Heat flow and depth versus age for the Mesozoic northwest Atlantic Ocean: results from the Sohm abyssal plain and implications for the Bermuda Rise

Heat flow in the Sohm abyssal plain is measured to be 53 mW/m² at an age of 163 Ma. This is 25% higher than predicted by conductive cooling models, even though the sediment-corrected basement depth of 6.5 km at this location is normal for its age. An analysis of existing heat flow, depth and geoid anomalies in the northwest Atlantic shows that there is little correlation between heat flow and depth throughout the entire region. Depth and geoid are clearly related to the Bermuda swell while the associated heat flow anomaly, once adjusted for variations with age, is limited to 5 mW/m² and only decays to the south. This means that the Bermuda swell is probably not caused by extensive thermal reheating within the lithosphere, but instead by dynamic uplift at its lower boundary due to the convective upwelling of a mantle plume. The regionally high heat flow in the northwest Atlantic may be a thermal remanent of previous plumes which passed beneath this region early in its history. Therefore, depth and heat flow anomalies from this region cannot be used to provide constraints on steady-state parameters of the lithosphere, such as the presence or absence of a long-term boundary layer at its base.     

Geophysical observations pertaining to solid-state convection in the terrestrial planets

We present a broad-based review of the observational evidence that pertains to or otherwise implies solid-state convection to be occurring (or have occurred) in the interiors of the terrestrial planets.For the Earth, the motion of the plates is prima facie evidence of large-scale mantle convection. Provided we understand upper-mantle thermal conductivity correctly, heat flow beneath the old ocean basins may be too high to be transported conductively from the upper mantle through the base of the lithosphere and therefore convection on a second smaller scale might be operative. The horizontal scale of plate dimensions implies, due to typical cell aspect ratios observed in convection, that the motion extends to the core-mantle boundary. Improved global data coverage and viscoelastic modeling of isostatic rebound due to Pleistocene deglaciation imply a uniform mantle viscosity, and thus indicate that whole-mantle convection could exist. Additionally, there is some seismic evidence of lithospheric penetration to depths deeper than 700 km. We discuss some salient features and assumption boundedness of arguments for convection confined to the upper mantle and for convection which acts throughout the mantle since the vertical length scale has a profound effect upon the relevance of geophysical observations. The horizontal form of mantle convection may be fully three-dimensional with complex planform and, therefore, searching for correlative gravity patterns in the ocean basins may not be useful without additional geophysical constraints. Many long-wavelength gravity anomalies may arise from beneath the lithosphere and must be supported dynamically, although thermal convection is not a unique explanation. Topography is an additional geophysical constraint, but for wavelengths greater than a few hundred kilometers, a general lack of correlation exists between oceanic residual gravity and topography, except at specific locations such as Hawaii. Theoretical calculations predict a complex relationship between these two observational types. Oceanic gravity data alone shows no regular planform and there is no correlation with any small-scale convective pattern predicted by laboratory experiments.All of the observational evidence argues against Martian plate tectonics occurring now or over much of the history of this planet, but lack of plate tectonics is not an argument against interior convection. The Tharsis uplift on Mars may have resulted from convective processes in the mantle, and the present-day gravity anomaly associated with Tharsis must be supported by the finite strength of the lithosphere or by mantle convection. Stresses imparted by the present topographic load would be greater than a kilobar, in excess of long-term finite strength. Observed fracture patterns are probably a direct result of this load, and the key question concerns the level of resultant strain relief. The global topographic and geomorphic dichotomy between the northern and southern hemisphere required a solid-state flow process to create the accompanying center-of-figure to center-of-mass offset.Lunar heat flow values, in analogy with oceanic heat flow on the Earth, strongly imply a convective mechanism of heat transport in the interior which, based on seismic Q values, is limited to the lower mantle. The presence of moonquakes in this region does not preclude solid-state convective processes. Lunar conductivity profiles provide no information on convection because of the difficulty in conductivity modeling, uniqueness of models, and the uncertainty in the conductivity-temperature relationship. The excess oblateness of the lunar figure over the hydrostatic value does not require convective support; in fact, such a mechanism is unlikely.The presence of a dipole magnetic field on Mercury does not provide a constraint on mantle convection unless its existence can be inextricably linked to a molten core. The non-hydrostatic shape of the equatorial figure, required for the observed $\text{3}\text{2}$ resonance between Mercury's rotational and orbital periods, is most likely related to surface processes, as opposed to convection. The $\text{3n}\text{2}$ resonance implies escape from a 2n resonance and, therefore, is related to the question of a molten core. Further dynamical data is needed to constrain interior models.Interpretation of limited radar imagery for the surface of Venus is enigmatic in terms of plate tectonics and therefore interior convection. Linear tensional and possibly compressional features are observed, but there are also crustal regions which appear to show large impact structures and are thus geologically old and may not have been recycled.     

Crustal structure and origin of the Cape Verde Rise

The Cape Verde Islands are located on a mid-plate topographic swell and are thought to have formed above a deep mantle plume. Wide-angle seismic data have been used to determine the crustal and uppermost mantle structure along a ~ 440 km long transect of the archipelago. Modelling shows that ‘normal’ oceanic crust, ~ 7 km in thickness, exists between the islands and is gently flexed due to volcano loading. There is no direct evidence for high density bodies in the lower crust or for an anomalously low density upper mantle. The observed flexure and free-air gravity anomaly can be explained by volcano loading of a plate with an effective elastic thickness of 30 km and a load and infill density of 2600 kg m^− 3. The origin of the Cape Verde swell is poorly understood. An elastic thickness of 30 km is expected for the ~ 125 Ma old oceanic lithosphere beneath the islands, suggesting that the observed height of the swell and the elevated heat flow cannot be attributed to thermal reheating of the lithosphere. The lack of evidence for high densities and velocities in the lower crust and low densities and velocities in the upper mantle, suggests that neither a crustal underplate or a depleted swell root are the cause of the shallower than expected bathymetry and that, instead, the swell is supported by dynamic uplift associated with the underlying plume.     

Thermal convection in a cylindrical annulus with a non-Newtonian outer surface

This study presents the results of numerical simulations of a model for lithospheremantle coupling in a terrestrial type planet. To first order, a geologically active terrestrial type planet may consist of a metallic core, silicate mantle and lithosphere, with the lithosphere being rheologically different from the mantle. Therefore we have developed a numerical model consisting of a thin non-Newtonian fluid hoop that is dynamically coupled to a thick Newtonian fluid cylindrical annulus. Thus the rheological dichotomy between mantle and lithosphere is built into the model. Time-dependent calculations show the existence of at least two regimes of behaviors. In one regime, the behavior of the hoop switches between periods characterized by low or high speeds, in response to changes in convective vigor and planform. This regime may apply to the planet Venus where the available evidence indicates that prior to 500 myr ago, the planet was resurfaced on a time scale of <100 myr. Since that time, large-scale tectonic activity on Venus has been sharply curtailed. In the other regime, which is more like plate tectonics on Earth, the hoop speeds rise and fall on short time scales.     

Anomalous subsidence on the rifted volcanic margin of Pakistan: No influence from Deccan plume

The role of hotter than ambient plume mantle in the formation of a rifted volcanic margin in the northern Arabian Sea is investigated using subsidence analysis of a drill site located on the seismically defined Somnath volcanic ridge. The ridge has experienced > 4 km of subsidence since 65 Ma and lies within oceanic lithosphere. We estimate crustal thickness to be 9.5–11.5 km. Curiously < 400 m of the thermal subsidence occurred prior to 37 Ma, when subsidence rates would normally be at a maximum. We reject the hypothesis that this was caused by increasing plume dynamic support after continental break-up because the size of the thermal anomalies required are unrealistic (> 600 °C), especially considering the rapid northward drift of India relative to the Deccan-Réunion hotspot. We suggest that this reflects very slow lithospheric growth, possibly caused by vigorous asthenospheric convection lasting > 28 m.y., and induced by the steep continent–ocean boundary. Post-rift slow subsidence is also recognized on volcanic margins in the NE Atlantic and SE Newfoundland and cannot be used as a unique indicator of plume mantle involvement in continental break-up.     

Constraints on the dynamics of mantle plumes from uplift of the Hawaiian Islands

The ∼0.2 mm/yr uplift of Hawaiian islands Lanai and Molokai and Hawaiian swell topography pose important constraints on the structure and dynamics of mantle plumes. We have formulated 3-D models of mantle convection to investigate the effects of plume-plate interactions on surface vertical motions and swell topography. In our models, the controlling parameters are plume radius, excess plume temperature, and upper mantle viscosity. We have found that swell height and swell width constraints limit the radius of the Hawaiian plume to be smaller than 70 km. The additional constraint from the uplift at Lanai requires excess plume temperature to be greater than 400 K. If excess plume temperature is 400 K, models with plume radius between 50 and 70 km and upper mantle viscosity between 10²⁰ and 3×10²⁰ Pa s satisfy all the constraints. Our results indicate that mantle plume in the upper mantle may be significantly hotter than previously suggested. This has important implications for mantle convection and mantle melting. In addition to constraining plume dynamics, our models also provide a mechanism to produce the observed uplift at Lanai and Molokai that has never been satisfactorily explained before.     

Planimetric and volumetric changes of reef islands in response to wave conditions

Reef islands are morphologically dynamic features located on atolls and platform reefs that are very sensitive to wave‐induced processes on different timescales. The planform morphological evolution of reef islands is widely described; however, the mechanisms of the volumetric variations in response to wave energy are still poorly documented. To assess their multitemporal vertical and horizontal mobility, we performed a series of synchronous measurements of the volumetric changes and incident wave energies at two reef islands and a shingle bank at the Rocas Atoll in the South Atlantic Ocean. The results show the differences in the magnitudes and locations of the sediment mobility between the reef islands. Whereas one island remained stable on all timescales, with only small volumetric changes concentrated at its extremities, the other island (Farol Island) showed high mobility, especially during the energetic northern swell season. The gross volumetric change reached 10.03 × 10³ m³ (5% of the total island volume) on a daily timescale; however, on a seasonal scale, the gross erosion was compensated by the gross accretion, indicating a cyclical seasonal pattern. Moreover, the observed volumetric changes induced by the waves on both daily and seasonal timescales did not result in large shoreline displacements. However, long‐term oceanward erosion and substantial lagoonward accretion were observed at Farol Island on a decadal scale, resulting in a pronounced change in its planform morphology. This appears to be promoted by at least three sediment transport pathways induced by waves at the atoll, including sediment adjustment between the reef islands. Our results show that reef islands on the same atoll can have very distinct morphological behaviors on daily, seasonal and decadal scales in response to the same boundary conditions. Copyright © 2017 John Wiley & Sons, Ltd.     

The thermal history of the Sichuan Basin,SW China: Evidence from the deep boreholes

The Sichuan Basin is a superimposition basin composed of terrestrial and marine sediments that is well known for its abundant petroleum resources. Thermal history reconstruction using paleogeothermal indicators, including vitrinite reflectance and thermochronological data, shows that different structural subsections of the Sichuan Basin have experienced various paleogeothermal episodes since the Paleozoic. The lower structural subsection comprising the Lower Paleozoic to Middle Permian (Pz-P₂ successions experienced a high paleogeothermal gradient (23.0–42.6°C/km) at the end of the Middle Permian (P₂, whereas the upper structural subsection comprising Late Permian to Mesozoic strata underwent a relatively lower paleogeothermal gradient (13.2–26.9°C/km) at the beginning of the denudation (Late Cretaceous or Paleocene in the different regions). During the denudation period, the Sichuan Basin experienced a successive cooling episode. The high paleogeothermal gradient resulted from an intensive thermal event correlated to the Emeishan mantle plume. The heat flow value reached 124.0 mW/m² in the southwestern basin near the center of the Emeishan large igneous province. The low geothermal gradient episode with heat flow ranging from 31.2 to 70.0 mW/m² may be related to the foreland basin evolution. The cooling event is a result of the continuous uplift and denudation of the basin.     

Evolution of the deformation and stress patterns of the East Asia continent under multi-driving force

Introduction Seismological observation and all kinds of ways of geodesy measurement imply that there is significant correlation between crustal movement and seismic activity (MEI, 1993). Therefore, it is very important to get the materials of the continental crust deformation and the patterns of stress field for the studying of the mechanism and prediction of the continental strong earthquake. Data of the continental deformation and patterns of stress field can be mainly obtained by the follo…     

Three-dimensional numerical simulations of crustal deformation and subcontinental mantle convection

3-D simulations of mantle convection allowing for continental crust are explored to study the effects of crustal thickening on lithosphere stability and of continents on large-scale mantle flow. Simulations begin with a crustal layer within the upper thermal boundary layer of a mantle convection roll in a 1 × 1 × 1 Cartesian domain. Convective stresses cause crust to thicken above a sheet-like mantle downwelling. For mild convective vigor an initial crustal thickness variation is required to induce 3-D lithospheric instability below the zone of crustal convergence. The amplitude of the required variation decreases with increasing convective vigor. Morphologically, instability is manifest in formation of drip-like thermals that exist within the large-scale roll associated with initial crustal thickening. A strong surface signature of the drips is their ability to cause deviations from local Airy compensation of topography. After the initial thickening phase, the crustal accumulation that forms serves as a model analog to a continent. Its presence leads to mantle flow patterns distinctly different from the steady-state roll that results in its absence. Large lateral thermal gradients are generated at its edge allowing this region to be the initiation site for continued small-scale thermal instabilities. Eventually these instabilities induce a restructuring of large-scale mantle flow, with the roll pattern being replaced by a square cell. Although preliminary and idealized, the simulations do show the fluid dynamical plausibility behind the idea that significant mantle variations can be generated along the strike of a largely 2-D mountain chain by the formation of the chain itself. The ability of a model continent to cause a change in fundamental convective planform also suggests that the effects of continental crust on mantle convection may be low-order despite the seemingly trivial volume of crust relative to mantle.     

Early Permian Tarim Large Igneous Province in northwest China

Tarim Large Igneous Province (TLIP) is the second Late Paleozoic LIPs in China after the recognition of Emeishan LIP, and is a hot research topic in geosciences. On the basis of the analysis of research history about TLIP, this paper summarizes the research result during last twenty years and suggests the key research area in the future. The residual distribution range of TLIP is up to 250000 km², and the largest residual thickness is 780 m. The eruption of basalt happened during 290–288 Ma and belongs to LIPs magmatic event with fast eruption of magma. The lithological units of the TLIP include basalt, diabase, layered intrusive rock, breccia pipe mica-olivine pyroxenite, olivine pyroxenite, gabbro, ultramafic dyke, quartz syenite, quartz syenite porphyry and bimodal dyke. The basalt and diabase of TLIP exhibit OIB-like trace element patterns and enrichment of LILE and HFSE, and mainly belong to high TiO₂ series. There is an obvious difference in isotope among the basalt from Keping and the basalt and dibase from the northern Tarim Basin. The basalt from Keping with negative ? _Nd and high REE value derives from enriched mantle, and the diabase and basalt from the northern Tarim Basin with positive ? _Nd and low REE value are related to depleted mantle. The crust uplifting in the Early Permian and the development of picrite and large scale dyke and formation of large scale V-Ti-Magnetite deposit in Wajilitag area support the view that the TLIP is related to mantle plume. The TLIP has a temporal-spatial relationship with Permian basic to ultra-basic igneous rock, which is distributed widely in Central Asia, and they represent a tectono-magmatic event with very important geodynamic setting. This paper also suggests that the deep geological process, the relation with mantle plume, mineralization, the relation with environmental change and biological evolution, and the geodynamics of the TLIP will be the key research topics in the future.     

4D Arctic: A Glimpse into the Structure and Evolution of the Arctic in the Light of New Geophysical Maps,Plate Tectonics and Tomographic Models

Knowledge about the Arctic tectonic structure has changed in the last decade as a large number of new datasets have been collected and systematized. Here, we review the most updated, publicly available Circum-Arctic digital compilations of magnetic and gravity data together with new models of the Arctic’s crust. Available tomographic models have also been scrutinized and evaluated for their potential to reveal the deeper structure of the Arctic region. Although the age and opening mechanisms of the Amerasia Basin are still difficult to establish in detail, interpreted subducted slabs that reside in the High Arctic’s lower mantle point to one or two episodes of subduction that consumed crust of possibly Late Cretaceous–Jurassic age. The origin of major igneous activity during the Cretaceous in the central Arctic (the Alpha–Mendeleev Ridge) and in the proximity of rifted margins (the so-called High Arctic Large Igneous Province—HALIP) is still debated. Models of global plate circuits and the connection with the deep mantle are used here to re-evaluate a possible link between Arctic volcanism and mantle plumes.     

United States Quaternary coastal sequences and molluscan racemization geochronology – What have they meant for each other over the past 45 years?

The field of amino acid racemization (AAR) geochronology had its beginnings in the 1960's with the analysis of Quaternary mollusks of known relative age from United States Atlantic coast sites. Subsequent AAR studies of sites from Florida, California and then the entire western and eastern coasts of the U.S. have documented two important concepts: 1) in both uplifted marine terrace and subsurface sections, it can be demonstrated that D/L values increase with increasing geologic age; 2) D/L values in samples of equal age increase with decreasing latitude (increasing temperature). These two north–south coastlines with broad latitude ranges (Pacific sites span more than 20°, Atlantic sites more than 15°) provide an ideal, rare framework to test these principles, and the contrast in thermal histories of these two coasts (one maritime, the other more continental) adds additional insights into issues of aminozone correlation over latitude ranges as small as 2°. The trend of D/L values vs. latitude (isochrons) is established using calibrations based primarily on U–Th coral dating, the best of these calibrated trends seen for multiple sites on the Pacific coast. Pacific coast isochrons follow smooth trends and have been used to create and evaluate kinetic models using the modern latitudinal temperature gradient for comparison. Atlantic coast isochrons are more difficult to reconcile with the modern temperature gradient and available U–Th coral ages, suggesting either complex effective temperatures or unknown diagenetic effects on the AAR results. In addition to these studies that used local or regional field studies to test and evaluate AAR methods, relative or numerical ages based on AAR studies on both coasts have contributed to research on coastal uplift or subsidence rates, coastal stratigraphy and sea-level history, age mixing, and diagenesis of carbonate fossils.     

Helium and argon isotopes of the Tertiary basic igneous rocks from Shandong Peninsula and implications for the magma origin

Helium (He) and Argon (Ar) isotopic compositions of the Tertiary basic igneous rocks were determined by the high temperature melting extraction method. The selected samples for the studies included al-kaline basalts and diabases from the Jiyang basin,and the surrounding Shanwang and Qixia outcrops in the Shandong Peninsula,eastern China. The results show that the Paleogene basalts and diabases from the Jiyang basin yielded a wide range of P4 PHe abundance of (73.70-804.16)×10 P-8 Pcm P3 P STP·g P-1 P,with P3 PHe/ P4 PHe ratios of 0.374-2.959 Ra,which was lower than the MORB but evidently higher than the con-tinental crust value. The Neogene alkaline basalts from the Jiyang basin,Shanwang and Qixia outcrops have variable P4 PHe abundances ((42.34-286.72)×10-8 Pcm P3 P STP·g-1 P),and "continental crust-like" P3 PHe/ P4 PHe ratios (0.013-0.074 Ra). All of them contain atmospheric-like P40 PAr/ P36 PAr ratio (395.4-1312.7),reflecting the mantle sources with air components. Their low P3 PHe/ P4 PHe ratios are interpreted as the enrichment of the radiogenic P4 PHe mainly inherited from the mantle. He and Ar systematics show the mixing of MORB-type,air and a P4 PHe enriched member in the mantle source,suggesting that these igneous rocks originated from the depleted asthenospheric mantle mixed with an EMI component. Therefore,the present He and Ar isotopes do not support the viewpoints that the Cenozoic igneous rocks of Eastern North China were the products of mantle plume(s) activities.     

Possible effects of lateral viscosity variations induced by plate-tectonic mechanism on geoid inferred from numerical models of mantle convection

In a traditional analytical method, the convective features of Earth’s mantle have been inferred from surface signatures obtained by the geodynamic model only with depth-dependent viscosity structure. The moving and subducting plates, however, bring lateral viscosity variations in the mantle. To clarify the effects of lateral viscosity variations caused by the plate-tectonic mechanism, I have first studied systematically instantaneous dynamic flow calculations using new density-viscosity models only with vertical viscosity variations in a three-dimensional spherical shell. I find that the geoid high arises over subduction zones only when the vertical viscosity contrast between the upper mantle and the lower mantle is O(10³) to O(10⁴), which seems to be much larger than the viscosity contrast suggested by other studies. I next show that this discrepancy may be removed when I consider the lateral viscosity variation caused by the plate-tectonic mechanism using two-dimensional numerical models of mantle convection with self-consistently moving and subducting plates, and suggest that the observed geoid anomaly on the Earth’s surface is significantly affected by plate-tectonic mechanism as a first-order effect.     

Petrogenetic model of the Permian Tarim Large Igneous Province

Over the last two decades great strides have been made in characterizing the spatial distribution, time sequence,geochemical characteristics, mantle sources, and magma evolution processes for various igneous rocks in the Early Permian Tarim Large Igneous Province(TLIP). This work has laid a solid foundation for revealing the evolutionary processes and genetic models of large igneous provinces(LIPs). This study systematically demonstrates the two-stage melting model for the TLIP based on our previous research work and predecessor achievements, and highlights the two types of magmatic rocks within the TLIP.The two-stage melting model suggests that the formation of the TLIP is mantle plume related. The early hot mantle plume caused the low-degree partial melting of the lithosphere mantle, while in the later stage, the plume partially melted due to adiabatic uplift and decompression. Therefore, this model carries signatures of both the "Parana" and "Deccan" models in terms of mantle plume activity. During the early stage, the mantle plume provided the heat required for partial melting of sub-continental lithosphere mantle(SCLM), similar to the "Parana Model", while later the plume acted as the main avenue for melting, as in the "Deccan Model". Basalts that erupted in the first stage have higher 87Sr/86 Sr, lower 143Nd/144 Nd ratios, and are enriched in large ion lithophile elements and high field strength elements, indicating a possible origin from the enriched continental lithosphere mantle,similar to the Parana type geochemical features. The basic-ultrabasic intrusive rocks in the second stage exhibit lower 87Sr/86 Sr,higher 143Nd/144 Nd ratios relative to the basalts, consistent with the involvement of a more depleted asthenospheric material,such as a mantle plume, similar to the Deccan type geochemical features. The first stage basalts can be further subdivided into two categories, i.e., Group 1 and Group 2 basalts. Group 2 basalts have lower 87Sr/86 Sr and higher 143Nd/144 Nd ratios than Group 1 basalts, and lie between compositions of the Group 1 basalts and second stage magmatism. Group 2 basalts may be the intermediate component of the TLIP, and the whole TLIP is the result of plume and lithosphere interaction. Developing this petrogenetic model for the TLIP aids in comprehensively understanding its magmatism and deep geological and geodynamic processes. Furthermore, this work enriches the theories describing the origin of large igneous province and mantle plume activity.     

Submerged notches and doline sediments as evidence for Holocene subsidence

The possibility of Holocene subsidence along the northern coast of the Corinth Gulf is often mentioned in the literature; however, systematic detailed evidence that submergence (e.g. of archaeological remains) does not simply depend from eustatic sea-level rise is most often missing. In this paper, a new detailed study of submerged tidal-notch profiles along the limestone coast has shown that periods of sea-level stability are intercalated with periods of rapid subsidence or gradual relative sea-level rise. It appears that most of the sites considered, seem to have been affected by a relatively recent co-seismic subsidence of about half a meter, whereas during the longer period, by stages of relative sea-level stability and/or gradual relative sea-level rise. This evidence of subsidence is confirmed by radiocarbon dating in doline sediments, suggesting that during certain periods, a relative sea-level rise was much faster than the raising suggested by glacio-eustatic or hydro-isostatic estimations. Juxtaposing a list of known earthquakes occurred in the area shows that several earthquakes (e.g. the 1981 one for the easternmost sites considered) are potential candidates for the recent co-seismic displacements and thus supporting the geomorphological interpretations.     

Numerical modeling of mantle diapirism as a cause of intracontinental rifting

The numerical model of mantle diapirism and active rifting is developed. The model describes the possibility of extension and thinning of the Earth’s crust under the action of a local 100-km long heat source in the sublithospheric mantle, which causes melting and rising of the magmatic diapir through the cratonic lithosphere. The model combines the mechanisms of the uplifting of the anomalously hot material due to its gravitational instability, underplating of magma beneath the continental crust, and its extension by the forces of the convective flows at the base of the plate. The obtained results shed light on some geological features of the joint formation of the large Vilyui igneous province and Vilyui sedimentary basin.     

Origin of the Deccan Trap flows at Mahabaleshwar inferred from Nd and Sr isotopic and chemical evidence

The Deccan flows at Mahabaleshwar are divisible into a lower and an upper group, based on Nd and Sr isotopic ratios, which define two correlated trends. This distinction is supported by incompatible element ratios and bulk compositions. The data reflect contamination in a dynamic system of magmas from an LIL-depleted,ε_JUV ≥ +8 mantle by two different negative ε_JUV endmembers, one undoubtedly continental crust, the other either continental crust or enriched mantle. The depleted mantle source, anomalously high in (⁸⁷Sr/⁸⁶Sr), may have been in the subcontinental lithosphere or a region of rising Indian Ocean MORB mantle.     

The geoid and single-cell mantle convection

The geoid shows an antisymmetric departure from the spheroid of best fit. A single zero-elevation contour divides its surface into nearly equal strips in one of which the elevation is everywhere positive and in the other everywhere negative. These two areas are interleaved roughly like the strips covering a tennis ball. This pattern may indicate global single-cell convection in the mantle. It is argued that on this convection hypothesis, the upcurrents underlie the low-geoid strip, although the opposite view could be supported. No simple relation is to be expected between the proposed whole-mantle convection and plate motions, because other constraints act on plates and because the asthenosphere will partially decouple the whole-mantle motions from the lithosphere. However, the proposed whole-mantle convective system is consistent with rapid northwestward motion of the Pacific plate, with fast spreading of the East Pacific Rise and with slow spreading of the North Atlantic Ridge. Seismological velocity anomalies in the mantle, while highly relevant to whole-mantle convection, do not at present decide for or against the hypothesis here advanced.