Pattern Of Mantle Convection Research Articles

The influence of boundary heterogeneity in experimental models of mantle convection with internal heat sources

The lateral variations in the thickness of the D″ layer observed by recent global seismology are suspected to control mantle dynamics. Namiki and Kurita (1999) performed laboratory experiments to explore how undulations of various sizes in the D″ layer affect convection patterns. In addition, we found that the undulation at the lower boundary can induce plumes and that there is a critical height above which the undulation controls convection patterns. Observed undulations at the top of the D″ layer exceed this critical height, suggesting that they may control the mantle convection patterns. The previous work, however, assumed basal heating. Since internal heating is an important source which drives mantle convection, we extend our previous study to a case incorporating internal heat sources using laboratory experiments. Internal heat generation is simulated by lowering the boundary temperatures at a constant rate. We observed that the undulation, which has the comparable thickness to the critical height determined in the purely basal heating case, can fix the location of the upwelling although the magnitude of the intensity of the upwelling is weaker. This situation is similar to the Earth’s hotspots and suggests that the undulation at the top of the D″ layer is the source region of the hotspots.

The influence of tectonic plates on mantle convection patterns, temperature and heat flow

SUMMARY The dynamic coupling between plate motion and mantle convection is investigated in a suite of Cartesian models by systematically varying aspect ratios and plate geometries. The aim of the study presented here is to determine to what extent plates affect mantle flow patterns, temperature and surface heat flux. To this end, we compare numerical convection models with free-slip boundary conditions to models that incorporate between one and six plates, where the geometries of the plates remain fixed while the plate velocities evolve dynamically with the flow. We also vary the widths of the plates and the computational domain in order to determine what constraint these parameters place on the mean temperature, heat flux and plate velocity of mantle convection models. We have investigated the influence of plates for three whole-mantle convection cases that differ in their heating modes (internally heated and basally heated) and rheologies (isoviscous and depth-dependent viscosity). We present a systematic investigation of over 30 models that exhibit increasingly complex behaviour in order to understand highly time-dependent systems using the insight gained from simpler models. In models with aspect ratios from 0.5 to 12 we find that for the same heating mode, variations in temperature can be as much as 40 per cent when comparing calculations with unit-width plates to models incorporating plates with widths equal to five times the model depth. Mean surface heat flux may decrease by 60 per cent over the same range of plate widths. We also find that internally heated mantle convection models incorporating plates exhibit novel behaviour that, we believe, has not been described previously in mantle convection studies. Specifically, in internally heated models, plate motion is characterized by episodic reversals in direction driven by changes in the mantle circulation from clockwise to counterclockwise and vice versa. These flow reversals occur in internally heated convection and are caused by a build-up of heat in the interiors of wide convection cells close to mantle downwellings. We find that flow reversals occur rapidly and are present in both single-plate and multiple-plate models that include internal heating. This behaviour offers a possible explanation for why the Pacific plate suddenly changed its direction some 43 Ma.

Quest for dynamic topography: Observations from Southeast Asia

Research Article| November 01, 2000 Quest for dynamic topography: Observations from Southeast Asia Paul Wheeler; Paul Wheeler 1Bullard Laboratories, Madingley Rise, Madingley Road, Cambridge CB3 0EZ, UK Search for other works by this author on: GSW Google Scholar Nicky White Nicky White 1Bullard Laboratories, Madingley Rise, Madingley Road, Cambridge CB3 0EZ, UK Search for other works by this author on: GSW Google Scholar Geology (2000) 28 (11): 963–966. https://doi.org/10.1130/0091-7613(2000)28<963:QFDTOF>2.0.CO;2 Article history received: 03 Apr 2000 rev-recd: 17 Jul 2000 accepted: 06 Aug 2000 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Paul Wheeler, Nicky White; Quest for dynamic topography: Observations from Southeast Asia. Geology 2000;; 28 (11): 963–966. doi: https://doi.org/10.1130/0091-7613(2000)28<963:QFDTOF>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Transient dynamic topography is maintained by time-varying density anomalies deep within Earth's mantle. There is considerable interest in predicting and measuring dynamic topography because its spatial and temporal variation will help to track changes in the pattern of mantle convection. Despite the availability of increasingly elaborate predictive models, there is little agreement about the amplitude, wavelength, and rate of change of dynamic topography. Here we analyze a hybrid data set of subsidence observations from continental and oceanic basins in Southeast Asia, where published dynamic models predict 1–2 km of regional subsidence. Residual subsidence was isolated by removing the well-defined effects of lithospheric extension and/or cooling. At long wavelengths (∼103 km), the amplitude of residual subsidence (i.e., maximum permissible dynamic subsidence) has an upper bound of only ∼300 m. The spatial distribution of permissible dynamic topography has no simple spatial or temporal relationship to the pattern of subduction over the past 65 m.y.; anomalous subsidence commenced 5–10 m.y. ago, accelerating to a present-day rate of ∼100 m/m.y. Our results show that density anomalies within the lower mantle have little apparent influence at Earth's surface. Instead, the distribution of residual subsidence reported here can be accounted for by minor variations in lithospheric thickness or by transient density variations beneath the lithospheric plate. The absence of measurable dynamic topography is surprising and indicates that mantle mass anomalies are supported elsewhere, presumably at internal boundaries within Earth. Our hybrid data set can thus be used as an independent constraint on the viscosity structure of the mantle. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The influence of boundary heterogeneity in experimental models of mantle convection

Recent global seismological observations have revealed lateral variations in the thickness of the D″ layer. We have performed laboratory experiments to explore how undulations of various sizes in the D″ layer affect convection patterns. We find that topography on the lower boundary can induce plumes and that there is a critical height above which topography controls convection patterns. Observed undulations in the D″ layer exceed this critical height, suggesting that they may control mantle convection patterns.

Three-Dimensional Mantle Convection With Continental Crust: First-Generation Numerical Simulations

The potential effects of continental crust on mantle convection are explored using three-dimensional numerical simulations. The simulations model the coupling between a thin, deformable layer of chemically buoyant crust and a deep layer of thermally convecting and chemically dense mantle. Simulations begin with a crustal layer embedded within the upper thermal boundary layer of a mantle convection roll in a 1 × 1 × 1 domain. Convective stresses cause crust to thicken above a sheetlike mantle downwelling. For mild convective vigor an initial crustal thickness variation is required to induce three-dimensional mantle instability below the zone of thickening. Instability is characterized by the formation of local driplike thermals that exist within the large-scale thermal roll associated with crustal thickening. In terms of surface effects the drips cause significant deviations from local Airy compensation. After initial thickening the crustal accumulation that forms serves as an analog to a continent. Its presence leads to mantle flow patterns different from the thermal roll that occurs in its absence. Large lateral thermal gradients are generated at the edge of a crustal accumulation, which, as a result, becomes the initiation site for small-scale thermals that circulate below a model continent. Eventually, these instabilities induce a restructuring of large-scale mantle flow morphology from a roll to a square cell. Although preliminary, the simulations support the idea that crustal thickening of the type associated with mountain belt formation can alter local mantle flow patterns that can affect subsequent crustal deformation. Thus problems of continental dynamics and local mantle dynamics may be more strongly coupled than is generally presumed. The ability of crustal accumulations to cause a change in large-scale convective planform also suggests that continental crust may have low-order effects on global mantle convection patterns despite its seemingly trivial relative volume.

Transitions in the style of mantle convection at high Rayleigh numbers

The pattern and style of mantle convection govern the thermal evolution, internal dynamics, and large-scale surface deformation of the terrestrial planets. In order to characterize the nature of heat transport and convective behaviour at Rayleigh numbers, Ra, appropriate for planetary mantles (between 10 4 and 10 8), we perform a set of laboratory experiments. Convection is driven by a temperature gradient imposed between two rigid surfaces, and there is no internal heating. As the Rayleigh number is increased, two transitions in convective behaviour occur. First we observe a change from steady to time-dependent convection at Ra≈10 5. A second transition occurs at higher Rayleigh numbers, Ra≈5×10 6, with large-scale time-dependent flow being replaced by isolated rising and sinking plumes. Corresponding to the latter transition, the exponent β in the power law relating the Nusselt number Nu to the Rayleigh number (Nu∼Ra β ) is reduced. Both rising and sinking plumes always consist of plume heads followed by tails. There is no characteristic frequency for the formation of plumes.

Slicing into the earth

Regional arrays of seismometers provide a powerful means of mapping the details of deep‐Earth structure. Our understanding of the geological processes at work within our planet depends on our ability to examine them; seismic techniques remain the best tool available. However, spatial aliasing due to the less‐than‐optimal distribution of global seismometers has long made it difficult to determine deep‐Earth structure from teleseismic waves. The temporary deployment of portable broadband seismometers can help by providing high‐resolution windows into the Earth. Patterns of global mantle convection create seismically observable features such as anisotropy at the top and bottom of the mantle, topography of upper mantle discontinuities, and heterogeneous structure at the core‐mantle boundary.

Implications of a metal‐bearing chemical boundary layer in D″ for mantle dynamics

At lower mantle conditions, the thermal conductivity of non‐metallic crystals is an order of magnitude lower than that of dense metallic minerals such as FeO, FeSi and Fe, which may be present in a chemical boundary layer at the base of the mantle. Because the core‐mantle boundary is nearly isothermal, variations in the thickness of a metal‐bearing layer induce lateral temperature variations of several hundred kelvin, which in turn affect the pattern of mantle convection. Upwellings should occur where the layer is thickest; the resulting stability of the metal + silicate layer with respect to the flow may also stabilize the pattern of mantle convection, and reduce its time‐dependence.

Upper mantle flow beneath the Northwest of China and its lithospheric dynamics

The regional isostatic gravity anomalies in the Northwest of China (80°–95°E, 35°–45°N) and corresponding problems of mantle dynamics are discussed in detail in this paper. It is assumed that the anomaly of isostatic gravity in this region is caused by the nonuniformity in density and that the boundary deformation is related to the thermal convection in the upper mantle. Using the correlation equation between the mantle flow and regional anomalies of isostatic gravity as a constraint, we have calculated the patterns of upper mantle convection. The result shows that the regional lithospheric tectonics is strongly correlated with the upper mantle flow. The uplift of the Tianshan and Kunlun Mountains corresponds to the upward flows in the mantle while the depression of the Tarim, Turpan and Junggar Basins corresponds to the downward flow. Meanwhile, we have also worked out a basic framework for the lithospheric dynamics of the Northwest of China. It is reasonable to infer that the mantle flow and the horizontal compression from the India plate may act as the main driving forces for the lithospheric tectonics of this region.

Increased mantle convection during the mid‐Cretaceous: A comparative study of mantle potential temperature

Mantle convection patterns of the past are not well known, yet an understanding of changing mantle convection characteristics is fundamental to understanding the evolution of plate tectonics. There are very few ways to examine mantle characteristics of the past. Changes in spreading rate and volcanic activity with time have been used to draw conclusions about historic changes in mantle activity. Mantle temperature has been found to be related to crustal thickness. With this relationship, crustal thicknesses may now yield new conclusions about historic changes in mantle characteristics. We have inferred changes in mantle convection patterns throughout the last 180 m.y. by examining variations in assumed crustal thickness within the Pacific basin. Crustal thicknesses were calculated from residual depth anomalies by assuming that residual depth anomalies are the result of isostatic compensation of variations in crustal thickness. Crustal thickness is determined at the time of crustal formation and is dependent upon the temperature of the mantle source material. Intraplate hot spot volcanism effects on crustal thickness were not ignored. Examination of variations in crustal thickness of crust of different ages can reveal information about changing temperatures of the mantle at the ridge through time. We have learned that mantle temperatures at the ridge during the mid‐Cretaceous were more variable than those temperatures at the ridge after the mid‐Cretaceous. Furthermore, we have inferred from the data that mantle temperatures at hot spots were higher during the mid‐Cretaceous than those at hot spots existing after the mid‐Cretaceous. We suggest that mantle convection at the ridge was more rapid during the mid‐Cretaceous causing a higher variability of temperatures at the ridge. We also note that this period of increased mantle convection is concurrent with the increased mantle temperatures at hot spots within the Pacific basin.

Surge-tectonic evolution of southeastern Asia: a geohydrodynamics approach

The repeated need for ad hoc modifications in plate-tectonic models to explain the evolution of southeastern Asia reveals their inability to fully explain the complex features and dynamics of this region. As one example, the hypothesis does not provide a mechanism to explain the 180° turns and twists along the strike of several foldbelts and island arcs in the region (e.g. Banda arc). Convection-cell configuration renders such 180° contortions and Rayleigh-Bénard-type convection impossible. However, during the last 10 years, new data bearing on the convection-cell problem have become available in the form of seismotomographic images of the earth's interior. These images show that (i) mantle diapirs as proposed by traditional plate-tectonic models do not exist; (ii) there is no discernible pattern of upper or lower mantle convection, and thus no longer an adequate mechanism to move plates; and (iii) the lithosphere above a depth of about 80 km is permeated by an interconnected network of low-velocity channels. Seismic-reflection studies of the low-velocity channels discovered on the seismotomographic images reveal that these channels have walls with a 7.1–7.8 km s −1 P-wave velocity. Commonly, the interiors of the channels are acoustically transparent, with much slower P-wave velocities, in places as low as 5.4 km s −1. The author and co-workers have interpreted the low velocities as evidence for the presence of partial melt in the channels, and they postulated that this melt moves preferentially eastward as a result of the earth's rotation. They named these channels “surge channels” and their new hypothesis for earth dynamics “surge tectonics”. Surge channels underlie every type of tectonic belt, which includes mid-ocean ridges, aseismic ridges, continental rifts, strike-slip fracture zones, and foldbelts. In southeastern Asia, surge channels—mainly foldbelts—lie between all platform and cratonic massifs. These massifs, platforms, and tectonics belts—the surge channels—form an anastomosing E-W pattern southern Asiatic Russia, Mongolia, western China, the Qinghai-Tibetan region, and northern India and Pakistan. Such an anastomosing pattern indicates that flow is an active process in the surge channels. Surface studies of phenomena that might be associated with the surge channels soon revealed that all active channels are characterized by higher-than-normal heat flow (> 55 mW m −2, thermal springs and elevated ground-water temperatures, volvanic phenomena, bands of microearthquakes, and linear belts of faults, fractures, and fissures. The latter are especially visible on satellite images. The bands of high heat flow, thermal springs, microearthquakes, and faults-fractures-fissures almost exactly coincide. The fault-fracture-fissure systems are interpreted to be streamlines caused by flow in the surge channels—a consequence of Stokes's Law (an expression of Newton's Second Law of Motion)-and show that Poiseuille flow must dominate in the channels. Hence, the mechanism producing the belts of linear faults-fractures-fissures is viscous drag, produced by fluid motions. The eastward flow of the magma in the channels is demonstrated clearly in the tectonic patterns of southeastern Asia. In the northern part of the region studied, the E-W striking anastomosing surge channels (tectonic belts) splay northeastward into the coastal regions of Russia. In the south, they splay southward and southeastward through the Malay Peninsula and Indonesia. The open horsetail structures thus created prove that flow is W-E. The presence of the two splay directions, NE and S-SE, indicates in addition that a barrier to eastward flow must lie directly east of Asia. In this author's opinion, this barrier is the existing Benioff zone, because the same NE and S-SE splay patterns are present on each of the paleotectonic maps that have been prepared for nine time intervals from the beginning of Sinian (latest Proterozoic) time to the present. The presence of the W-E flow patterns through 850 Ma of geological time, patterns that remain essentially unchanged, means simply that tectonic explanations of Asian geology need revision. The patterns that have been mapped indicate that W-E flow across Asia has persisted essentially unchanged for 850 Ma. Surge tectonics is the only hypothesis yet proposed that explains these patterns and their persistence.

Recent deformation rates on Venus

Constraints on the recent geological evolution of Venus may be provided by quantitative estimates of the rates of the principal resurfacing processes, volcanism and tectonism. This paper focuses on the latter, using impact craters as strain indicators. The total postimpact tectonic strain lies in the range 0.5–6.5%, which defines a recent mean strain rate of 10−18–10−17 s−1 when divided by the mean surface age. Interpretation of the cratering record as one of pure production requires a decline in resurfacing rates at about 500 Ma (catastrophic resurfacing model). If distributed tectonic resurfacing contributed strongly before that time, as suggested by the widespread occurrence of tessera as inliers, the mean global strain rate must have been at least ∼10−155 S−1, which is also typical of terrestrial active margins. Numerical calculations of the response of the lithosphere to inferred mantle convective forces were performed to test the hypothesis that a decrease in surface strain rate by at least two orders of magnitude could be caused by a steady decline in heat flow over the last billion years. Parameterized convection models predict that the mean global thermal gradient decreases by only about 5 K/km over this time; even with the exponential dependence of viscosity upon temperature, the surface strain rate drops by little more than one order of magnitude. Strongly unsteady cooling and very low thermal gradients today are necessary to satisfy the catastrophic model. An alternative, uniformitarian resurfacing hypothesis holds that Venus is resurfaced in quasi‐random “patches” several hundred kilometers in size that occur in response to changing mantle convection patterns. Under such a model, the observed crater strain distribution indicates that about 1% of the planet's surface is tectonically active at any time. However, this model requires a very weak crustal rheology to achieve surface velocities ∼100 mm/yr appropriate to the required “patch” size. Without well‐developed lateral weak zones, Venus is essentially a one‐plate planet, but one in which the lithosphere is able to respond to topography produced by mantle convection through faulting and limited horizontal movement. The net rate of tectonic activity is logarithmically intermediate between Earth and Mars: about 100 times slower than plate tectonics, but up to 100 times faster than planets where tectonic stress arises largely from lithospheric cooling and contraction.

Asymmetric Phase Effects and Mantle Convection Patterns

Recent high-pressure experiments and thermodynamic calculations have shown that the Clapeyron slope of the spinel-perovskite phase transition at a depth of 660 kilometers in the Earth's mantle changes from negative to positive at temperatures above 1700 degrees to 2000 degrees C. In numerical experiments that account for this phase behavior, cold downwelling flows were impeded at the phase boundary, but hot plumes ascended to the upper mantle with ease. The resultant mantle convection was partially layered and strongly time-dependent. Mantle layering was weaker when the mantle was hotter and when the Rayleigh number was larger.

特集 火山活動と地殻応力場 中央海嶺のマグマ供給システム

Recent progress in studies on magma plubming system beneath mid-ocean ridges is reviewed. Mid-ocean ridges are not continuous, homogeneous series of crests, but are segmented by various topographical offsets of several orders of magnitude. Such topographical ridge segmentation is a surface manifestation of along-strike variations in the pattern of mantle convection and the supply of magma.Upwelling of the mantle material beneath the spreading center takes the form of a diapir, rather than sheet-like flow. Such diapiric upwelling has an along-axis dimension of several hundred to several tens of kilometers, which corresponds to first-and second-order segments, but some are as small as the scale of third-order segments. Where magma upwelling is intense, hot lithosphere and high magmatic productivity produces thick crusts, resulting in low gravity anomaly.When the spreading rate is low, mantle flow is cooled on route to the bottom of the lithosphere and ceases to melt at depths. Thus magmas produced beneath slow spreading ridges have low degrees of melting. On the contrary, fast spreading ridges are followed by intense mantle upwelling which melts significantly the mantle column from deeper part of the upper mantle up to just beneath the crust.Magmatic budget is so large at fast spreading ridges as to maintain large, steady-state magma chambers along the ridge axes with extensions comparable to second-and third-order segments. Such magma chambers have a very small melt pocket one to two kilometers wide and up to several hundred meters thick, underlain by a large mush of crystals and melt. Cumulate layers on the top of the mush sink and deform to S-shaped or downwarping layers as seen in large layered plutonic bodies in some ophiolites. However, low spreading ridges do not have enough supply of magma but only possess short-lived small magma chambers consisting of crystal mush.Volcanic landforms consist of major topographic features associated with small volcanic edifices : the former is a large shieled volcano several tens of kilometers long and is probably formed by episodic eruptions every several tens of thousand years ; the latter is a small conical lava cone comprising pillow flows which is constructed during a short-period eruption but some are active as long as 1.8 million years which are unequivocally central volcanoes.Rifting episodes at spreading axes depend on the spreading rate and the width of the volcanic zone. Slow spreading ridges such as Iceland have rifting every 100 years, but fast spreading ridges such as East Pacific Rise rift every year or two. Such rifting episodes continue for several years and are associated with multiple dikes with an injection interval of a month up to a year.Diapiric mantle upwelling, structure of magma chambers and volcanic landforms seen in mid-ocean ridges resemble those in some large ophiolites such as the Troodos, Cyprus and the Semail, Oman, and post-Teritary volcanism in Iceland.

Effect of depth‐dependent viscosity on the pattern of mantle convection

Three‐dimensional structures of thermal convection are studied by numerical experiments in a rectangular domain of a plane fluid layer. The main question is the joint effect of the heating mode and the depth‐dependent viscosity on the planform of the circulation. Heating mode is defined by the ratio of internal and basal heat inputs. For viscosity‐depth functions, simple models of the mantle viscosity are used. If the Rayleigh number is of the order 105–106 (based on the maximum viscosity), this choice produces cellular flow with a closed network of cold descending sheets around isolated upwellings, independently of the heating mode. The central upwellings are concentrated in narrow cylindrical plumes if there is a significant basal heat input. Convection of the Earth's mantle may occur with descending currents represented by subduction and ascending currents in the form of plumes situated below hotspots.

Pattern of mantle convection and Pangaea break-up, as revealed by the evolution of the African plate

A tensional stress regime has governed the tectonic evolution of most of the African continent. The long history of crustal extension is documented by the widespread occurrence of continued intraplate rifting and volcanism since Triassic times and by the large-scale plate movements following the break-up of Pangaea. A remarkable result of the present investigation is the recognition that the extension displays a continent-wide and plate-wide radial pattern, centred in equatorial Africa near 10°E/0°N, the African Centre A. The radial pattern is evident from the diverging block movements, documented by the intra-continental rifting in Africa, by the break-up of Pangaea and the subsequent, diverging drift of North America and the Gondwana fragments away from Africa, entailing a radial growth of the African plate. All these phenomena are manifestations of what may be referred to as the ‘African lithospheric divergence’. The African plate is surrounded, to 85%, by active oceanic ridges. Ridge-push forces exerted on the African plate have to be balanced by some other forces in order to explain the tensional stress regime which, even at present, dominates large parts of the African continent . Shear traction exerted by the convecting asthenosphere on the base of the African lithosphere offers an explanation for the observed phenomena. It is proposed that relatively warm and less dense mantle material rises from the deep mantle below the African plate. In the upper mantle and at the base of the African lithosphere, the ascending material diverges and flows radially away from the centre of ascent. Upper mantle flow below the sea-floor spreading axes is unidirectional and horizontal and is directed away from the African centre A. It is proposed that the ascending flow beneath Africa forms part of a very large, bicellular circulation system in the Earth's mantle. The origin of geotectonic cycles is probably interrelated with the onset, build-up, main phase and decay of such large-scale convection systems.

Hotspots and sunspots: surface tracers of deep mantle convection in the Earth and Sun

The pattern of new appearances of hotspots on the Earth is investigated using available age data. The worldwide total of Cenozoic and Mesozoic hotspots, predicted by extrapolation from the observed number of continental flood basalts, is ∼ 40, in agreement with Crough's counts. There are found to be no true antipodal pairs. In one interpretation of the data, new appearances of hotspots occur first at high latitudes in both hemispheres and then migrate toward the equator; shortly before the migration cycle is finished, the next cycle begins. A relative deficiency of hotspots, however, is observed in very low equatorial and very high polar regions. In addition, hotspots occupy two large hemispherical groups in longitude that slowly drift in the same direction as the axial body rotation. An observed magnetic superchron seems to end when a new hotspot migration cycle in latitude begins; but consecutive superchrons show opposite polarity. Although the hotspot data are rather sparse, their suggested patterns resemble the well-known patterns of complexes of active regions on the Sun, as exemplified by proxy data, such as sunspots. Both the Earth and the Sun possess large convecting, rotating and magnetized mantles, in which the characteristic surface tracers are believed to reflect the patterns of convective phenomena very deep in the mantle (even though the tracers themselves—hotspots and sunspots—are certainly not analogs of each other). The present study supports existing theoretical ideas that the large-scale patterns of deep mantle convection in the Earth and Sun may fundamentally resemble each other, despite the enormous difference in their molecular viscosities. Magnetic polarity reversal histories also show close similarities in the two bodies, suggesting the operation of basically similar convective dynamos. However, the Sun has displayed no known analog of the Earth's magnetically disturbed intervals.

Rifting of Africa and pattern of mantle convection beneath the African plate

Abstract The Mesozoic and Cenozoic tectonic setting of the African continent is characterized by an extensional stress regime, as is evident from the evolution of the West, Central and East African rift systems, the opening of the Indian and Atlantic oceans and the development of Tethys domain, as well as from the results of geophysical investigations. The most prominent feature of the Bouguer gravity anomaly map of Africa is a great belt of negative Bouguer anomalies which extends across the entire African continent from Ethiopia in the East to Benguela in the West. In conjunction with the Red Sea spreading zone this belt appears to encircle a “centre” in Cameroon. It probably corresponds to a distinct zone of lithospheric thinning and extension. The entire African continent is characterized by higher-than-normal elevations. It is located inside the African residual geoid high. Earthquake fault-plane solutions confirm the extensional character of the present tectonic regime of eastern and southern Africa where normal faulting predominates. The average orientation of T axes is N-S in South Africa, NW-SE in Zambia, Mozambique and Madagascar, E-W in Kenya and Ethiopia, and NE-SW in the Red Sea region. From north to south, the orientation of T axes rotates systematically in a clockwise sense by 130° and describes a radial pattern centred on equatorial West Africa. This indicates that the African lithosphere has been subject to continent-wide radial extension around a hypothetical West African “spreading centre”, referred to as the African Spreading Centre A (ASC-A), which coincides with the geographical centre of the African plate. Shear traction exerted by the convecting mantle on the base of the lithosphere is considered to be a major driving force of plate movements. In the case of Africa, the continent-wide regularities in the pattern of lithospheric extension, the elevated position of the continent, as well as the evolution of the mid-oceanic ridge system surrounding Africa, provide distinct boundary conditions for the pattern of large-scale mantle flow beneath the African plate. It is proposed that relatively warm and less dense mantle material rises below Africa, forming a single mega-plume or a number of plumes. At the base of the African lithosphere, the ascending material diverges and flows radially away from the centre of ascent, corresponding to the ASC-A. This ascending flow forms part of a very large mantle convection cell which occupies the Eurafrican hemisphere. The descending branch of this convection cell is located at a distance of 80–90° from the ASC-A. The mid-oceanic ridges surrounding the African continent are located between the upwelling and the downwelling branches of this convection cell, at a distance of 50–60° from the ASC-A. Upper mantle flow below the sea-floor spreading axes is uni-directional and horizontal and is directed away from the ASC-A.

On the scale of mantle heterogeneity

A data set comprising 3313 residuals measured from digital records of 429 earthquakes is analyzed for the heterogeneity of the Earth's mantle. The smoothed SS residuals reveal a large pattern of heterogeneity and agree well with the major tectonic features on the Earth's surface and the residual patterns predicted by upper mantle and whole mantle models. The spectrum of SS residuals indicates that the heterogeneity in the Earth's mantle, which probably reflects the patterns of mantle convection, is dominated by long-wavelength features.Our analysis of the power of SS residuals as a function of harmonic degree shows a distinct difference in the rate of decrease of the power below and above degree 8. The rate is first slower than l−1, and then it exceeds l−2. This is also true for SS residuals predicted for model SH425.2 and Love wave phase velocities at 100 s measured by other researchers. This indicates that there is a preference for features with dimensions larger than 2500–3500 km.We find that the power spectrum of the velocity anomalies associated with the subducted slabs is two orders of magnitude lower for 1 ≤ l ≤ 8 than that of velocity anomalies obtained by inversion of waveform data.For the ocean basins, correlation of the SS residuals with the square root of age of the oceanic lithosphere is highly significant. However, the age correlation predicts only 10% of the variance. This means that the thermal signature of the spreading center represents only a small fraction of the thermal anomalies introduced into the upper mantle by convection.

Geological correlations with the interior density structure of Venus

A model has been constructed by stipulating that Venusian long‐wavelength topography and geoid elevations are explained by the sum of density anomalies associated with mantle convection and lithospheric compensation mechanisms. For convenience the latter are represented by a single Airy surface because individual lithospheric components cannot be resolved at the wavelengths considered in this paper. Mantle convection is represented at a single depth by mass perturbations that drive viscous flow. We invert spherical harmonic representations of the topography and the geoid to create global representations of the mantle convection pattern and near‐surface density anomalies. The mantle convection pattern matches well with previous numerical simulations of mantle convection that show a pattern of isolated upwellings amidst an interconnected network of downwellings. The near‐surface density anomalies show no particular pattern. The inversion results have been compared with a global synthesis of Magellan data. Impact crater density does not correlate with the mantle convection pattern or the near‐surface density anomalies. Large volcanic structures are associated with upwellings. Planitiae are associated with downwellings. Coronae are anticorrelated with high‐amplitude upwellings and downwellings and apparently are passive features associated with rifting that is related to mantle convection in a complex fashion. Tesserae are associated with negative lithospheric density anomalies interpreted as areas of thickened crust. Tesserae show no definitive relationship to the present mantle convection pattern.