Geoid Anomalies Research Articles

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.

El Niño–Southern Oscillation signals in sea level, surface mass redistribution, and degree‐two geoid coefficients

We use a coupled Earth system model to simulate and quantify the impact of the El Niño–Southern Oscillation (ENSO) on monthly to interannual variations of steric and eustatic global mean sea level (GMSL), surface mass loading, and on the corresponding degree‐two geoid coefficients (C21, S21, and C20). GMSL is dominated by eustatic variations on monthly to interannual timescales, but less than 10% of the eustatic variance is related to ENSO. In contrast, steric GMSL correlates linearly in phase with ENSO with an explained variance of nearly 46%. Together these results imply that total GMSL variations are only weakly correlated with ENSO. Despite this small correlation, we find a distinct ENSO pattern of sizable surface mass load anomalies. Over the continents, this pattern is similar to typical ENSO‐related precipitation anomalies. Over the oceans, the pattern features a global, albeit weaker, response, with generally increased loading in the Arctic and Pacific oceans, and decreased loading in the Atlantic and Indian oceans. These surface loading anomalies lead to statistically significant ENSO‐related variations in the S21 and C20 geoid coefficients, but not in C21. In analyzing the individual subsystem contributions, we find that S21 is influenced by both ocean mass redistribution and soil moisture loading, whereas C20 is mainly influenced by soil moisture loading. Our results highlight the importance of high‐amplitude regional loading anomalies that integrate to low‐degree geoid anomalies.

The Composition and Evolution of Lithospheric Mantle: a Re-evaluation and its Tectonic Implications

The composition of the subcontinental lithospheric mantle (SCLM) is broadly related to the tectonothermal age of the overlying crust, suggesting a secular change in SCLM-forming processes. Most estimated compositions of Archean SCLM, based on well-studied suites of xenoliths and xenocrysts, are depleted garnet lherzolites with high orthopyroxene/olivine. However, these compositions make it difficult to account for the high shear-wave velocities measured in the cores of large cratons, and predict deeper geoid anomalies and higher elevations than are observed in most cratons. Global and regional seismic tomography indicates that most cratonic xenolith suites represent material from the lower-velocity margins of lithospheric blocks. This implies that previous compositional estimates are strongly biased toward metasomatized material. We suggest that most Archean SCLM originally consisted of highly depleted dunites/harzburgites, similar to the Archean orogenic massifs of western Norway. Incorporation of such rocks in the cold upper parts of the cratonic SCLM satisfies the seismic and gravity data, suggesting that large volumes of these rocks are preserved in the cores of cratons, but are poorly sampled by volcanic rocks. The roots of most Proterozoic shields probably consist of refertilized Archean SCLM; the juvenile SCLM beneath Proterozoic and Phanerozoic mobile belts reflects only moderate depletion of Primitive Mantle compositions. Rather than a gradual evolution in SCLM-forming processes, we suggest a sharp dichotomy between Archean and younger tectonic regimes. The differences in buoyancy and viscosity between these two types of SCLM have played a major role in the construction, preservation and recycling of continental crust. If originally Archean SCLM is more widespread than currently recognized, models of crustal growth rates and recycling may need to be revised.

High-harmonic geoid signatures related to glacial isostatic adjustment and their detectability by GOCE

Abstract The Earth’s asthenosphere and lower continental crust can regionally have viscosities that are one to several orders of magnitude smaller than typical mantle viscosities. As a consequence, such shallow low-viscosity layers could induce high-harmonic (spherical harmonics 50–200) gravity and geoid anomalies due to remaining isostasy deviations following Late-Pleistocene glacial isostatic adjustment (GIA). Such high-harmonic geoid and gravity signatures would depend also on the detailed ice and meltwater loading distribution and history. ESA’s Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite mission, planned for launch in Summer 2008, is designed to map the quasi-static geoid with centimeter accuracy and gravity anomalies with milligal accuracy at a resolution of 100 km or better. This might offer the possibility of detecting gravity and geoid effects of low-viscosity shallow earth layers and differences of the effects of various Pleistocene ice decay scenarios. For example, our predictions show that for a typical low-viscosity crustal zone GOCE should be able to discern differences between ice-load histories down to length scales of about 150 km. One of the major challenges in interpreting such high-harmonic, regional-scale, geoid signatures in GOCE solutions will be to discriminate GIA-signatures from various other solid-earth contributions. It might be of help here that the high-harmonic geoid and gravity signatures form quite characteristic 2D patterns, depending on both ice load and low-viscosity zone model parameters.

Lithosphere structure underneath the Tibetan Plateau inferred from elevation, gravity and geoid anomalies

The Tibetan Plateau is the product of crustal thickening caused by collision between India and Asia. Plate tectonic reconstructions suggest continuous northward movement of the Indian plate relative to stable Eurasia at nearly 50 mm/yr for the last 50 My. The plateau is now at ~5 km elevation with steep topographic gradients across the southern and northern margins. These steep topographic gradients are also related to large lateral variations in the geoid and gravity anomalies. In a SSW to NNE cross section, the Bouguer gravity anomaly decreases over a distance of 500 km from about 0 mGal in the India plate to ~−500 mGal on the Tibetan Plateau. The geoid anomaly also presents steep gradients on both the Himalayan front and the northern margin, reaching values between 20–30 m on the plateau, suggesting a pronounced thinning of the lithospheric mantle. Uplift late in the tectonic evolution of the plateau, the widespread extension, and the associated magmatism have been attributed to convective removal of the lower part of lithospheric mantle and its replacement by hotter and lighter asthenosphere. Here we present a two-dimensional lithospheric thermal and density model along a transect from the Indian plate to Asia, crossing the Himalaya front and the Tibetan Plateau. The model is based on the assumption of local isostatic equilibrium, and is constrained by the topography, gravity and geoid anomalies and by thermal data within the crust. Our results suggest that the height of the Tibetan Plateau is compensated by thick crust in the south and by hot upper mantle to the north. The Tibetan Plateau as a whole cannot be supported isostatically only by thickened crust; a thin and hot lithosphere beneath the northern plateau is required to explain the high topography, gravity, geoid and crustal temperatures. The lithosphere reaches a maximum depth of ~260 km beneath the southern Plateau, and thins abruptly northward to ~100 km under the central and northern Plateau. The lithosphere depth increases again beneath the Qaidam basin and the Qilian Shan to ~160 km.

Validation of ERS-1 and High-Resolution Satellite Gravity with in-situ Shipborne Gravity over the Indian Offshore Regions: Accuracies and Implications to Subsurface Modeling

Geoid and gravity anomalies derived from satellite altimetry are gradually gaining importance in marine geoscientific investigations. Keeping this in mind, we have validated ERS-1 (168 day repeat) altimeter data and very high-resolution free-air gravity data sets generated from Seasat, Geosat GM, ERS-1 and TOPEX/POSEIDON altimeters data with in-situ shipborne gravity data of both the Bay of Bengal and the Arabian Sea regions for the purpose of determining the consistencies and deviations. The RMS errors between high resolution satellite and ship gravity data vary from 2.7 to 6.0 mGal, while with ERS-1 data base the errors are as high as 16.5 mGal. We also have generated high resolution satellite gravity maps of different regions over the Indian offshore, which eventually have become much more accurate in extracting finer geological structures like 85° E Ridge, Swatch of no ground, Bombay High in comparison with ERS-1satellite-derived gravity maps. Results from the signal processing related studies over two specific profiles in the eastern and western offshore also clearly show the advantage of high resolution satellite gravity compared to the ERS-1 derived gravity with reference to ship gravity data.

Does mantle convection currently exist on Mercury?

The full equations of convective motion over the age of the solar system can be solved in a few hours (2D) to several weeks (3D) with numerical simulations in Cartesian or spherical geometry. Using the 2D, Cartesian geometry code, ConMan, we model the thermal evolution of Mercury. Our preferred calculation has an initial Rayleigh number of 8 × 10 4, no mantle internal heating, and a constant core heat flux of 9.75 mW/m 2 in a non-Newtonian rheology. These values all fall within the range of previously estimated parameters from 1D calculations, and allow for weak mantle convection to persist until present, supporting the outer core dynamo explanation of Mercury's intrinsic magnetic field. Further, the peak dynamic topography and geoid anomalies from our model are about 179 m and 37 m, respectively—values that are not unrealistic and should be well within the range of observation by the MESSENGER spacecraft. Future spacecraft observations, yielding new and more accurate data, will help us better understand the thermal evolution of Mercury and the origin of its magnetic field.

Paleolithospheric structure revealed by continental geoid anomalies

Lithospheric geoid anomalies record changes in elevation and potential energy experienced by continental lithosphere. Estimates of local isostatic equilibration and potential energy, in tandem with lithosphere-related geoid anomalies, can be used to estimate paleolithospheric thickness, providing a clearer understanding of how and why continental topography is developed. We employ several simplifying assumptions about the crustal and mantle lithosphere density and structure (and readily acknowledge that our results are therefore first-order approximations) to predict the pre-orogenic structure of the lithosphere. At the outset we emphasize that while this approach does not provide an exhaustive evaluation of the deformation mechanism, it does serve to quantify the relative role played by the variations in the crustal and upper mantle components of the lithosphere. In this way we are able to use independent measurement of lithospheric geoid anomalies, current (post-orogenic) elevation and lithospheric structure, and paleoelevation information to estimate topographic development and structural support over time. Application of this technique to the southwestern United States indicates that the uplift of the Colorado Plateau is the result of processes in both the crust and mantle lithosphere and that the lithosphere of the pre-orogenic Southern Basin and Range was thinned relative to the Northern Basin and Range and Colorado Plateau. Although we use the southwestern U.S. as an example, this method can help constrain uplift mechanisms for any region for which the structure and geoid anomaly of the modern lithosphere is well understood.

Topography and geoid anomalies due to density heterogeneities at the base of the thermal lithosphere: application to oceanic swells and small-wavelength geoid lineations

SUMMARY The surface topography, gravity and geoid associated with loads situated at the base of the thermal lithosphere are computed. The model lithosphere is composed of a viscous lower layer with depth-dependent viscosity overlain by an elastic lid. When the viscosity varies strongly with depth, the amplitude of the surface topography decreases rapidly as the wavenumber characterizing the mass anomaly increases. The response for this two-layer lithosphere differs substantially from the response computed for the loading from below of an elastic lid. If the 200km wavelength geoid anomalies observed in the oceans are due to convective instabilities at the base of the lithosphere, a positive geoid anomaly with an amplitude reaching 30 to 50cm is expected over a cold downwelling for ages greater than 10 Myr. No detectable topography associated with the lineations should be present for ages greater than 20 Myr. Our model predicts that some topographic anomalies might be visible for younger ages. If 1000 km broad swells are due to light material at a depth of about 100 km, the associated geoid-over-topography ratios are quite reduced compared to those predicted by the long-wavelength linear relationship usually employed for estimating the apparent compensation depth.

A rapid method to map the crustal and lithospheric thickness using elevation, geoid anomaly and thermal analysis. Application to the Gibraltar Arc System, Atlas Mountains and adjacent zones

We present a method based on the combination of elevation and geoid anomaly data together with thermal field to map crustal and lithospheric thickness. The main assumptions are local isostasy and a four-layered model composed of crust, lithospheric mantle, sea water and the asthenosphere. We consider a linear density gradient for the crust and a temperature dependent density for the lithospheric mantle. We perform sensitivity tests to evaluate the effect of the variation of the model parameters and the influence of RMS error of elevation and geoid anomaly databases. The application of this method to the Gibraltar Arc System, Atlas Mountains and adjacent zones reveals the presence of a lithospheric thinning zone, SW–NE oriented. This zone affects the High and Middle Atlas and extends from the Canary Islands to the eastern Alboran Basin and is probably linked with a similarly trending zone of thick lithosphere constituting the western Betics, eastern Rif, Rharb Basin, and Gulf of Cadiz. A number of different, even mutually opposite, geodynamic models have been proposed to explain the origin and evolution of the study area. Our results suggest that a plausible slab-retreating model should incorporate tear and asymmetric roll-back of the subducting slab to fit the present-day observed lithosphere geometry. In this context, the lithospheric thinning would be caused by lateral asthenospheric flow. An alternative mechanism responsible for lithospheric thinning is the presence of a hot magmatic reservoir derived from a deep ancient plume centred in the Canary Island, and extending as far as Central Europe.

Buckling instabilities of subducted lithosphere beneath the transition zone

A sheet of viscous fluid poured onto a surface buckles periodically to generate a pile of regular folds. Recent tomographic images beneath subduction zones, together with quantitative fluid mechanical scaling laws, suggest that a similar instability can occur when slabs of subducted oceanic lithosphere encounter enhanced resistance to penetration at 660 km depth. Beneath subduction zones such as Central America and Java, the fast anomalies in elastic wavespeed are too broad (≥400 km) to be explained by simple compressional thickening of subducted lithosphere that is initially 50–100 km thick. By contrast, a periodic buckling mechanism can account for both the characteristic ‘wedge’ shapes of the anomalies and for their widths, which agree closely with those predicted by the scaling laws. In addition to providing a plausible physical interpretation of tomographic observations beneath some subduction zones, buckling instabilities have important consequences for other aspects of global geodynamics such as tectonic plate reconstructions, interpretation of geoid anomalies, and modeling of secular changes in Earth's moment of inertia.

Lithospheric structure in the Atlantic–Mediterranean transition zone (southern Spain, northern Morocco): a simple approach from regional elevation and geoid data

A new methodology for automatic modelling of crustal and lithospheric thickness or density variations that integrates both elevation and geoid anomalies is presented. We used this algorithm to image the main lithospheric heterogeneities within the Atlantic–Mediterranean transition zone. Main assumptions in the modelling are local isostasy and a two-layered model comprising crust and lithospheric mantle with constant densities, plus sea water and asthenosphere. The results show a wide zone of thickened lithosphere, oriented NE–SW and located beneath the western Betics and Rif, the Gulf of Cadiz and the northwestern Atlantic Moroccan margin. In addition, a zone of thinned lithosphere with the same orientation is shown beneath the Atlas Mountains and the eastern Alboran Basin. To cite this article: J. Fullea Urchulutegui et al., C. R. Geoscience 338 (2006).

Effects of mantle upwelling in a compressional setting: the Atlas Mountains of Morocco

AbstractWe discuss the implications of a lithospheric model of the Moroccan Atlas Mountains based on topography, heat flow, gravity and geoid anomalies, taking into account the regional geology. The NW African cratonic lithosphere, some 160–180 km thick, thins to c. 80 km beneath the Atlas fold‐thrust belts, in contrast with the shortening regime prevailing there since the early Cenozoic. This fact explains several geological and geophysical features as high topography with modest tectonic shortening, the occurrence of alkaline magmatism contemporaneous to compression, the absence of large crustal roots to support elevation, the scarce development of foreland basins, and a marked geoid high. The modelled lithosphere thinning is related to a thermal upwelling constrained between the Iberia–Africa convergent plate boundary and the Saharan craton.

A splash in Earth gravity from the 2004 Sumatra Earthquake

The 26 December 2004 Sumatra earthquake displaced a large amount of material in the radial direction with respect to the Earth's center, and the earthquake is thus expected to have permanently changed the gravity in a broad region surrounding its causative fault.This article examines gravity changes, which are here quantified by geoid anomaly patterns depicting the displacement of the sea surface's hydrostatic component after the earthquake. The article focuses on the feasibility of the detection of these patterns by gravity space missions. Models of changes in the rotation and oblateness of the Earth due to global gravitational effects of the earthquake have already been reported [Chao and Gross, 2005].

Improvement in the Determination of the Marine Geoid by Estimating the Bathymetry from Altimetry and Depth Soundings

The contribution of bathymetry to the estimation of gravity field related quantities is investigated in an extended test area in the Mediterranean Sea. The region is located southwest of the island of Crete, Greece, bounded between 33ˆ ≤ ϕ ≤ 35ˆ and 15ˆ ≤ λ ≤ 25ˆ. Gravity anomalies from the KMS99 gravity field and shipborne depth soundings are used with a priori statistical characteristics of depths in a least-squares collocation procedure to estimate a new bathymetry model. Two different global bathymetry models, namely JGP95E and Sandwell and Smith V8, are used to derive the depth a priori statistical information, while the estimated model is compared against both the global ones and the shipborne depth soundings to assess whether there is an improvement. Various marine geoid models are estimated using ERS1 and GEOSAT Geodetic Mission altimetry and shipborne gravity data. In that process, the effect of the bathymetry is computed using both the estimated and the original depths through a residual terrain modeling reduction. The TOPEX/Poseidon Sea Surface Heights, known for their high accuracy and precision, and the GEOMED solution for the geoid in the Mediterranean are used as control for the validation of the new geoid models and to assess the improvement that the estimated depths offer to geoid modeling. The results show that the newly estimated bathymetry agrees better (by about 30 to 300 m) with the shipborne depth soundings and provides smoother residual geoid heights and gravity anomalies (by about 8–20%) than those from global models. Finally, the achieved accuracy in geoid modeling ranges between 6 and 10 cm (1σ).

The structure and the surface manifestation of mantle plumes in depth-dependent three-dimensional models

Thermal convection has been modeled using a 3D Cartesian model, in order to study the structure of mantle plumes and their surface manifestation. Hotspots can be regarded as the surface manifestation of mantle plumes. The main characteristic features of the hotspots are their volcanism, topographic, geoid and heat flow anomalies. The following characteristics of the hotspot-generator plumes are studied: their geometrical size, temperature distribution, the horizontal extent and maximum amplitude of the surface features: geoid, topographic and heat flow anomalies. The aim of the model calculations is to investigate the effect of depth-dependent viscosity and internal heating in case of scaling the non-dimensional result for upper and whole mantle convection. When upper mantle convection is supposed, the calculated amplitudes of the anomalies fit well to the observed values, but their lateral extent is too small. The extent of the anomalies suggests whole mantle plume. Taking into account the high viscosity of the lithosphere, the low viscosity of the asthenosphere and the D″ layer at the CMB and using internal heating helps to gain better fit to the observations. The most complex whole mantle model predicts quite realistic features, except the topographic height.

MiFil: A method to characterize seafloor swells with application to the south central Pacific

We propose a filtering method to characterize large‐scale depth anomalies. The MiFil method (for minimization and filtering) requires two stages: a first one to roughly remove the volcano component by minimizing the depth anomaly and a second one to smooth the shape and totally remove the small spatial length scale remaining topography using a median filter. The strength of this method, directly applicable on two‐dimensional grids, is that it does not require any assumption on the location, amplitude, or width of the large‐scale feature to characterize, except its minimal width. We only consider the spatial length scale of the features to remove. Application to volcanic chains of the south central Pacific is presented, and the results lead to a better understanding of the tectonics and volcanism emplacement of the zone. The Society is the only “classical” hot spot that corresponds to the simple interaction of a plume with the lithosphere and for which a buoyancy flux of 1.58 ± 0.15 Mg s−1 is obtained. The Marquesas volcanic chain, although quite comparable, presents a swell morphology that prevents such interpretation and quantification. For the Tuamotu and Cook‐Austral volcanic chains, no reliable quantification can be made because the depth and geoid anomalies are caused by several phenomena occurring at different depths that cannot be separated.

Contemporary kinematics of the southern Aegean and the Mediterranean Ridge

SUMMARY This study focuses on the kinematics of the southern Aegean and the Mediterranean Ridge (MR). A quantification of the deformation of the MR is essential for both evaluating physical models of accretionary wedges in general and for obtaining a self-consistent model of the surface deformation over the entire Nubia‐Eurasia (NU‐EU) plate boundary zone in the eastern Mediterranean. Previous kinematic studies have not properly considered the deformation field south of the Hellenic arc. Although this study focuses on the deformation field of the MR, we also discuss the kinematics of the southern Aegean, because the geometry and movement of the Hellenic arc determine to a large extent the kinematic boundary conditions for kinematic studies of the MR. We calculate a continuous velocity and strain rate field by interpolating model velocities that are fitted in a least-squares sense to published Global Positioning System (GPS) velocities. In the interpolation, we use information from a detailed data set of onshore and offshore active faulting to place constraints on the expected style and direction of the model strain rate field. In addition, we use the orientations of tracks left by seamounts travelling into the wedge to further constrain the offshore deformation pattern. Our model results highlight the presence of active shear partitioning within the Mediterranean ridge. High compressional strain rates between the ridge crest and the deformation front accommodate approximately 60‐ 70 per cent of the total motion over the wedge, and the outward growth rate of the frontal thrust is ∼ 4m m yr −1 . Strain partitioning within the wedge leads to 19‐23 mm yr −1 of dextral motion at the wedge‐backstop contact of the western MR, whereas the Pliny and Strabo trenches in the eastern MR accommodate 21‐23 mm yr −1 of sinistral motion. The backstop of the western MR is kinematically part of the southern Aegean, which moves as a single block [the Aegean block (AE)] at 33‐34 mm yr −1 in the direction of S24 ◦ W ± 1 ◦ towards stable Nubia (NU). Our model confirms that there is a clear divergence between the western and eastern Hellenic arc and we argue for a causal relation between the outward motion of the arc and the gradient in the regional geoid anomaly. Our results suggest that a significant driving source of the surface velocity field lies south of the Hellenic arc and only for the southeastern Aegean could there be some effect as a result of gravitational collapse associated with density differences within the overriding plate.

Geoid and Gravity in Earth Sciences – An Overview

Precise global geoid and gravity anomaly information serves essentially three different kinds of applications in Earth sciences: gravity and geoid anomalies reflect density anomalies in oceanic and continental lithosphere and the mantle; dynamic ocean topography as derived from the combination of satellite altimetry and a global geoid model can be directly transformed into a global map of ocean surface circulation; any redistribution or exchange of mass in Earth system results in temporal gravity and geoid changes. After completion of the dedicated gravity satellite missions GRACE and GOCE a high standard of global gravity determination, both of the static and of the time varying field will be attained. Thus, it is the right time to investigate the future needs for improvements in the various fields of Earth sciences and to define the right strategy for future gravity field satellite missions.

Plume‐induced topography and geoid anomalies and their implications for the Tharsis rise on Mars

The topography and geoid anomalies produced from an upwelling plume are sensitive to the depth of plume buoyancy, Dp, and elastic lithospheric thickness, Te, both of which control the degree of compensation. We formulated spherically axisymmetric convection models of mantle plumes with temperature‐ and pressure‐dependent viscosity to dynamically determine Dp and Te and the resulting topography and geoid anomalies from the plume. From 10 cases with different Rayleigh number, activation energy, and activation volume, we determined Te ranging from 100 km to 180 km and Dp that is approximately twice Te. The ratio of geoid to topography, RG/T, from these plume models is either negative or slightly positive (<0.03) at wavelengths that are most relevant to the Tharsis rise (low harmonics from degrees 2 to 4). The modeled RG/T is significantly smaller than that observed for the Tharsis rise, suggesting that Tharsis is unlikely to be dynamically supported.