Deep Mantle Structure Research Articles

Widespread D″ ${\mathbf{D}}^{\mathbf{{\prime\prime}}}$ Anisotropy Beneath North America and the Northeastern Pacific and Implications for Upper Mantle Anisotropy Measurements

AbstractObservations of seismic waves that have passed through the Earth's lowermost mantle provide insight into deep mantle structure and dynamics, often on relatively small spatial scales. Here we use SKS, S2KS, S3KS, and PKS signals recorded across a large region including the United States, Mexico, and Central America to study the deepest mantle beneath large swaths of North America and the northeastern Pacific Ocean. These phases are enhanced via beamforming and then used to investigate polarization‐ and propagation direction‐dependent shear wave speeds (seismic anisotropy). A differential splitting approach enables us to robustly identify contributions from anisotropy. Our results show strong seismic anisotropy in approximately half of our study region, indicating that anisotropy may be more prevalent than commonly thought. In some regions, the anisotropy may be induced by flow driven by sinking cold slabs, and in other, more compact regions, by upwelling flow. Measured splitting due to lowermost mantle anisotropy is sufficiently strong to be non‐negligible in interpretations of SKS splitting due to upper mantle anisotropy in certain regions, which may prompt future re‐evaluations of upper mantle anisotropy beneath North and Central America.

A Comparison of Gravimetric, Isostatic, and Spectral Decomposition Methods for a Possible Enhancement of the Mantle Signature in the Long-Wavelength Geoidal Geometry

A long-wavelength geoidal geometry characterizes the most pronounced features of the Indian Ocean geoid low and the West Pacific and North Atlantic geoid highs. These large geoid undulations (globally roughly within ±100 m) are mainly attributed to a deep mantle structure and large lithospheric density and geometry variations (such as the African superswell), while maximum geoid modifications by a topographic relief of Himalaya and Tibet are up to ~30 m. To enhance the mantle signature in a long-wavelength geoidal geometry, gravimetric, isostatic, and spectral decomposition methods can be applied. In this study, we demonstrate that isostatic schemes yield isostatic geoid models that closely resemble a long-wavelength geoidal geometry. The gravimetric method, on the other hand, modifies the mantle geoid significantly. Further modifications of the mantle geoid by removing gravitational contributions of lithospheric mantle density and lithospheric thickness variations should (optimally) enhance the signature of the deep mantle in the sub-lithospheric mantle geoid. Our results confirm this assumption by revealing (large-scale) positive anomalies in the Central Pacific and along the Atlantic Ocean that are coupled by two negative anomalies in the East Pacific and South Eurasia. A gravimetric method thus better enhances the mantle signature in the geoidal geometry than isostatic and spatial decomposition methods. Nonetheless, our results also indicate the presence of possibly large errors in geoid modelling results that limit their full implementation in gravimetric studies of the Earth’s mantle density structure without using tomographic images of the mantle and additional geophysical, geothermal, and geochemical constraints.

The deep structure of the Trans-European Suture Zone (based on seismic survey and GSR data) and some insights in to its development

Deep crust and mantle structure of the Trans-European Suture Zone (TESZ) is considered on the basis of geological and geophysical investigations in the Baltic Sea-Black Sea section. The crustal structure of TESZ was studied on the basis of wide-angle depth seismic sounding (WDS), which was performed by international scientific teams with the participation of the Institute of Geophysics of NAS of Ukraine (IGF NASU). TESZ mantle structure was studied down to a depth of 800 km by the 3D P-velocity model of the Eurasian mantle according to the Taylor approximation method developed in the Institute of Geophysics of NASU. It is concluded that the deep crustal and mantle structure of the zone is a result of the simultaneous action of plate- and plum tectonic processes. TESZ was formed on two major collision alstages: in the late Ordovician — early Silurianas a result of the accession of the Avalonia microcontinent to the East European Platform (EEP), and in the late Carboniferous – early Permian with the accession of the European Hercynian (Varisian) terranes to EEP. The TESZ crustal structure is a trough of 150 (sometimes up to 200) km wide and several to 21 km deep, built by the allochthonous complex of paleozoids that underwent Caledonian and Hercynian orogens beyond the trough. Mantle structure of the TESZ, according to seismic tomographic studies, is of dual nature: on the one hand, the zone is traced subvertically to a depth of 700 km, on the other, within the zone there are everywhere inclined layers — slips to the depth of 350—600 km, that is the traces of subduction processes, which precededorac companied TESZ formation. Both structural features overlapeachother, which complicates paleotectonic and geohistorical analysis of TESZ formation. TESZ sinking to greater depths in the mantle can be explained by its increased permeability for advection of ultra-deep mantle fluids, established hereborogensic tomographic and paleomagnetic methods. Several variants of TESZ formation are assumed — A- or B-subduction during north eastern plate thrusting under the south western one in all variants.

The influence of uncertain mantle density and viscosity structures on the calculations of deep mantle flow and lateral motion of plumes

SUMMARY Mantle plumes form from thermal boundary layers, such as Earth's core–mantle boundary. As plumes rise towards the surface, they are laterally deflected by the surrounding mantle flow that is governed by deep mantle density and viscosity structures. The lateral motions of mantle plumes carry information of deep mantle structure and dynamics and are used to setup reference frames by which absolute plate motions are reconstructed. In this study, we compare two methods to compute deep mantle flow and lateral motion of plumes. In mantle convection (MC) models, the mantle flow field and lateral motions of plumes are determined by solving conservation equations forward-in-time from given initial conditions. In plume advection (PA) models, approximate viscosity and present-day density structures are used to calculate present-day mantle flow which is then propagated backward-in-time assuming zero thermal diffusion, and plume conduits are represented by continuous lines and are passively advected within the background mantle flow. The question is how assumptions in PA models influence the predictions of deep mantle flow and plume lateral motions. Here, we perform purely thermal MC models and thermochemical MC models with intrinsically dense materials in the lowermost mantle. The deep mantle flow and plume lateral motions are determined accurately in each MC model. We also perform PA models using the approximated present-day viscosity and temperature structures in these MC models. We find that PA models without considering temperature-dependence of viscosity and/or only using long wavelength present-day temperature structure (up to degree 20) often lead to an average of ∼50–60 per cent and ∼60–200 per cent differences of present-day mantle flow velocities than purely thermal MC models and thermochemical MC models, respectively. By propagating inaccurate flow fields backward-in-time in PA models often cause even larger errors of mantle flow velocities in the past. Even using the same parameters and starting from the same present-day mantle flow fields as in MC models, the PA models still show an average of ∼10–30 per cent misfit of mantle flow velocities after ∼40 Ma. In addition, we show that errors of mantle flow fields in PA models can cause ∼100–600 per cent differences of plume lateral motions than that constrained in MC models in the past 60 Ma. Even we use the mantle flow in MC models to advected virtual plumes in PA models, the virtual plumes could still show ∼50–300 per cent difference of lateral motions than dynamic plumes in MC models if the virtual plumes do not start with the same locations and/or shapes as plumes in MC models. We also find virtual plumes in PA models initiated at different locations and/or with different shapes can be later advected to similar locations, suggesting that the lateral motions of plumes in PA models can be non-unique. Therefore, it is important to consider the build-in assumptions of PA models when interpreting their predictions on deep mantle flow field and plume lateral motions. The accuracy of PA models would improve as we gain better understanding on Earth's deep mantle structure and dynamics.

Did the Neoproterozoic Revolution Extend to the Deep Mantle?

Did the Neoproterozoic Revolution Extend to the Deep Mantle?

Structure and dynamics of the Tonga subduction zone: New insight from P-wave anisotropic tomography

The Tonga-Lau-Fiji region is important to study plate-plume and subduction-ridge interactions, but its deep mantle structure is still not very clear. Here we present high-resolution tomography of 3-D P-wave azimuthal anisotropy down to 400 km depth of the Tonga subduction zone derived from arrival-time data of local earthquakes recorded at seafloor and land seismometers. The subducting Tong slab is imaged as high-velocity anomalies at depths of 100-400 km, whereas large-scale low-velocity anomalies down to 400 km depth are revealed in the mantle wedge beneath the backarc basin and volcanic arc. Trench-parallel anisotropy beneath the Lau Basin extends southwards to ∼140 km depth at ∼20.5°S, representing the extent of both southward flow of the Samoan plume and toroidal flow by the slab rollback. At depths of 140-400 km, the Lau Basin and Fiji Plateau mainly exhibit plate-parallel fast-velocity directions (FVDs) north of ∼20.5°S, indicating strong corner flow in the mantle wedge driven by the slab subduction and dehydration. The Tonga slab exhibits trench-parallel FVDs at depths of <200 km, reflecting fossil fabric formed during the plate spreading stage, whereas, at greater depths, the slab mainly exhibits trench-normal FVDs, which may reflect complicated deformations within the slab. These results suggest that the Samoan plume has a significant impact on the Tonga-Lau-Fiji region, leading to variations in the scale and depth extent of mantle flows.

Early Cambrian renewal of the geodynamo and the origin of inner core structure

Paleomagnetism can elucidate the origin of inner core structure by establishing when crystallization started. The salient signal is an ultralow field strength, associated with waning thermal energy to power the geodynamo from core-mantle heat flux, followed by a sharp intensity increase as new thermal and compositional sources of buoyancy become available once inner core nucleation (ICN) commences. Ultralow fields have been reported from Ediacaran (~565 Ma) rocks, but the transition to stronger strengths has been unclear. Herein, we present single crystal paleointensity results from early Cambrian (~532 Ma) anorthosites of Oklahoma. These yield a time-averaged dipole moment 5 times greater than that of the Ediacaran Period. This rapid renewal of the field, together with data defining ultralow strengths, constrains ICN to ~550 Ma. Thermal modeling using this onset age suggests the inner core had grown to 50% of its current radius, where seismic anisotropy changes, by ~450 Ma. We propose the seismic anisotropy of the outermost inner core reflects development of a global spherical harmonic degree-2 deep mantle structure at this time that has persisted to the present day. The imprint of an older degree-1 pattern is preserved in the innermost inner core.

Geochemical Constraints on the Structure of the Earth's Deep Mantle and the Origin of the LLSVPs

AbstractGeophysical analysis of the Earth's lower mantle has revealed the presence of two superstructures characterized by low shear wave velocities on the core‐mantle boundary. These Large Low Shear Velocity Provinces (LLSVPs) play a crucial role in the dynamics of the lower mantle and act as the source region for deep‐seated mantle plumes. However, their origin, and the characteristics of the surrounding deep mantle, remain enigmatic. Mantle plumes located above the margins of the LLSVPs display evidence for the presence of this deep‐seated, thermally and/or chemically heterogeneous mantle material ascending into the melting region. As a result, analysis of the spatial geochemical heterogeneity in ocean island basalts provides constraints on the structure of the Earth's lower mantle and the origin of the LLSVPs. In this study, we focus on the Galápagos Archipelago in the eastern Pacific, where bilateral asymmetry in the radiogenic isotopic composition of erupted basalts has been linked to the presence of LLSVP material in the underlying plume. We show, using spatial variations in the major element contents of high‐MgO basalts, that the isotopically enriched south‐western region of the Galápagos mantle—assigned to melting of LLSVP material—displays no evidence for lithological heterogeneity in the mantle source. As such, it is unlikely that the Pacific LLSVP represents a pile of subducted oceanic crust. Clear evidence for a lithologically heterogeneous mantle source is, however, found in the north‐central Galápagos, indicating that a recycled crustal component is present near the eastern margin of the Pacific LLSVP, consistent with seismic observations.

Composition and Pressure Effects on Partitioning of Ferrous Iron in Iron-Rich Lower Mantle Heterogeneities

Both seismic observations of dense low shear velocity regions and models of magma ocean crystallization and mantle dynamics support enrichment of iron in Earth’s lowermost mantle. Physical properties of iron-rich lower mantle heterogeneities in the modern Earth depend on distribution of iron between coexisting lower mantle phases (Mg,Fe)O magnesiowüstite, (Mg,Fe)SiO3 bridgmanite, and (Mg,Fe)SiO3 post-perovskite. The partitioning of iron between these phases was investigated in synthetic ferrous-iron-rich olivine compositions (Mg0.55Fe0.45)2SiO4 and (Mg0.28Fe0.72)2SiO4 at lower mantle conditions ranging from 33–128 GPa and 1900–3000 K in the laser-heated diamond anvil cell. The resulting phase assemblages were characterized by a combination of in situ X-ray diffraction and ex situ transmission electron microscopy. The exchange coefficient between bridgmanite and magnesiowüstite decreases with pressure and bulk Fe# and increases with temperature. Thermodynamic modeling determines that incorporation and partitioning of iron in bridgmanite are explained well by excess volume associated with Mg-Fe exchange. Partitioning results are used to model compositions and densities of mantle phase assemblages as a function of pressure, FeO-content and SiO2-content. Unlike average mantle compositions, iron-rich compositions in the mantle exhibit negative dependence of density on SiO2-content at all mantle depths, an important finding for interpretation of deep lower mantle structures.

The interaction between mantle plumes and lithosphere and its surface expressions: 3-D numerical modelling

SUMMARY The rise of mantle plumes to the base of the lithosphere leads to observable surface expressions, which provide important information about the deep mantle structure. However, the process of plume–lithosphere interaction and its surface expressions remain not well understood. In this study, we perform 3-D spherical numerical simulations to investigate the relationship between surface observables induced by plume–lithosphere interaction (including dynamic topography, geoid anomaly and melt production rate) and the physical properties of plume and lithosphere (including plume size, plume excess temperature, plume viscosity, and lithosphere viscosity and thickness). We find that the plume-induced surface expressions have strong spatial and temporal variations. Before reaching the base of the lithosphere, the rise of a plume head in the deep mantle causes positive and rapid increase of dynamic topography and geoid anomaly at the surface but no melt production. The subsequent impinging of a plume head at the base of the lithosphere leads to further increase of dynamic topography and geoid anomaly and causes rapid increase of melt production. After reaching maximum values, these plume-induced observables become relatively stable and are more affected by the plume conduit. In addition, whereas the geoid anomaly and dynamic topography decrease from regions above the plume centre to regions above the plume edge, the melt production always concentrates at the centre part of the plume. We also find that the surface expressions have different sensitivities to plume and lithosphere properties. The dynamic topography significantly increases with the plume size, plume excess temperature and plume viscosity. The geoid anomaly also increases with the size and excess temperature of the plume but is less sensitive to plume viscosity. Compared to the influence of plume properties, the dynamic topography and geoid anomaly are less affected by lithosphere viscosity and thickness. The melt production significantly increases with plume size, plume excess temperature and plume viscosity, but decreases with lithosphere viscosity and thickness.

Deep mantle structure and origin of Cenozoic intraplate volcanoes in Indochina, Hainan and South China Sea

SUMMARYCenozoic basalts with ages ranging from 28.5 to &amp;lt; 0.1 Ma are widely distributed in the Indochina block, the South China Sea basin and the Leiqiong area in South China including the Leizhou Peninsula and the northern Hainan Island, which form the southeastern Asian basalt province (SABP). These Cenozoic basalts share common petrological and geochemical characteristics. However, the origin of the Cenozoic intraplate volcanism in the SABP is still a controversial issue. In this work, we apply a novel technique of multiscale global tomography to study the whole-mantle 3-D P-wave velocity (Vp) structure beneath the SABP. Our results show that low-Vp anomalies prevail in the whole mantle beneath the SABP. Although the strongest low-Vp zones exist beneath Hainan, significant low-Vp anomalies are also visible in the mantle beneath other parts of the SABP. These low-Vp anomalies appear somehow independent, rather than deriving from a single plume. We deem that a cluster of plumes rather than a single plume existed in the Cenozoic and may still exist now in the mantle beneath the SABP, though the Hainan plume may be the strongest one. A geochemical study suggested that the Hainan plume upwelling might be slowing down and close to exhausting its source zone. This geochemical inference is supported by our tomographic images showing that the low-Vp zones under Hainan are weak and intermittent in the lower mantle (∼700–2889 km depths). The low-Vp zones in the mantle beneath other SABP Cenozoic volcanoes are also weak, suggesting that those mantle plumes, if any, are also dying or already dead. As compared with a strong single plume, each member in a plume cluster should be small and weak, and so hard to exist long. The SABP is surrounded by subduction zones. The hot mantle upwelling beneath the SABP might be caused by collapsing of subducted slabs down to the lowermost mantle.

Distinct formation history for deep-mantle domains reflected in geochemical differences

The Earth’s mantle is currently divided into the African and Pacific domains, separated by the circum-Pacific subduction girdle, and each domain features a large low shear-wave velocity province (LLSVP) in the lower mantle. However, it remains controversial as to whether the LLSVPs have been stationary through time or dynamic, changing in response to changes in global subduction geometry. Here we compile radiogenic isotope data on plume-induced basalts from ocean islands and oceanic plateaus above the two LLSVPs that show distinct lead, neodymium and strontium isotopic compositions for the two mantle domains. The African domain shows enrichment by subducted continental material during the assembly and breakup of the supercontinent Pangaea, whereas no such feature is found in the Pacific domain. This deep-mantle geochemical dichotomy reflects the different evolutionary histories of the two domains during the Rodinia and Pangaea supercontinent cycles and thus supports a dynamic relationship between plate tectonics and deep-mantle structures. Earth’s deep-mantle domains are geochemically distinct. The African domain is enriched in subducted material, which suggests a different history from the Pacific domain and a dynamic relationship between plate tectonics and deep-mantle structures.

Effects of Heat‐Producing Elements on the Stability of Deep Mantle Thermochemical Piles

AbstractGeochemical observations of ocean island and mid‐ocean ridge basalts suggest that abundances of heat‐producing elements (HPEs: U, Th, and K) vary within the mantle. Combined with bulk silicate Earth models and constraints on the Earth's heat budget, these observations suggest the presence of a more enriched (potentially deep and undepleted) reservoir in the mantle. Such a reservoir may be related to seismically observed deep mantle structures known as large low shear velocity provinces (LLSVPs). LLSVPs might represent thermochemical piles of an intrinsically denser composition, and many studies have shown such piles to remain stable over hundreds of Myr or longer. However, few studies have examined if thermochemical piles can remain stable if they are enriched in HPEs, a necessary condition for them to constitute an enriched HPE reservoir. We conduct a suite of mantle convection simulations to examine the effect of HPE enrichment up to 25× the ambient mantle on pile stability. Model results are evaluated against present‐day pile morphology and tested for resulting seismic signatures using self‐consistent potential pile compositions. We find that stable piles can form from an initial basal layer of dense material even if the layer is enriched in HPEs, depending on the density of the layer and degree of HPE enrichment, with denser basal layers requiring increased HPE enrichment to form pile‐like morphology instead of a stable layer. Thermochemical piles or LLSVPs may therefore constitute an enriched reservoir in the deep mantle.

Pangea and the Lower Mantle

AbstractWe show that the peripheral Pangea subduction zone closely followed a polar great circle. We relate it to the band of faster‐than‐average velocities in lowermost mantle. Both structures have an axis of symmetry in the equatorial plane. Assuming geologically long‐term stationarity of the deep mantle structure, we propose to use the axis of symmetry of Pangea to define an absolute reference frame. This reference frame is close to the slab remnants and NNR frames of reference but disagrees with hot spot‐based frames. We apply this model to the last 400 Myr. We show that a hemispheric supercontinent appeared as early as 400 Ma. However, at 400 Ma, the axis of symmetry was situated quite far south and progressively migrated within the equatorial plane that it reached at 300 Ma. From 300 to 110–100 Ma, it maintained its position within the equatorial plane. We propose that the stationarity of Pangea within a single hemisphere surrounded by subduction zones led to thermal isolation of the underlying asthenosphere and consequent heating as well as a large accumulation of hot plume material. We discuss some important implications of our analysis concerning the proposition that the succession of supercontinents and dispersed continents is controlled by an alternation from a degree 1 to a degree 2 planform.

Optimum choice of parameters and numerical schemes for a regional gravimetric Moho recovery

Despite practical limitations of isostatic theories to model the Moho geometry are well-known, gravimetric methods are often used in terrestrial studies of crustal structure in regions with a low seismic data coverage. Moreover, these methods are indispensable in planetary studies. Various gravimetric methods have been proposed. The Airy and Pratt theories are defined based on adopting a local compensation mechanism. The Vening Meinesz theory assumes a regional isostatic flexural model. The Vening Meinesz regional isostatic model generally describes a respond of the lithosphere to a load more realistically than the Airy model over continents. The Pratt method, on the other hand, better describes a compensation mechanism of the oceanic lithosphere. The application of a particular isostatic model also depends on applied numerical procedures, parameters for inversion, input data specifications, and many other aspects. In this study, we address some basic aspects by applying local and regional isostatic models for a Moho recovery. We also conduct a spectral analysis to assess a spectral resolution of gravity data that is optimal for a Moho recovery. Furthermore, we inspect the influence of low-degree spherical harmonics of gravity field on a Moho geometry. Gravimetric results are validated using seismic data at the European plate. Our results confirm a better performance of a regional compensation principle. We also demonstrate that a different thickness of the oceanic and continental crustal thickness should be taken into account as a priori information. Spectral analysis indicates that gravity data used for a Moho inversion should optimally have a spatial resolution between degrees 60 to 180. Results also show that low-degree spherical harmonics do not modify significantly the Moho geometry, particularly over regions with a relatively homogenous structure of deep mantle.

The interplay of dynamic topography and eustasy on continental flooding in the late Paleozoic

Global sea level change can be inferred from sequence stratigraphic and continental flooding data. These methods reconstruct sea level from peri-cratonic and cratonic basins that are assumed to be tectonically stable and sometimes called reference districts, and from spatio-temporal correlations across basins. However, it has been understood that long-wavelength (typically hundreds of km) and low-amplitude (<2 km) vertical displacements of the Earth's surface due to mantle flow, namely dynamic topography, can occur in the absence of crustal deformation. Dynamic topography can drive marine inundation or regional emergence of continents and must be taken into consideration for eustasy estimates. Our analysis indicates that the long-term trend in global-scale maximum flooding over the late Paleozoic generally correlates with global sea level curves. The first-order flooding history of North America correlates with some estimates of eustasy. The Paleozoic inundation of South America does not follow long-term sea level variations. The flooding lows during the Early Carboniferous and high during the Late Carboniferous are at odds with estimates of eustasy and can be explained by dynamic uplift and subsidence, respectively. Our dynamic topography models indicate that the Yangtze Platform of South China experienced significant dynamic subsidence during the transition from Permian to Triassic largely due to proto-Pacific subduction and its northward motion to collide with North China. The reference districts – Western New York, Oklahoma and Kansas, and West Texas in North America – were to some degree affected by dynamic uplift and subsidence associated with long-lived Panthalassa subduction zones, closure of the Rheic Ocean and large-scale upwelling above the African deep-mantle structure during late Paleozoic times. This indicates that some published global sea level curves may include non-eustatic signals such as dynamic uplift or subsidence. The interpretation of stratigraphic data gathered from these reference districts should be treated with caution to estimate global sea level variations.

Sources vs processes: Unraveling the compositional heterogeneity of rejuvenated-type Hawaiian magmas

Spatial isotope variations in oceanic island basalts hold clues to the geochemical structure of mantle plumes. This signal is however susceptible to contributions from non-plume sources and modifications by physical processing in the upper mantle, i.e. source hybridization and segregation through gradients in melting conditions. A comprehensive survey of lava isotope systematics is therefore necessary to reveal spatiotemporal patterns at the scale of a single hotspot and disentangle plume sources from upper mantle physical and chemical contributions. We present Hf and Pb isotope data on 72 previously characterized onland and submarine lavas from the islands of Ni‘ihau and Kaua‘i, and from the North Arch Volcanic Field. These lavas cover the entire ∼ 5 Ma eruptive history of Ni‘ihau and Kaua‘i, allowing for a detailed assessment of the temporal geochemical evolution of Hawaiian volcanism. Furthermore, the broad lateral coverage of volcanic activity offered by these lavas constrains the spatial variability of sources across the volcanic chain, allowing investigation of the role of an upper asthenosphere contribution to plume volcanism. Early, shield-building (tholeiitic) lavas from Kaua‘i and Ni‘ihau overlap in Nd–Hf–Sr–Pb isotopes, contrasting with the across-plume asymmetry seen in younger shield lavas (i.e. the LOA-KEA double chain), and their isotope systematics suggest sampling of both LOA and KEA components. In contrast, late Kaua‘i and Ni‘ihau rejuvenated lavas do not overlap in isotope composition. They share common isotope characteristics with rejuvenated lavas from North Arch, Ka‘ula and O‘ahu, and together form two isotopically distinct, southwestern and northeastern groups. The two groups have steep slopes (>1.7) in a εNd − εHf plot and require distinct depleted components. The high εHf for a given εNd, high 87Sr/86Sr, and trace element systematics of the southwestern group (Ni‘ihau, Ka‘ula and O‘ahu) indicate a depleted component intrinsic to the plume. In contrast, lavas from the northeastern group (Kaua‘i and North Arch) indicate a source component isotopically identical to Pacific MORB. The shift from a spatially broad and homogeneous plume source in the shield stage of Kaua‘i and Ni‘ihau (6–3.5 Ma) to a bilaterally heterogeneous source during the rejuvenated stage (<3.5 Ma) is synchronous with the emergence of the LOA and KEA geochemical double chain in younger Hawaiian shield lavas. Our data reveal a shift to Pacific asthenosphere contribution to Kaua‘i volcanism through time, while Ni‘ihau exclusively samples the plume. These observations are inconsistent with a scenario where plume geochemical structure is the primary control on spatial heterogeneities in lava chemistry, and instead point to a more complex and dynamic interaction between plume, Pacific asthenosphere and lithosphere. Therefore, deep mantle structure might not be the primary control on the spatial distribution of isotope systematics in oceanic island lavas, making a link between the latter and contrasts imaged in the lower mantle through seismic tomography tenuous.

Origin and evolution of the deep thermochemical structure beneath Eurasia

A unique structure in the Earth’s lowermost mantle, the Perm Anomaly, was recently identified beneath Eurasia. It seismologically resembles the large low-shear velocity provinces (LLSVPs) under Africa and the Pacific, but is much smaller. This challenges the current understanding of the evolution of the plate–mantle system in which plumes rise from the edges of the two LLSVPs, spatially fixed in time. New models of mantle flow over the last 230 million years reproduce the present-day structure of the lower mantle, and show a Perm-like anomaly. The anomaly formed in isolation within a closed subduction network ∼22,000 km in circumference prior to 150 million years ago before migrating ∼1,500 km westward at an average rate of 1 cm year−1, indicating a greater mobility of deep mantle structures than previously recognized. We hypothesize that the mobile Perm Anomaly could be linked to the Emeishan volcanics, in contrast to the previously proposed Siberian Traps.

Landscape evolution in Africa during the Cenozoic and Quaternary—the legacy and limitations of Lester C. King

African landscape evolution since 66 Ma reflects interactions among parts of the Earth system from the Core to the Biosphere. We stress changes in those interactions in three events that have dominated landscape development: (i) a climatic revolution when the circumpolar current and the East Antarctic ice sheet first formed ∼37 Ma; (ii) a tectonic revolution at ∼32 Ma dominated by elevation of ∼30 topographic structural swells continent-wide; and (iii) a second climatic revolution in a Northern Hemisphere cooling event (at ∼2.7 Ma) that triggered Sahara desert initiation and the beginning of glacial cycles in the Northern Hemisphere (∼2.15 Ma). We recognize the following distinct features of the great Afro-Arabian continent (∼40 M km2) that show its relationship to Earth structure and processes: deep mantle structure and dynamics, plate motion with respect to that structure, especially plate-arrest (∼32 Ma). The topographic, erosional, geomorphic, and depositional modifications following that tectonic event were strongly influenced by changes around the continent in oceanic and atmospheric circulation that affected the entire continent. Atmospheric circulation changes since ∼34 Ma have involved zonal winds, the ITCZ, desert formation and destruction, the evolution of the persistent (since ∼35 Ma) Antarctic ice sheet, and since ∼2 Ma of the rapidly cycling Eurasian ice sheets. We explain that a widely supported idea that ancient erosion surfaces have survived at high elevations in Africa is the result of a failure to recognize that the present elevations of the continent’s swells are dynamically maintained, but do not display a thermochronological signature because they are young (less than ∼32 My), so that conduction of heat from shallow convection cells has not yet reached the Earth’s surface.

Oceanic mantle gravimetric response to the seafloor spreading of the Southwest Sub‐basin, South China Sea

AbstractThe residual mantle gravity anomaly (RMA) is a straightforward response of the deep mantle structure of the lithosphere. We acquired RMA from newly updated free‐air gravity anomaly by incorporating abundant geophysical evidences, such as the International Ocean Drilling Program expedition 349, the interpretation of 10 seismic profiles and the bathymetric data in the Southwest Sub‐basin (SWSB) of the South China Sea. By inverting the crustal thickness by the application of the isostatic flexure analysis, comparing with ocean bottom seismic tomography result, then determining a detail crustal thickness of the study region, we calculated the RMA of the oceanic basin in the SWSB. The main features of the RMA and their relation to the palaeo‐spreading pattern are discussed, and the implications of the asymmetric lithospheric deformation due to variations in melt extraction are proposed. In the light of the asymmetric RMA at the north and south sides of the fossil spreading centre in the SWSB, we preferred asymmetric melt retention depth and extraction rate changes during the oceanic lithosphere creation of the South China Sea. Copyright © 2016 John Wiley &amp; Sons, Ltd.