Geoid Highs Research Articles

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.

Constraints on Mantle Viscosity From Slab Dynamics

AbstractThe radial viscosity of the mantle is generally thought to increase by ∼10–100 times from the upper to lower mantle with a putative, abrupt increase at 660 km depth. Recently, a low viscosity channel (LVC) between 660 and 1,000 km has been suggested. We conduct a series of time‐dependent flow models with viscosity either increasing or decreasing at 660 km depth while tracking slab structure, state‐of‐stress, and geoid. We find that a LVC will lower the amplitude of long wavelength (>5,000 km) geoid highs over slabs, with amplitudes <10 m in height, while increasing the slab dip angle and downdip tension in the upper 300 km of slabs. A viscosity increase at 660 km gives rise to strong downdip compression throughout a slab and this pattern will largely go away with the introduction of the LVC. In addition, the endothermic phase change at 660 km depth can substantially affect the stress distribution within slabs but has a minor influence on the geoid. Models that fit the observed long wavelength geoid and observed focal mechanism in the western Pacific favor models without the presence of the LVC between 660 km and 1,000 km depths.

Geoid low and highs of the Indian Ocean and Western Pacific: Implications to mantle convection

As geoid anomalies have capacity to unravel deep-seated density structures, it has acquired special significance in recent years for this purpose. Geoid anomalies of India and SE Asia are characterized by the Indian Ocean geoid low and geoid highs of the Western Pacific. The spectral analysis of the Indian Ocean geoid low (IOGL) suggests depths of ∼1300, ∼700 and ∼340km for relatively large wavelength components. They are related to middle mantle and transition zones in the upper mantle, respectively where olivine changes to spinel with sharp increase in the velocity and density. This geoid low is caused by the subducted Indian lithosphere that rolled back after subduction and got accumulated in this section. In this process, the subducting slab would acquire same temperature as surroundings and would be characterized by low velocity and density as it primarily consists of the low density rocks of continental lithosphere. The dehydration of subducting slab of different temperatures at different levels would set in convection cells. In this case, mid-to-upper mantle circulation (convection) is envisaged that has caused Central Indian Ocean Deformation Zone with large scale faults, folds, high heat flow and seismic activity. Presence of such convection at the tail end of the Indian plate drives it relatively faster (∼5cm/year). The IOGL may mark the site of first collision between the Indian and the Asian plates.The geoid highs of the Western Pacific coincide with the subduction zones in this sector. Its spectral analysis provided average depths to the large wavelength sources as ∼2335, ∼845 and ∼523km. They are attributed to sources above D″ layer, and the transition zones in the upper-middle mantle within the error limit of 10–15% depending on the wavelengths. Deeper level of sources for the geoid highs of Pacific Ocean may be attributed to high angle of subduction in this sector where cold subducting slab of high velocity and density can get easily detached and sink into the lower mantle. In this case due to high angle of subduction of oceanic lithosphere, the subducting slab would be at lower temperature compared to surroundings maintaining it brittle even in the transition zone that has given rise to deep focus earthquakes almost up to 700km. The high angle subduction along the subduction zones in the Western Pacific causes faster subduction that drives the Pacific plate faster (∼4–10cm/year). In this case, the dehydration of the subducting slab would set in whole mantle convection between the lower and the upper mantle in the back arc regions of the mantle wedge. It has plausibly given rise to extensional regime forming back arc basins on large scale such as Japan Sea, Philippine Sea, Lau basin and others.

Geodynamics of NW India: Subduction, Lithospheric Flexure, Ridges and Seismicity

Abstract Spectral analysis of digital data of the Bouguer anomaly map of NW India suggests maximum depth of causative sources as 134 km that represents the regional field and coincides with the upwarped lithosphere – asthenosphere boundary as inferred from seismic tomography. This upwarping of the Indian plate in this section is related to the lithospheric flexure due to its down thrusting along the Himalayan front. The other causative layers are located at depths of 33, 17, and 6 km indicating depth to the sources along the Moho, lower crust and the basement under Ganga foredeep, the former two also appear to be upwarped as crustal bulge with respect to their depths in adjoining sections. The gravity and the geoid anomaly maps of the NW India provide two specific trends, NW-SE and NE-SW oriented highs due to the lithospheric flexure along the NW Himalayan fold belt in the north and the Western fold belt (Kirthar – Sulaiman ranges, Pakistan) and the Aravalli Delhi Fold Belt (ADFB) in the west, respectively. The lithospheric flexures also manifest them self as crustal bulge and shallow basement ridges such as Delhi – Lahore – Sagodha ridge and Jaisalmer – Ganganagar ridge. There are other NE-SW oriented gravity and geoid highs that may be related to thermal events such as plumes that affected this region. The ADFB and its margin faults extend through Ganga basin and intersect the NW Himalayan front in the Nahan salient and the Dehradun reentrant that are more seismogenic. Similarly, the extension of NE-SW oriented gravity highs associated with Jaisalmer – Ganganagar flexure and ridge towards the Himalayan front meets the gravity highs of the Kangra reentrant that is also seismogenic and experienced a 7.8 magnitude earthquake in 1905. Even parts of the lithospheric flexure and related basement ridge of Delhi – Lahore – Sargodha show more seismic activity in its western part and around Delhi as compared to other parts. The geoid highs over the Jaisalmer – Ganganagar ridge passes through Kachchh rift and connects it to plate boundaries towards the SW (Murray ridge) and NW (Kirthar range) that makes the Kachchh as a part of a diffused plate boundary, which, is one of the most seismogenic regions with large scale mafic intrusive that is supported from 3-D seismic tomography. The modeling of regional gravity field along a profile, Ganganagar – Chandigarh extended beyond the Main Central Thrust (MCT) constrained from the various seismic studies across different parts of the Himalaya suggests crustal thickening from 35-36 km under plains up to ~56 km under the MCT for a density of 3.1 g/cm3 and 3.25 g/cm3 of the lower most crust and the upper mantle, respectively. An upwarping of ~3 km in the Moho, crust and basement south of the Himalayan frontal thrusts is noticed due to the lithospheric flexure. High density for the lower most crust indicates partial eclogitization that releases copious fluid that may cause reduction of density in the upper mantle due to sepentinization (3.25 g/cm3). It has also been reported from some other sections of Himalaya. Modeling of the residual gravity and magnetic fields along the same profile suggest gravity highs and lows of NW India to be caused by basement ridges and depressions, respectively. Basement also shows high susceptibility indicating their association with mafic rocks. High density and high magnetization rocks in the basement north of Chandigarh may represent part of the ADFB extending to the Himalayan front primarily in the Nahan salient. The Nahan salient shows a basement uplift of ~ 2 km that appears to have diverted courses of major rivers on either sides of it. The shallow crustal model has also delineated major Himalayan thrusts that merge subsurface into the Main Himalayan Thrust (MHT), which, is a decollment plane.

Long and short wavelengths of Indian Ocean geoid and gravity lows: Mid-to-upper mantle sources, rapid drift and seismicity of Kachchh and Shillong plateau, India

Spectral analysis of the Indian Ocean geoid low provides depth to the large wavelength sources as ∼1300, ∼700 and 340km that are supported from the spectral analysis and the modeling of the corresponding large wavelength regional gravity anomaly with negative density contrasts at these levels. The three levels coincide with the sharp changes in the gradient of the seismic velocities related to the olivine–spinel transformation of successively increasing Fe/Mg ratio as depth increases, known as transition zones. The first two segments are supported from continuous wavelet transform analysis of the large wavelength component of the corresponding gravity field. The low density rocks in this section appear to be related to the subducted Indian/Tethyan lithosphere that roll back and drifted southwards after subduction as inferred from tomography experiments. The relatively short wavelength sources of the spectrum of the geoid data at depths of 162 and 85km suggest sources along the lithosphere – asthenosphere boundary (LAB) under the Indian continent and surrounding oceans, respectively. A low viscosity zone has been envisaged below 660km discontinuity that may correspond to the low density rocks in this section which is popularly referred to as graveyards of the subducted rocks under geoid lows. The subducted slab is hydrated due to dehydration from metamorphism that causes upwelling in the mid-to-upper mantle which is likely to set in baby plumes. Presence of fluid may transform part of olivine to serpentine further reducing the bulk density of rocks in this section. They would make this region buoyant that appears to be responsible for the Central Indian Ocean Deformation Zone with large scale folding, faulting, seismicity, and high heat flow. The same also appear to be responsible for the rapid drift of the Indian plate.Short wavelength component of the Indian Ocean geoid low reflects most of the surface/shallow tectonics of the region similar to the gravity anomaly providing an additional data set for this purpose. Kachchh and the Shillong plateau inspite of being in the intra plate region are highly seismogenic that compare almost to the activity along the plate boundaries. Tectonics and residual geoid anomalies of the Indian continent and adjoining regions suggest that the geoid highs of Kachchh and the Shillong plateau along the NW and the NE corners of India are connected to the geoid highs of the plate boundaries through several lineaments and faults that also show geoid highs and trends of seismic activity leading to plate boundaries. Geoid highs indicating high density mafic rocks suggest their connections/extensions to plate boundaries indicating them to be plausibly part of diffused plate boundaries. Besides, both these regions are affected by (i) prominent lithospheric flexure of the Indian plate (ii) large scale mafic intrusions and (iii) interaction of proterozoic mobile belts with effects of present day Himalayan orogeny that make them more vulnerable for seismic activity.

Gravity above subduction zones and forces controlling plate motions

Short‐ and intermediate‐wavelength gravity and geoid anomalies are used to provide constraints on the mechanical structure of subduction zones and on the forces involved. This study is based on 2D Cartesian dynamically self‐consistent models with Newtonian or power law rheologies. We show that both strong decoupling of the two convergent plates (shear stresses of the order of 107 Pa) and weakened bending lithosphere are necessary to reproduce the observed geoid and gravity data. Good fits are found for relatively low failure stresses (≈30–50 MPa). For all models providing reasonable predictions of gravity, only a small fraction of the downgoing slab weight is transmitted to the surface plates. About 10% of the energy is dissipated in the contact zone between the two plates, 10% to 20% in the bending region, and more than 70% in the sublithospheric mantle. The basal tractions (on the order of ≈1−4 × 1012 N/m) induce a net motion of the plates, with the subducting lithosphere moving faster than predicted by the no‐net‐motion principle. A marked positive geoid anomaly is predicted above subduction zones at intermediate wavelength (λ = 2000–4000 km) in the case of pure whole mantle convection. Such large geoid highs are not observed. Introducing “partial layering” (i.e., mass anomalies hampering mantle flow through the transition zone) is necessary to reconcile model predictions and observations for these wavelengths.

The Pangea conundrum

Geodynamic models for supercontinent assembly, whereby the dispersing continental fragments of a supercontinent break up and migrate from geoid highs to reassemble at geoid lows, fail to account for the amalgamation of Pangea. Such models would predict that the oceans created by continental breakup in the early Paleozoic (e.g., Iapetus, Rheic) would have continued to expand as the continents migrated toward sites of mantle downwelling in the paleo-Pacific, reassembling an extroverted supercontinent as this ocean closed. Instead, Pangea assembled as a result of the closure of the younger Iapetus and Rheic Oceans. Geodynamic linkages between these three oceans preserved in the rock record suggest that the reversal in continental motion may have coincided with the Ordovician emergence of a super-plume that produced a geoid high in the paleo-Pacific. If so, the top-down geodynamics used to account for the breakup and dispersal of a supercontinent at ca. 600–540 Ma may have been overpowered by bottom-up geodynamics during the amalgamation of Pangea.

Effects of plate and slab viscosities on the geoid

The effects of plate rheology (strong plate interiors and weak plate margins) and stiff subducted lithosphere (slabs) on the geoid and plate motions, considered jointly, are examined with three-dimensional spherical models of mantle flow. Buoyancy forces are based on the internal distribution of subducted lithosphere estimated from the last 160 Ma of subduction history. While the ratio of the lower mantle/upper mantle viscosity has a strong effect on the long-wavelength geoid, as has been shown before, we find that plate rheology is also significant and that its inclusion yields a better geoid model while simultaneously reproducing basic features of observed plate motion. Slab viscosity can strongly affect the geoid, depending on whether a slab is coupled to the surface. In particular, deep, high-viscosity slabs beneath the northern Pacific that are disconnected from the surface as a result of subduction history produce significant long-wavelength geoid highs that differ from the observation. This suggests that slabs in the lower mantle may be not as stiff as predicted from a simple thermally activated rheology, if the slab model is accurate.

Correlation of Venus Surface Features and Geoid

The distributions of pairs of features on Venus are compared using simple statistical tests. We consider coronae types, crater types, and chasmata; some of these are obviously associated, and some are possibly associated. Coronae are grouped to represent evolutionary stages, tectonic and volcanic modification of craters is considered, and the chasmata are represented by five great circle arcs. These arcs fit rift or extension zones at the 90% level, totaling 55,000 km, comparable to the length of spreading ridges on Earth. Comparing these sets with Venus' geoid field, we find that all the chasmata correlate positively with geoid highs and negatively with lows, with the peaks in the geoid field occurring at the intersections of the arcs. Modified craters also exhibit this relation to the geoid. The domal coronae, the youngest and currently active, may be sites of upwelling. These correlate strongly with geoid highs and negatively with geoid lows, whereas later-stage coronae, the majority, do not show this correlation. Strong association of the youngest coronae with Themis–Atla suggests that the most recent activity occurs along this arc.

The Global Distribution of Wrinkle Ridges on Venus

New digital mapping of over 65,000 wrinkle ridges of the plains of Venus shows a strong correlation between the location and orientation of these long, low-amplitude compressive anticlines and major features of the geoid and long-wavelength topography. Regions with wrinkle ridges occupy 43% of the plains and are strongly skewed toward low elevations and negative geoid anomalies. About 93% of the wrinkle ridge plains lie below mean planetary radius and 72% have negative geoid anomalies. In contrast, the extensional rift zones of Venus are strongly skewed toward high elevations and positive geoid anomalies. Thus compressive deformation dominates the topographic and geoid lows, whereas extensional rifting is generally restricted to the highs. These observations are consistent with geoid-based stress models that predict compression in geoid lows and extension in geoid highs. The orientations of wrinkle ridges are generally consistent over regions extending for 1000–10,000 km. Many regions have multiple sets of wrinkle ridges of different orientations reflecting multiple episodes and directions of compression; however, about 80% of the regions display a single dominant wrinkle ridge orientation, with secondary orientations subordinate. The dominant wrinkle ridge orientations in many regions follow the contours of long-wavelength topography and geoid or lie along the axes of troughs in the geoid. Thus the maximum horizontal compression recorded by the folds is commonly parallel to the present-day gradient in geoid and topography, although some regions are strong exceptions, perhaps reflecting changes in topography and geoid with time. The dominant wrinkle ridge trends ring several major geoid and topographic swells, especially Western Aphrodite Terra and Lada Terra, with ring diameters of 75–120° (8000–13,000 km). In addition there are smaller rings of wrinkle ridges around the swells in Themis, Eistla, and Bell regiones, especially Gula Mons, with ring diameters of 25–45° (2500–5000 km). In contrast, there are no rings of compressive deformation surrounding some other geoid/topographic highs, including Beta and Atla regiones, which have been proposed to be the youngest swells and display the largest peak geoid/topography ratios and largest geoid gradients. These observations are consistent with the idea that not all the major topographic/geoid swells existed on Venus at the time of wrinkle ridge formation.

High‐resolution, two‐dimensional vertical tomography of the central Pacific mantle using ScS reverberations and frequency‐dependent travel times

A vertical tomogram is constructed for the mantle between Tonga and Hawaii. Using a complete Gaussian‐Bayesian approach, we inverted a data set comprising 304 ScS reverberation travel times with Fréchet kernels computed by the paraxial ray approximation and 1122 frequency‐dependent phase delays of turning and surface waves with Fréchet kernels calculated by a normal‐mode coupling algorithm. The model parameters include shear speed variations, perturbations to shear wave radial anisotropy in the uppermost mantle, and the topographies of the 410‐ and 660‐ km discontinuities. Tests demonstrate that the data set resolves lateral structures in the upper mantle with scale lengths of several hundred kilometers and greater. The most significant feature in our model is a well resolved regular pattern of high and low shear velocities in the upper mantle. These variations have a horizontal wavelength of 1500 km, a vertical dimension of 700 km, and an amplitude of about 3%, and they show a strong positive correlation with seafloor topography and geoid height variations. The geoid highs correspond to a series of northwest trending swells associated with the major hotspots of the Society, Marquesas, and Hawaiian Islands. Where these swells cross the corridor, they are underlain by high shear velocities throughout the uppermost mantle, so it is unlikely that their topographies are supported by thermal buoyancy. Although chemical buoyancy generated by basaltic volcanism and the formation of its low‐density peridotitic residuum can explain the positive correlation between topography and velocity variation at the shallow depth, an additional mechanism is needed to account for the shear velocity pattern at depths greater than 200 km. It is therefore hypothesized that the central Pacific is underlain by a system of Richter‐type convective rolls that are oriented subparallel to the absolute plate velocity, have unit aspect ratio transverse to this orientation, and are confined above the 660‐km discontinuity. This convection pattern appears to be strongly correlated with the locations of the Tahitian, Marquesan, and Hawaiian hotspots, which raises interesting questions for Morgan's hypothesis that these hotspots are the surface manifestations of deep mantle plumes.

Shallow bias of paleomagnetic inclinations in the Paleozoic and Precambrian

An updated analysis of the global paleomagnetic database shows that the frequency distributions of paleomagnetic inclinations for the Cenozoic and Mesozoic eras (younger than 250 Ma) are compatible with a random geographical sampling of a time-averaged geomagnetic field that closely resembles that of a geocentric axial dipole. In contrast, the frequency distributions of paleomagnetic inclinations for the Paleozoic and Precambrian eras (prior to 250 Ma) are over-represented by shallow inclinations. After discounting obvious secondary causes for the bias, such as from data averaging, sedimentary inclination error, inhomogeneous lithological distributions, and tropical remagnetization, we show that the anomalous inclination distributions for the Paleozoic and Precambrian can be explained by a geomagnetic field source model which includes a relatively modest (∼25%) contribution to the axial dipole from a zonal octupole field and an arbitrary zonal quadrupolar contribution. The apparent change by around 250 Ma to a much more axial dipolar field geometry might be due to the stabilization of the geodynamo from growth of the inner core to some critical threshold size, a gross speculation which would imply that either the threshold size was rather large or the inner core nucleated rather late in Earth history. Alternatively, if a geocentric axial dipole model is assumed or can eventually be demonstrated independently, the anomalous inclination distributions for the Paleozoic and Precambrian may reflect a tendency of continental lithosphere to be cycled into the equatorial belt, perhaps because geoid highs associated with long-term continental aggregates influence true polar wander.

Localization of the gravity field and the signature of glacial rebound

The negative free-air gravity anomaly centred on Hudson Bay, Canada, shows a remarkable correlation with the location of the Laurentide ice sheet, suggesting that this gravity anomaly is the result of incomplete post-glacial rebound. This region, however, is also underlain by higher-than-average mantle seismic velocities, suggesting that the gravity low might result instead from dynamic topography associated with convective downwellings. Here we analyse the global gravity field as a simultaneous function of geographic location and spectral content. We find that the Hudson Bay gravity low is unique, with anomalously high amplitude in the spectral band where the power from the Laurentide ice load is greatest and the relaxation times predicted for viable models of viscous relaxation are longest. We estimate that about half of the Hudson Bay gravity anomaly is the result of incomplete post-glacial rebound, and derive a mantle viscosity model that explains both this gravity signature and the characteristic uplift rates for the central Laurentide and Fennoscandian regions. This model has a jump in viscosity at 670 km depth, comparable to that in dynamic models of the geoid highs over subducted slabs, but lacks a low-viscosity asthenosphere, consistent with a higher viscosity in the upper mantle beneath shields than in oceanic regions.

Driving Forces for Limited Tectonics on Venus

The very high correlation of geoid height and topography on Venus, along with the high geoid topography ratio, can be interpreted as local isostatic compensation and/or dynamic compensation of topography at depths ranging from 50 to 350 km. For local compensation within the lithosphere, the swell-push force is proportional to the first moment of the anomalous density. Since the long-wavelength isostatic geoid height is also proportional to the first moment of the anomalous density, the swell push force is equal to the geoid height scaled by −g2/2πG. Because of this direct relationship, the style (i.e., thermal, Airy, or Pratt compensation) and depth of compensation do not need to be specified and can in fact vary over the surface. Phillips (1990) showed that this simple relationship between swell-push force and geoid also holds for dynamic uplift by shear traction on the base of the lithosphere caused by thermal convection of an isoviscous, infinite half-space mantle. Thus for all reasonable isostatic models and particular classes of dynamic models, the geoid height uniquely determines the magnitude of the swell-push body force that is applied to the venusian lithosphere.Given this body force and assuming Venus can be approximated by a uniform thickness thin elastic shell over an inviscid sphere, we calculate the present-day global strain field using equations given in Banerdt (1986); areas of positive geoid height are in a state of extension while areas of negative geoid height are in a state of compression. The present-day model strain field is compared to global strain patterns inferred from Magellan-derived maps of wrinkle ridges and rift zones. Wrinkle ridges, which are believed to reflect distributed compressive deformation, are generally confined to regions with geoid of less than 20m while rift zones are found primarily along geoid highs. Moreover, much of the observed deformation matches the present-day model strain orientations suggesting that most of the rifts on Venus and many of the wrinkle ridges formed in a stress field similar to the present one. In several large regions, the present-day model strain pattern does not match the observations. This suggests that either the geoid has changed significantly since most of the strain occurred or our model assumptions are incorrect (e.g., there could be local plate boundaries where the stress pattern is discontinuous). Since the venusian lithosphere shows evidence for limited strain, the calculation also provides an estimate of the overall strength of the lithosphere in compression and extension which can be compared with rheological models of yield strength versus depth. At the crests of the major swells, where evidence for rifting is abundant, we find that the temperature gradient must be at least 7K/km.

Free-surface formulation of mantle convection-II. Implication for subduction-zone observables

SUMMARY Viscous and viscoelastic models for a subduction zone with a faulted lithosphere and internal buoyancy can self-consistently and simultaneously predict long-wavelength geoid highs over slabs, short-wavelength gravity lows over trenches, trench-forebulge morphology, and explain the high apparent strength of oceanic lithosphere in trench environments. The models use two different free-surface formulations of buoyancydriven flows (see, for example, Part I): Lagrangian viscoelastic and pseudo-free-surface viscous formulations. The lower mantle must be stronger than the upper in order to obtain geoid highs at long wavelengths. Trenches are a simple consequence of the negative buoyancy of slabs and a large thrust fault, decoupling the overriding from underthrusting plates. The lower oceanic lithosphere must have a viscosity of less than Pa s in order to be consistent with the flexural wavelength of forebulges. Forebulges are dynamically maintained by viscous flow in the lower lithosphere and mantle, and give rise to apparently stiffer oceanic lithosphere at trenches. With purely viscous models using a pseudo-free-surface formulation, we find that viscous relaxation of oceanic lithosphere, in the presence of rapid trench rollback, leads to wider and shallower back-arc basins when compared to cases without viscous relaxation. Moreover, in agreement with earlier studies, the stresses necessary to generate forebulges are small (- 100 bars) compared to the unrealistically high stresses needed in classic thin elastic plate models.

Geoid lineations of 1000 km wavelength over the central Pacific

Altimeter profiles of the ERS‐1 and Topex‐Poseidon satellites have been used to compute a geoid surface from which we have extracted medium‐wavelength geoid anomalies over the central Pacific. In this region, the geoid shows prominent elongated anomalies of 20–40 cm in amplitude, with a spacing of ∼1000 km and oriented N60°W. In the south central Pacific, the Polynesian hotspot swells seem to be located on the linear geoid highs. However, the latter extend much farther eastward, preceeding the active hotspots. To the north, other geoid lines are visible but they do not coincide with known tectonic features. The wavelet transform applied to raw geoid data clearly detects a strong signal at the 1000–1200 km wavelength. The amplitude of the lineations increases with age, by a factor of 3 between 10 and 50 my. Analysis of seafloor topography corrected for age and sediments reveals topography anomalies correlated with the geoid lineations. Admittance and coherence calculations give high coherence (0.9) in the 1000–1200 km waveband and admittance values of 1.5 m/km at 10 my and 3 m/km at 60 my. The mechanism producing the lineations is unclear. Their characteristics however are not incompatible with a convective origin.

Spherical disharmonies in the Earth sciences and the spatial solution: Ridges, hotspots, slabs, geochemistry and tomography correlations

There is increasing use of statistical correlations between geophysical fields and between geochemical and geophysical fields in attempts to understand how the Earth works. Typically, such correlations have been based on spherical harmonic expansions. The expression of functions on the sphere as spherical harmonic series has many pitfalls, especially if the data are nonuniformly and/or sparsely sampled. Many of the difficulties involved in the use of spherical harmonic expansion techniques can be avoided through the use of spatial domain correlations, but this introduces other complications, such as the choice of a sampling lattice. Additionally, many geophysical and geochemical fields fail to satisfy the assumptions of standard statistical significance tests. This is especially problematic when the data values to be correlated with a geophysical field were collected at sample locations which themselves correlate with that field. This paper examines many correlations which have been claimed in the past between geochemistry and mantle tomography and between hotspot, ridge, and slab locations and tomography using both spherical harmonic coefficient correlations and spatial domain correlations. No conclusively significant correlations are found between isotopic geochemistry and mantle tomography. The Crough and Jurdy (short) hotspot location list shows statistically significant correlation with lowermost mantle tomography for degree 2 of the spherical harmonic expansion, but there are no statistically significant correlations in the spatial case. The Vogt (long) hotspot location list does not correlate with tomography anywhere in the mantle using either technique. Both hotspot lists show a strong correlation between hotspot locations and geoid highs when spatially correlated, but no correlations are revealed by spherical harmonic techniques. Ridge locations do not show any statistically significant correlations with tomography, slab locations, or the geoid; the strongest correlation is with lowermost mantle tomography, which is probably spurious. The most striking correlations are between mantle tomography and post‐Pangean subducted slabs. The integrated locations of slabs correlate strongly with fast areas near the transition zone and the core‐mantle boundary and with slow regions from 1022–1284 km depth. This seems to be consistent with the “avalanching” downwellings which have been indicated by models of the mantle which include an endothermic phase transition at the 670‐km discontinuity, although this is not a unique interpretation. Taken as a whole, these results suggest that slabs and associated cold downwellings are the dominant feature of mantle convection. Hotspot locations are no better correlated with lower mantle tomography than are ridge locations.

Global decoupling of crust and mantle: Implications for topography, geoid and mantle viscosity on Venus

The surface of Venus is so hot that the lower crust may be weak enough to allow decoupling of mantle and crust. An analytic model of such decoupling assumes that the shallow mantle forms the top boundary layers of large scale mantle convection cells. Crustal flow is driven by the motion of the mantle and by topographically induced pressure gradients. The model predicts that the lowest lowlands are sites of mantle upwelling and thinner than average crust. Highlands are places where mantle downwells and the crust is thick. Surface heat flow is inversely correlated with elevation, consistent with recent estimates of brittle layer thickness variations on Venus. If the average crustal thickness is about 20 km then the average lower crustal viscosity must be close to 1018 Pa s to allow decoupling. The observed amplitude of geoid highs over highlands requires an Earth‐like increase in mantle viscosity with depth.

Antipodal hotspot pairs on the Earth

The results of statistical analyses performed on three published hotspot distributions suggest that significantly more hotspots occur as nearly antipodal pairs than is anticipated from a random distribution, or from their association with geoid highs and divergent plate margins. The observed number of antipodal hotspot pairs depends on the maximum allowable deviation from exact antipodality. At a maximum deviation of ≤700 km, 26% to 37% of hotspots form antipodal pairs in the published lists examined here, significantly more than would be expected from the general hotspot distribution. Two possible mechanisms that might create such a distribution include: (1) symmetry in the generation of mantle plumes, and (2) melting related to antipodal focusing of seismic energy from large‐body impacts.

On the origin of deviatoric stresses in the lithosphere

Mantle circulation models capable of predicting satisfactory plate velocities and geoid are tested for their ability to generate adequate lithospheric stresses. The model Earth mantle is driven by mass heterogeneities defined by seismic tomography and slab distribution. Its surface is divided into 11 freely moving plates. The mantle circulation produces shear stresses at the base of each plate. These are used to compute a dynamical component of the lithospheric stresses which is added to another component induced by variations of the Moho depth inside the lithosphère itself. The extensional state of the continents caused by their thick crust is more than compensated by a compressional tendency related to their high seismic velocity roots, interpreted as dense roots. Strong upwellings beneath equatorial regions with geoid highs could also generate anomalous topography and extensional stresses. The long‐wavelength stress pattern proposed by our models cannot easily be compared with available data: the internal loads are still poorly defined, the tested Earth model may be too simple, and the observations are inadequately distributed.