs (of which PGZs mark the margins at the CMB) have also existed for at least that long. LLSVPs could thus form the isolated reservoir invoked by some to explain the distinctive isotopic compositions of terrestrial rocks. PGZs lie at places where the boundaries of: (i) The outer core, (ii) one of the LLSVPs or LSVPs, and (iii) the seismically faster part of the deep mantle meet. Horizontal temperature gradients across the steeply inclined margins to the LLSVPs, the interiors of which are hotter than the surrounding mantle, at the CMB are key controls for the generation of plumes. Near the CMB the association of the high temperature of the outer core with an inclined thermal boundary layer at the margins of LLSVPs facilitates the generation of mantle plumes in the PGZs. [Copyright &y& Elsevier]

There is a clear correlation between downward projected large igneous province (LIP) eruption sites of the past 200 Myr and the margins of the large low-velocity provinces (LLVPs) at the base of the mantle. We established this correlation by using palaeomagnetic as well as fixed and moving hotspot reference frames. Our finding indicates that the majority of the LIPs have been generated by plumes that rose from the D″ zone at the edges of the LLVPs. Most LIP eruption sites project radially downwards to the core–mantle boundary (CMB) within ±10° of the 1 per cent slow shear wave velocity contour in the SMEAN tomographic model. Steep shear wave velocity gradients have been mapped near the CMB along much of the lengths of the LLVP margins close to that contour which marks a faster/slower boundary (FSB) within the D″ zone. The observation that eruption sites of LIPs as old as 200 Myr can be linked to this prominent present day seismic structure shows that the FSBs of the two LLVPs have occupied their current positions for at least as long and that the process that leads to the generation of deep-seated plumes has been localized on the FSBs at the margins of the African and Pacific LLVPs for the same interval. The persistence of the LLVPs over 200 Myr is consistent with independent evidence that they are compositionally distinct and are not just simply hotter than the material making up the rest of the D″ zone. [ABSTRACT FROM AUTHOR]

Because of their slow relative motion, hotspots, mainly in the Pacific, are often used as a reference frame for defining plate motions. A coherent motion of all Pacific hotspots relative to the deep mantle may, however, bias the hotspot reference frame. Numerical results on the advection of plumes, which are thought to cause the hotspots on the Earth's surface, in a large-scale mantle flow field are therefore presented. Bringing the results into agreement with observations also leads to conclusions regarding the viscosity structure of the Earth's mantle, as well as the sources and distribution of plumes. The abrupt change in direction of the Hawaiian-Emperor chain implies an uppermantle viscosity under the Pacific of ∼ 1.5 × 10[sup 20] Pa s or less. Slow relative motion of hotspots requires high lower-mantle viscosity, unless hotspots are located at large-scale stationary upwellings that are currently unresolved by seismic tomography. For our preferred model, we obtain coherent motion of Pacific hotspots in a reference frame of no net rotation, as well as coherent motion relative to African hotspots, caused by return flow antiparallel to plate motion. Advection and regional differences in life expectancy can to a large part explain the distribution of plumes in relation to ridges, subduction zones (present and past) and seismic anomalies. Plume conduits are substantially tilted in the lower mantle. The surface motion of hotspots is often smaller than the advection rate of plume conduits in the lowermost mantle. [ABSTRACT FROM AUTHOR]

Abstract: Integration of geophysical and geological data show that the Yellowstone hotspot resulted from a mantle plume interacting with the overriding North America plate, a process that has highly modified continental lithosphere by magmatic and tectonic processes and produced the 16-17Ma, 700-km-long Yellowstone-Snake River Plain (YSRP) silicic volcanic system. Accessibility of the YSRP allowed large-scale geophysical projects to seismically image the hotspot and evaluate its kinematic properties using geodetic measurements. Seismic tomography reveals a crustal magma reservoir of 8% to 15% melt, 6km to 16km deep, beneath the Yellowstone caldera. An upper-mantle low-P-wave-velocity body extends vertically from 80km to 250km beneath Yellowstone, but the anomalous body tilts 60°WNW and extends to 660km depth into the mantle transition zone. We interpret this conduit-shaped low-velocity body as a plume of up to -3.5% Vp and -5.5% Vs perturbation that corresponds to a 1-2% partial melt. Models of whole mantle convection reveal eastward upper-mantle flow beneath Yellowstone at relatively high rates of 5cm/yr that deflects the ascending plume into its west-tilted geometry. A geodynamic model of the Yellowstone plume constrained by Vp and Vs velocities and attenuation parameters suggests low excess temperatures of up to 120K, corresponding to a maximum 2.5% melt, and a small buoyancy flux of 0.25Mg/s, i.e., properties of a cool, weak plume. The buoyancy flux is many times smaller than for oceanic plumes, nonetheless, plume buoyancy has produced a ~400-km-wide, ~500-m-high topographic swell centered on the Yellowstone Plateau. Contemporary deformation derived from GPS measurements reveals SW extension of 2-3mm/yr across the Yellowstone Plateau, one-fourth of the total Basin-Range opening rate, which we consider to be part of Basin-Range intraplate extension. Locally, decadal episodes of subsidence and uplift, averaging ~2cm/yr, characterize the 80-year Yellowstone caldera monitored history and are modeled as hydrothermal-magmatic sources. Moreover a recent episode, 2004-2009, of accelerated uplift of the Yellowstone caldera at rates up to 7cm/yr has been modeled as resulting from magmatic recharge of a 10-km-deep sill at the top of the crustal magma reservoir. Regionally, gravitational potential energy of the Yellowstone swell drives the lithosphere southwest and “downhill” from the Yellowstone Plateau 400km where it coalesces with Basin-Range province-wide westward extension. Based on the geometry and its assumed 660km depth, we extrapolate the plume source southwest to its original location at 17Ma and 600km southwest and 200km north of the YSRP. Importantly, this location is beneath the southern part of the Columbia Plateau flood basalt field of the same age and implies that the Yellowstone mantle plume may be the common source for both of these large volcanic fields. Our time-progression model suggests that the original plume head rose vertically behind the Juan de Fuca plate, but at ~12Ma it lost the protection of the subducting plate from eastward mantle flow and encountered cooler, thicker continental lithosphere, becoming entrained in eastward upper-mantle flow. These results reveal that Yellowstone plume-plate processes have had a profound effect on Late Cenozoic geologic evolution and topography of a large part of the western U.S. [Copyright &y& Elsevier]

Abstract: We present a high-resolution tomographic image in the upper mantle for the Hainan plume determined by using both local and teleseismic data. The arrival-time data from teleseismic events are measured precisely from seismograms recorded by nine permanent seismic stations in Hainan island and Leizhou peninsula. Our results show that striking low-velocity (low-V) anomalies of up to −5% in the crust and −2% in the mantle are visible under the Hainan hotspot from the surface down to 250km depth. The Hainan plume is imaged as a continuous, NW–SE tilting, low-V column with a diameter of about 80km. Given the denser data coverage and finer parameterization, our result represents a significant improvement with respect to earlier, regional and global tomographic studies. Such an inclination of the Hainan plume can be explained by numerical simulations. The tilted plume conduit in the upper mantle could be associated with the subduction of oceanic plates, especially the Philippine Sea plate, and the extrusion of the Eurasian plate. Most likely, the tilted plume conduit has buoyantly risen from the lower mantle. [Copyright &y& Elsevier]

Abstract: In a recent article, [Boschi, L., Becker, T.W., Steinberger, B., 2007. Mantle plumes: dynamic models and seismic images. Geochem. Geophys. Geosyst. 8, Q10006. doi:10.1029/2007GC001733] (BBS07) have re-evaluated the degree to which slow seismic tomography anomalies correlate with the possible locations of plume-like mantle upwellings connected to surface hotspots. They showed that several, but not all, hotspots are likely to have a deep mantle origin. Importantly, they found that when advection of plume conduits in mantle flow is considered, such correlations are significantly higher than when conduits are assumed to be vertical under hotspots. The validity of these statements depends, however, on the definition of statistical significance. BBS07 evaluated the significance of correlation through simple Student’s t tests. Anderson (personal communication, July 2007) questioned this approach, given that the true information content of published tomography models is generally unknown, and proposed, instead, to evaluate the significance of correlation by comparing tomographic results with Monte Carlo simulations of randomly located plumes. Following this approach, we show here that the correlation found by BBS07 between advected plumes and slow anomalies in S-velocity tomography is less significant than previously stated, but still significant (at the 99.7% confidence level). We also find an indication that the seismic/geodynamic correlation observed by BBS07 does not only reflect the natural tendency of plumes to cluster in slow/hot regions of the mantle: although realistically advected, and thereby biased towards such regions, our random plumes correlate with slow tomographic anomalies significantly less than the plume models of BBS07. A less significant correlation with plume models characterizes P-velocity tomography; the correlation is, however, enhanced, if flow is computed from tomographic models with amplified heterogeneity, possibly accounting for the known resolution limits of global seismic data. In summary, the conclusions of BBS07 are confirmed: even at relatively long wavelengths, tomographic models are consistent with the presence of a number of tilted, whole-mantle plume-shaped slow anomalies, connected to surface hotspots. [Copyright &y& Elsevier]

• The Melanesian Border Plateau was constructed from four volcanic sources. • At least two hotspots as well as tectonic drivers sourced MBP volcanism. • We propose the term oceanic mid-plate superstructures for some compound volcanoes. • OMSs appear common on lithosphere that drifted over the pacific LLSVP. The ocean basins contain numerous volcanic ridges, seamounts and large igneous provinces (LIPs). Numerous studies have focused on the origin of seamount chains and LIPs but much less focus has been applied to understanding the genesis of large volcanic structures formed from a combination or series of volcanic drivers. Here we propose the term Oceanic Mid-plate Superstructures (OMS) to describe independent bathymetric swells or volcanic structures that are constructed through superimposing pulses of volcanism, over long time periods and from multiple sources. These sources can represent periods when the lithosphere drifted over different mantle plumes and/or experienced pulses of volcanism associated with shallow tectonic drivers (e.g. plate flexure; lithospheric extension). Here we focus on the Melanesian Border Plateau (MBP), one example of an OMS that has a complex and enigmatic origin. The MBP is a region of shallow Pacific lithosphere consisting of high volumes of volcanic guyots, ridges and seamounts that resides on the northern edge of the Vitiaz Lineament. Here we reconcile recently published constraints to build a comprehensive volcanic history of the MBP. The MBP was built through four distinct episodes: (1) Volcanism associated with the Louisville hotspot likely generating Robbie Ridge and some Cretaceous seamounts near the MBP. (2) Construction of oceanic islands and seamounts during the Eocene when the lithosphere passed over the Rurutu-Arago hotspot. (3) Reactivation of previous oceanic islands/seamounts and construction of new volcanos in the Miocene when the lithosphere passed over the Samoa hotspot. (4) Miocene to modern volcanism driven by lithospheric deformation and/or westward entrainment of enriched plume mantle due to toroidal mantle flow driven by the rollback of the Pacific plate beneath the Tonga trench. The combination of these processes is responsible for ∼222,000 km2 of intraplate volcanism in the MBP and indicates that this OMS was constructed from multiple volcanic drivers. [ABSTRACT FROM AUTHOR]

How cratons, the ancient cores of continents, evolved since their formation over 2.5 Ga ago is debated. Seismic tomography can map the thick lithosphere of cratons and, with its resolution now reaching regional scales in most continents, it reveals a more complex, fragmented cratonic lithosphere than seen previously. Joint analysis of the information on the present state of the lithosphere from tomography and its past state from diamondiferous kimberlites reveals surprisingly common lithospheric erosion over the last few hundred million years. Large igneous provinces or hotspot tracks, attributed to deep-mantle plumes, tend to occur near the lithosphere-loss locations. This suggests that the cratonic roots foundered once modified by mantle plumes. Our results imply that the total volume of cratonic lithosphere has decreased substantially since its Archean formation, with the fate of each craton depending on its plate-tectonic movements relative to mantle plumes.ReferencesSteinberger, B., Bredow, E., Lebedev, S., Schaeffer, A. and Torsvik, T.H., 2019. Widespread volcanism in the Greenland–North Atlantic region explained by the Iceland plume. Nature Geoscience, 12(1), p.61.Lebedev, S., Schaeffer, A.J., Fullea, J. and Pease, V., 2018. Seismic tomography of the Arctic region: inferences for the thermal structure and evolution of the lithosphere. Geological Society, London, Special Publications, 460(1), pp.419-440.Ravenna, M., Lebedev, S., Fullea, J. and Adam, J.M.C., 2018. Shear‐Wave Velocity Structure of Southern Africa's Lithosphere: Variations in the Thickness and Composition of Cratons and Their Effect on Topography. Geochemistry, Geophysics, Geosystems. [ABSTRACT FROM AUTHOR]

At the base of Earth's mantle, just above the boundary with the core, lie two large and nearly antipodal provinces characterized by anomalously slow seismic wave speeds, termed the Large Low Shear-wave Velocity Provinces (LLSVPs). One of these structures lies beneath Africa, and the other beneath the Pacific Ocean. Accumulating evidence has increasingly revealed that the LLSVPs play an important role in mantle dynamics, most notably in that they are spatially associated with most deep-seated mantle plumes, and they coincide with the broad, antipodal upwellings of Earth's degree 2 mantle flow. In affecting large-scale convection in the mantle, and the nucleation of plumes that can fragment the lithosphere, these lowermost mantle structures may furthermore influence long-term plate motions. A key question is therefore: how stable have these features been through time? Here we review multiple independent lines of evidence that indicate that the LLSVPs have been in approximately the same position for the last 300 Ma, and possibly considerably longer. Such long-term stability of the LLSVPs remains a contentious interpretation, and so we also discuss some of the important criticism that has been raised in response to it. Given the broad implications that this discussion carries for our understanding of the nature and history of mantle convection and plate tectonics, we regard the further interrogation of LLSVP stability beyond 300 Ma to be one of the foremost imperatives in modern geophysics. The way forward is not yet clear: innovative observational methods and novel experiments together with further numerical simulations are needed to resolve the question of Earth's degree-2 stability. [ABSTRACT FROM AUTHOR]