Mantle Anomalies Research Articles

Recent seismicity of Italy: Active tectonics of the central Mediterranean region and seismicity rate changes after the Mw 6.3 L'Aquila earthquake

In this paper we present a new image of the instrumental seismicity of Italy, obtained by refining hypocentral determinations for about 100,000 earthquakes that occurred in the period 2005–2012. The improved locations yield new constraints on active tectonics of the central Mediterranean area, where prolonged interaction between nested plates and continental slivers led to the development of the Alpine and Apennines systems. Intermediate-depth and deep earthquakes define a lateral heterogeneous process of delamination and sinking of the continental lithosphere active beneath the mountain belts. Shallow seismicity prevalently occurs beneath elevated topography and correlates with low velocity mantle anomalies, suggesting a superposition of gravity-related forces to the Eurasia–Africa plate convergence. The delamination process drives a paired system of compression and extension that stretches the mountain range while shortening the external side of the belts. The updated seismic catalog permits us to resolve a sharp variation of seismic rates that occurred in recent years, timely after the 2009 Mw 6.3 L'Aquila earthquake. The increase of seismic rates is reasonably due to regional-scale perturbation of the stress field induced by fluid flow and pore-pressure variations within the crust, probably related to deep dehydration processes active beneath the mountain belt.

Perturbing effects of sub-lithospheric mass anomalies in GOCE gravity gradient and other gravity data modelling: Application to the Atlantic-Mediterranean transition zone

The GOCE (Gravity field and steady-state Ocean Circulation Explorer) mission, launched on March 2009, included a new type of satellite instrument, an on-board three-axis gradiometer able to measure the Earth's gravity gradients at the satellite height (255km). The potential of this new type of measurement and its derived products (i.e., global gravity field models), together with other land-based geophysical observables (Bouguer and geoid anomalies), to image sub-lithospheric thermal and compositional anomalies is evaluated in this study. We focus on the Atlantic-Mediterranean Transition Region (AMTR), the diffuse, transpressive contact between the Iberian Peninsula and North Africa. The present-day lithospheric structure in the area is characterized by a wide band of active deformation, large lateral variations in the lithosphere and the presence of a positive seismic-velocity anomaly in the uppermost mantle beneath the Betics and the westernmost Alboran Basin-Gibraltar Arc. Here, the perturbing effects of deep, sub-lithospheric density anomalies in GOCE gravity gradients and other land-based geophysical data are assessed, and its impact in lithospheric-scale geophysical-petrological modeling within a thermodynamically consistent framework analyzed. Some of the gravity gradients computed at the satellite altitude are rather sensitive to the presence of even a relatively small sub-lithospheric cold slab, like the one observed in the AMTR, showing the potential of the new GOCE data to map upper mantle anomalies. Lithospheric models ignoring the AMTR sub-lithospheric heterogeneities could be significantly biased. In particular, extreme changes of 4–6km and 60–70km in the crustal and lithospheric thickness, respectively, are required to include sub-lithospheric mantle contributions to land-based and satellite gravity data.

Deep earth recycling in the Hadean and constraints on surface tectonics

The Hadean mantle was efficiently heated from high internal heat production, high rates of impact bombardment, and primordial heat from accretion. As a result a strong case is made for extremely high internal temperatures, low internal viscosities, and extremely vigorous mantle convection. Previous studies of mixing in such high-Rayleigh number convective environments indicate that chemically heterogeneous mantle anomalies should have efficiently remixed into the mantle on timescales of less than 100 Myr. However, ¹⁴²Nd and ¹⁸²W isotope studies indicate that heterogeneous mantle domains survived, without mixing, for over 2 Gyr—at odds with expected mixing rates. Similarly, concentrations of platinum group elements in Archean komatiites, purportedly due to the later veneer of meteoritic addition on the Earth, only achieve current levels at 2.7 Ga—indicating a time lag of almost 1 to 2 Gyr in mixing this material thoroughly into the mantle. Previous studies have sought to explain slow Archean mantle mixing via mantle layering due to endothermic phase changes, or anomalously viscous blobs of material, with limited efficacy. Here we pursue another explanation for inefficient mantle mixing in the Hadean: tectonic regime. A number of lines of evidence suggest that resurfacing in the Archean was episodic, and extending these models to Hadean times implies the Hadean was characterized by long periods of tectonic quiescence. We explore mixing times in 3D spherical-cap models of mantle convection, which incorporate vertically stratified and temperature-dependent viscosities. We show that mixing in stagnant-lid regimes is, at the extreme, over an order of magnitude less efficient than mobile-lid mixing, and for plausible Rayleigh numbers and internal heat production, the lag in Hadean convective recycling can be explained. The attractiveness of this model is that it not only explains the long-lived ¹⁴²Nd and ¹⁸²W anomalies, but also posits an explanation for the delay between accretion of the late veneer—between 4.5 to 3.8 Ga on a stagnant surface—and its fully mixed signature apparent in elevated PGEs in 2.7 Ga komatiites. It also provides an explanation for the 400 Myrs of immobility of the mafic protolith from which the Jack Hill zircons were sourced, and retards early heat loss from the mantle, providing a solution to the “Archean thermal catastrophe” of parameterized Earth evolution models.

Lithospheric fabric variations in central North America: Influence of rifting and Archean tectonic styles

Upper mantle fabric, as detected by shear wave splitting measurements, records lithospheric strain history that may be related to past tectonic events. We present SKS (core‐refracted shear wave) splitting measurements taken around the western Great Lakes, North America: the Archean Superior Province (SP) and surroundings. We analyze 40 sites in the USA and 5 in Canada and outline a region of strong anisotropy corresponding to a high‐velocity zone in tomographic models, which we dub the Western Superior Mantle Anomaly (WSMA) and interpret to represent a preserved fabric from accretion of the SP. To the southwest of the WSMA, we locate a region of rotated fast direction corresponding to a linear low‐velocity feature observed by tomography and which may represent a failed branch of the Mid‐Continent Rift. Further south, within the Minnesota River Valley Terrane, the split times drop to near zero, suggesting isotropic lithosphere resulting from vertical (diapiric) rather than horizontal tectonic processes.

Multi‐scale dynamics and rheology of mantle flow with plates

Fundamental issues in our understanding of plate and mantle dynamics remain unresolved, including the rheology and state of stress of plates and slabs; the coupling between plates, slabs and mantle; and the flow around slabs. To address these questions, models of global mantle flow with plates are computed using adaptive finite elements, and compared to a variety of observational constraints. The dynamically consistent instantaneous models include a composite rheology with yielding, and incorporate details of the thermal buoyancy field. Around plate boundaries, the local resolution is 1 km, which allows us to study highly detailed features in a globally consistent framework. Models that best fit plateness criteria and plate motion data have strong slabs with high stresses. We find a strong dependence of global plate motions, trench rollback, net rotation, plateness, and strain rate on the stress exponent in the nonlinear viscosity; the yield stress is found to be important only if it is smaller than the ambient convective stress. Due to strong coupling between plates, slabs, and the surrounding mantle, the presence of lower mantle anomalies affect plate motions. The flow in and around slabs, microplate motion, and trench rollback are intimately linked to the amount of yielding in the subducting slab hinge, slab morphology, and the presence of high viscosity structures in the lower mantle beneath the slab.

Mantle structure beneath Africa and Arabia from adaptively parameterized P-wave tomography: Implications for the origin of Cenozoic Afro-Arabian tectonism

While the Cenozoic Afro-Arabian Rift System (AARS) has been the focus of numerous studies, it has long been questioned if low-velocity anomalies in the upper mantle beneath eastern Africa and western Arabia are connected, forming one large anomaly, and if any parts of the anomalous upper mantle structure extend into the lower mantle. To address these questions, we have developed a new image of P-wave velocity variations in the Afro-Arabian mantle using an adaptively parameterized tomography approach and an expanded dataset containing travel-times from earthquakes recorded on many new temporary and permanent seismic networks. Our model shows a laterally continuous, low-velocity region in the upper mantle beneath all of eastern Africa and western Arabia, extending to depths of ~500–700km, as well as a lower mantle anomaly beneath southern Africa that rises from the core-mantle boundary to at least ~1100km depth and possibly connects to the upper mantle anomaly across the transition zone. Geodynamic models which invoke one or more discrete plumes to explain the origin of the AARS are difficult to reconcile with the lateral and depth extent of the upper mantle low-velocity region, as are non-plume models invoking small-scale convection passively induced by lithospheric extension or by edge-flow around thick cratonic lithosphere. Instead, the low-velocity anomaly beneath the AARS can be explained by the African superplume model, where the anomalous upper mantle structure is a continuation of a large, thermo-chemical upwelling in the lower mantle beneath southern Africa. These findings provide further support for a geodynamic connection between processes in Earth's lower mantle and continental break-up within the AARS.

Sutton hotspot: Resolving Ediacaran-Cambrian Tectonics and true polar wander for Laurentia

Hotspot tracks represent plate motions relative to mantle sources, and paleomagnetic data from magmatic units along those tracks can quantify motions of those mantle anomalies relative to the Earth's magnetic field and rotational axis. The Ediacaran Period is notable for rapid and large paleomagnetic apparent polar wander (APW) for many continents. Whereas magmatic units attributed to the mantle plume suggest a practically stationary hotspot track, paleolatitudes of Laurentia for that interval vary dramatically; geologic and paleomagnetic data are at odds unless true polar wander (TPW) is invoked to explain a majority of APW. Here we test the plume-TPW hypothesis by generating the predicted Sutton hotspot track for a station- ary plume under a moving plate along the Laurentian margin during the interval from 615 to 530 Ma. Our model is the first to provide a kinematic framework for the extensive large igneous province associated with opening the Iapetus Ocean.

Extremely thin crust in the Indian Ocean possibly resulting from Plume-Ridge Interaction

The thickness of the crust created at ocean spreading centres depends on the spreading rate and melt production in the mantle. It is ~5–8 km for a crust formed at slow and fast spreading centres and 2–4 km at ultra-slow spreading centres away from hotspots and mantle anomalies. The crust is generally thin at the fracture zones and thick beneath hotspots and large igneous provinces. Here we present results for the crust generated at the fast Wharton spreading centre 55–58 Ma ago using seismic reflection and refraction data. We find that the crust over a 200 km segment of the Wharton Basin is only 3.5–4.5 km thick, the thinnest crust ever observed in a fast spreading environment. A thin crust could be produced by the presence of depleted and/or cold mantle. Numerical simulations and recent laboratory experiments studying the impact of a hot plume under a lithosphere show that a curtain of weak cold downwellings of depleted mantle material is likely to develop around the edges of the hot plume pond. Because of a strongly temperature-dependent viscosity of lithospheric material, the hotter, therefore less viscous, bottom of the lithosphere can be mobilized by an impinging plume. If sampled by a spreading centre, the locally cold and depleted mantle should result in low production of melt. We suggest that the observed thin crust in the Wharton Basin is likely to have been formed by the interaction between the Kerguelen mantle plume and the Wharton spreading centre ~55 Ma ago.

Precise determination of post-stishovite phase transition boundary and implications for seismic heterogeneities in the mid-lower mantle

The phase transition boundary between stishovite and CaCl2-type structure in pure SiO2 was investigated at 45–75GPa and 300–2490K based on the in situ X-ray diffraction measurements in a laser-heated diamond-anvil cell (LHDAC). Results show that the boundary has a positive dP/dT slope of 11.1MPa/K and the transition occurs about 70GPa at 2200K along the typical mantle geotherm. This corresponds to a depth of 1700km, somewhat shallower than the previous estimate. Seismic heterogeneities have been found in a wide depth range from 800 to 1850km in the upper to middle layers of the lower mantle. The post-stishovite phase transition is known to be ferroelastic-type, which causes a considerable reduction in shear modulus, leading to a large slow shear velocity anomaly. The almost pure SiO2 phase included in subducted crustal materials may be therefore responsible for the relatively deep mid-lower mantle seismic heterogeneities. The shallow lower mantle anomalies are possibly caused by the similar phase transition in SiO2 phase containing Al2O3 and H2O or by the phase transition in Al-bearing CaSiO3 perovskite from cubic to tetragonal structure.

Rhenium–osmium isotopes and platinum-group elements in the Rum Layered Suite, Scotland: Implications for Cr-spinel seam formation and the composition of the Iceland mantle anomaly

The Rum Layered Suite is a layered mafic–ultramafic body that was emplaced during Palaeogene North Atlantic margin rifting. It is a classic open-system magma chamber, constructed of 16 repeated coupled peridotite–troctolite units, some of which have laterally extensive ~ 2 mm-thick platinum-group element (PGE) enriched (~ 2 µg g − 1 ) Cr-spinel seams at their bases. In order to investigate Cr-spinel seam petrogenesis and enrichment of the PGE, abundances of these elements and Re–Os isotopes have been determined at three stratigraphic levels of the Rum Layered Suite that represent major magma replenishment events. Individual units preserve a range of initial 187Os/ 188Os ratios, demonstrating heterogeneity in the composition of replenishing magmas. Data for both the Cr-spinel seams and overlying silicates reveal that the processes that formed the Cr-spinel also concentrated the PGE, following magma replenishment. There is no evidence for structurally-bound PGE in Cr-spinel. Instead, the PGE budget of the Rum Layered Suite is linked to base metal sulphides, especially pentlandite, and to PGE alloys contained within the Cr-spinel seams, but which exist as separate phases at Cr-spinel grain boundaries. The range in initial Os isotope compositions ( γ Os = 3.4 to 36) in the Rum Layered Suite can be successfully modelled by 5–8% assimilation of Lewisian gneiss coupled with changing PGE contents in the replenishing magmas associated with sulphide removal. Initial 187Os/ 188Os ratios for Rum rocks range from 0.1305 to 0.1349 and are atypical of the convecting upper mantle, but are within the range for recently erupted picrites and basalts from Iceland and Palaeogene picrites and basalts from Baffin Island, Greenland and Scotland. Thus, the Os isotope data suggest that the North Atlantic Igneous Province magmas were collectively produced from a mantle source with components that remained relatively unchanged in Os isotopic composition over the past 60 Ma, and that likely contain a recycled lithospheric component.

Kinematic dynamo action in a sphere with weak differential rotation

SUMMARY The geodynamo is controlled by two fundamental symmetries: reflection through the equator and change of sign of magnetic field. It may also be influenced by lower mantle anomalies, which have a dominant cos 2φ longitude dependence; if so this would impart a further symmetry restriction of 180° rotation about the polar axis. Understanding these symmetry relations may help interpret magnetic observations and guide non-linear dynamic studies. This paper explores the kinematic dynamo action of a two-parameter class of steady flows with the four relevant spatial symmetries: axial and equatorial dipole and quadrupole, denoted Da, De, Qa, Qe, respectively. Previous work focussed on steady solutions and dynamo waves linked to the mean-field theory of Braginsky; here, we present some new steady solutions with Qe symmetry and new oscillatory solutions of all symmetries. All have only modest differential rotation and are unrelated to Braginsky theory. The results strengthen several previous findings: the vast majority of Da and Qa solutions are steady, in stark contrast to the findings of mean-field theory; differential rotation favours axial symmetry; symmetry is determined largely by the locations of flux concentration; and oscillatory solutions occur when radial and azimuthal components of field overlap. Some oscillatory solutions reverse by migration of flux in latitude, in similar fashion to those related to dynamo waves in mean-field theory, but others reverse by migration in longitude and yet more seem to reverse in place. The new dynamos raise the percentage of flows that generate magnetic field of any symmetry to 47 per cent. Many flows generate more than one symmetry and some generate all four symmetries; some flows generate two or three different symmetries at the same magnetic Reynolds number. About half the physically realizable symmetries (lowest critical magnetic Reynolds number) are Da, the remaining half being mainly Qa and De in equal proportion. Only 11 per cent of the dynamos are oscillatory, the rest are steady. This suggests steady flows favour steady magnetic fields, although for the Qe symmetry we find slightly more oscillatory solutions.

Factors responsible for the high position of the Siberian platform

A new model accounting for the origin of anomalously high elevations of the Siberian platform (SP) topography is presented. It is shown that the traditional interpretation of these topographic anomalies is at variance with the available evidence for the geological history of the SP development. The ideas elaborated in the paper are based on the concept of the formation of a mantle plume that has led to the supply of large volumes of molten material into the upper crust and surface basalt eruptions. A new approach is proposed for the construction of a density model of the Siberian upper mantle. A density model of the crust based on the available seismic and petrological data is constructed at the first stage. The calculated anomalous gravity field produced by this model is then subtracted from the observed field. The resulting residual mantle anomalies are used, together with seismological data, for the construction of an upper mantle density model. The formation of the present SP topography is shown to have been controlled by the thickening of the crust due to underplating caused by the development of a giant mantle plume at 251 Ma.

History and causes of post-Laramide relief in the Rocky Mountain orogenic plateau

The Rocky Mountain orogenic plateau has the highest mean elevation and topographic relief in the contiguous United States. The mean altitude exceeds 2 km above sea level and relief increases from 30 m in the river valleys of the Great Plains to more than 1.6 km deep in the canyons and basins of the Rocky Mountains and Colorado Plateau. Despite over a century of study, the timing and causes of elevation gain and incision in the region are unclear. Post-Laramide development of relief is thought to either result from tectonic activity or climatic change. Interpretation of which of these causes dominated is based upon reconstruction of datums developed from, and supported by, paleoelevation proxies and interpretations of landscape incision. Here we reconstruct a datum surface against which regional incision can be measured in order to evaluate late Cenozoic tectonic and climatic infl uences. The distribution, magnitude, and timing of post-Laramide basin fi lling and subsequent erosion are constrained by depositional remnants, topographic markers, and other indicators across the region. We suggest that post-Laramide basin fi lling resulted from slow subsidence during Oligocene to Miocene time. Incision into this basin fi ll surface began in late Miocene time and continues today. The pattern of incision is consistent with control by localized extensional tectonism superimposed upon regional domal surface uplift. Localized extension is associated with the projection of the Rio Grande Rift into the central Rockies, and the domal uplift generally coincides with the position of buoyant mantle anomalies interpreted at depth. If the magnitudes of incision directly refl ect magnitudes of surface elevation gain, they are less than can be resolved by existing paleoelevation proxy methods. In addition, the combination of post-Laramide subsidence followed by regional surface uplift reduces the net magnitude of surface elevation change since Laramide time.

A dynamic model for the Iceland Plume and the North Atlantic based on tomography and gravity data

SUMMARY The North Atlantic around Iceland is characterized by a large geoid high and an anomalous shallow ocean floor; both observations are presumably due to upwelling of hot material in the mantle. We extracted this region from a global tomography data set to study the structure of the mantle in more detail. The tomography data reveal a confined low-velocity structure from the core–mantle boundary (CMB) to the upper mantle, stretching from a location at the CMB below southwest Greenland towards the upper mantle in a strongly eastward inclination. In the present study we compare the observed gravity potential field in the North Atlantic with a modelled field based on mantle temperature variations estimated from tomography. Seismic traveltime residuals are converted to temperature variations assuming a linear relation between seismic velocity and density and a pressure-dependent thermal expansivity. We found a maximum excess temperature in the plume conduit at the CMB of ∼250 °C, weakly decreasing towards the phase transition zone (PTZ). In the PTZ a temperature rise of ∼50–70 °C is found, which is in agreement with the latent heat release by the olivine phase transitions. The 3-D temperature field is then used as the driving force for viscous flow in a Cartesian convection model. The model dimensions are chosen four times as large as the tomographic section to allow resolution for long wavelengths and convective return flow. For a number of constant viscosity and temperature- and depth-dependent viscosity cases the dynamic topography, gravity anomaly and geoid undulations are calculated and compared with the EGM96 potential field coefficients in the wavelength range of 400 to 4000 km. The observation data were also corrected for ocean lithosphere cooling and isostatic compensation of continental crust. The best agreement between observation and modelled data (78 per cent fit for geoid and 47 per cent for gravity) is obtained for a temperature-dependent viscosity of about one order of magnitude for 500 °C temperature variation and an increase of viscosity with depth by no more than a factor of 50 from the upper to the lower mantle. The generally good spatial agreement supports the tomographic model, at least for the upper mantle, and indicates that the East Greenland margin as well as the outer Faroer Ridge are dynamically supported. Low-density material west of the Kolbeinsey Ridge might be linked to low-density anomalies below the Greenland Shield. The presence of lower mantle anomalies causes a large-scale geoid high of ∼3 to 5 m in agreement with observations, but our approach cannot further constrain the spatial distribution of anomalies in the lower mantle.

Regional seismic wave propagation (Lg and Sn) and Pn attenuation in the Arabian Plate and surrounding regions

Continuous recordings of 17 broadband and short-period digital seismic stations from a newly established seismological network in Saudi Arabia, along with digital recordings from the broadband stations of the GSN, MEDNET, GEOFON, a temporary array in Saudi Arabia, and temporary short period stations in Oman, were analysed to study the lithospheric structure of the Arabian Plate and surrounding regions. The Arabian Plate is surrounded by a variety of types of plate boundaries: continental collision (Zagros Belt and Bitlis Suture), continental transform (Dead Sea fault system), young seafloor spreading (Red Sea and the Gulf of Aden) and oceanic transform (Owen fracture zone). Also, there are many intraplate Cenozoic processes such as volcanic eruptions, faulting and folding that are taking place. We used this massive waveform database of more than 6200 regional seismograms to map zones of blockage, inefficient and efficient propagation of the Lg and Sn phases in the Middle East and East Africa. We observed Lg blockage across the Bitlis Suture and the Zagros fold and thrust belt, corresponding to the boundary between the Arabian and Eurasian plates. This is probably due to a major lateral change in the Lg crustal waveguide. We also observed inefficient Lg propagation along the Oman mountains. Blockage and inefficient Sn propagation is observed along and for a considerable distance to the east of the Dead Sea fault system and in the northern portion of the Arabian Plate (south of the Bitlis Suture). These mapped zones of high Sn attenuation, moreover, closely coincide with extensive Neogene and Quaternary volcanic activity. We have also carefully mapped the boundaries of the Sn blockage within the Turkish and Iranian plateaus. Furthermore, we observed Sn blockage across the Owen fracture zone and across some segments of the Red Sea. These regions of high Sn attenuation most probably have anomalously hot and possibly thin lithospheric mantle (i.e. mantle lid). A surprising result is the efficient propagation of Sn across a segment of the Red Sea, an indication that active seafloor spreading is not continuous along the axis of the Red Sea. We also investigated the attenuation of Pn phase (QPn) for 1–2 Hz along the Red Sea, the Dead Sea fault system, within the Arabian Shield and in the Arabian Platform. Consistent with the Sn attenuation, we observed low QPn values of 22 and 15 along the western coast of the Arabian Plate and along the Dead Sea fault system, respectively, for a frequency of 1.5 Hz. Higher QPn values of the order of 400 were observed within the Arabian Shield and Platform for the same frequency. Our results based on Sn and Pn observations along the western and northern portions of the Arabian Plate imply the presence of a major anomalously hot and thinned lithosphere in these regions that may be caused by the extensive upper mantle anomaly that appears to span most of East Africa and western Arabia.

A resolved mantle anomaly as the cause of the Australian‐Antarctic Discordance

We present evidence for the existence of an Australian‐Antarctic Mantle Anomaly (AAMA), which trends northwest‐southeast (NW‐SE) through the Australian‐Antarctic Discordance (AAD) on the Southeast Indian Ridge (SEIR), is confined to the upper 120 km of the mantle beneath the AAD, and dips shallowly to the west so that it extends to a depth of about 150 km west of the AAD. Average temperatures within the AAMA are depressed about 100°C relative to surrounding lithosphere and suggest very rapid cooling of newly formed lithosphere at the AAD to an effective thermal age between 20 and 30 Ma. A convective down welling beneath the AAD is not consistent with the confinement of the AAMA in the uppermost mantle. In substantial agreement with the model of Gurnis et al. [1998], we argue that the AAMA is the suspended remnant of a slab that subducted at the Gondwanaland‐Pacific convergent margin more than 100 Myr ago, foundered in the deeper mantle, and then ascended into the shallow mantle within the past 30 Myr, cutting any ties to deeper roots. The stability of the AAMA and its poor correlation with residual topography and gravity imply that it is approximately neutrally buoyant. The thermally induced density anomaly can be balanced by bulk iron depletion of less than 0.8%, consistent with the warmer conditions of formation for the Pacific than Indian lithosphere. We hypothesize that the low temperatures in the AAMA inhibit crustal formation and the AAD depth anomaly is formed at the intersection of the SEIR and the AAMA. The northward migration of the SEIR overriding the cold NW‐SE trending AAMA therefore presents a simple kinematic explanation for both the V‐shaped residual depth anomaly in the southeast Indian Ocean and the western migration of the AAD along the SEIR. Neither explanation requires the Pacific asthenospheric mantle to push westward and displace Indian asthenosphere. The AAMA may also act as a barrier to large‐scale flows in the shallow asthenosphere and may therefore define a boundary for mantle convection and between the Indian and Pacific isotopic provinces. The westward dip of the AAMA would also favor along‐axis flow from the Indian Ocean asthenosphere to the AAD that may contribute to the penetration of Indian Ocean mid‐ocean ridge basalts into the AAD.

Lithosphere structure of Europe and Northern Atlantic from regional three-dimensional gravity modelling

SUMMARY Large-scale 3D gravity modelling using data averaged on a 1 ◦ grid has been performed for the whole European continent and part of the Northern Atlantic. The model consists of two regional layers of variable thickness—the sediments and the crystalline crust, bounded by reliable seismic horizons—the ‘seismic’ basement and the Moho surface. Inner heterogeneity of the model layers was taken into account in the form of lateral variation of average density depending on the type of geotectonic unit. Density parametrization of the layers was made using correlation functions between velocity and density. For sediments, sediment consolidation with depth was taken into account. Offshore a sea water layer was included in the model. As a result of the modelling, gravity effects of the whole model and its layers were calculated. Along with the gravity modelling an estimation of isostatic equilibrium state has been carried out for the whole model as well as for its separate units. Residual gravity anomalies, obtained by subtracting the gravity effect of the crust from the observed field, reach some hundred mGal (10 −5 ms −2 ) in amplitude; they are mainly caused by density heterogeneities in the upper mantle. A mantle origin of the residual anomalies is substantiated by their correlation with the upper-mantle heterogeneities revealed by both seismological and geothermal studies. Regarding the character of the mantle gravity anomalies, type of isostatic compensation, crustal structure, age and supposed type of endogenic regime, a classification of main geotectonic units of the continent was made. As a result of the modelling a clear division of the continent into two large blocks—Precambrian East-European platform (EEP) and Variscan Western Europe—has been confirmed by their specific mantle gravity anomalies (0 ÷ 50 × 10 −5 ms −2 and −100 ÷− 150 × 10 −5 ms −2 correspondingly). This division coincides with the Tornquist‐ Teisseyre Zone (TTZ), marked by a gradient zone of mantle anomalies. In the central part of the EEP (over the Russian plate) an extensive positive mantle anomaly, probably indicating a core of ancient consolidation of the EEP, has been distinguished. To the west and to the east of this anomaly positive mantle anomalies occur, which coincide with a deep suture zone (TTZ) and an orogenic belt (the Urals). Positive mantle anomalies of the Alps, the Adriatic plate and the Calabrian Arc, correlating well with both high-velocity domains in the upper mantle and reduced temperatures at the subcrustal layer, are caused by thickened lithosphere below these structures. Negative mantle anomalies, revealed in the Western Mediterranean Basin and in the Pannonian Basin, are the result of thermal expansion of the asthenosphere shallowing to near-Moho depths below these basins.

Constraints on the correlation ofP- andS-wave velocity heterogeneity in the mantle fromP,PP,PPPandPKPab traveltimes

We investigate the correlation of large-scale P- and S-velocity heterogeneity in the mantle by determining how well 106,000 compressional P, PP, PPP, and PKPab traveltimes can be explained by S-wave velocity model S20RTS(scaled using a depth dependent factor) and by a model in which the lateral P-velocity variations are different. We first assess the assumption that P-wave traveltimes can be explained by a model in which lateral P-velocity variations (δν P ) are identical to S-velocity variations (δυ S ) in model S20RTS. For a given depth, we project δυ S from S20RTS into model S2P using a depth-dependent scaling factor R defined as: δυ S = R(z) x δυ P . We find, by grid search, that the highest reduction of data variance is obtained when R increases linearly from 1.25 at the surface to 3.0 at the core-mantle boundary. A comparison of S-wave (+SS) and P-wave (+PP) traveltimes for identical source-receiver pairs also indicates that R increases with depth. Significantly higher variance reduction is not obtained when R is parametrized with an additional degree of freedom. Therefore, the precise shape of R cannot be constrained by our data. P- and PP-wave traveltime anomalies with respect to the scaled model S2P yield coherent geographic variations. This indicates that there are large-scale lateral P-velocity variations in the mantle that are different from those in model S2P. We estimate these variations by inverting P-wave traveltime anomalies with respect to model S2P for a degree 12 model of P-velocity heterogeneity. This model, P 12 s 2 p , indicates where in the well-sampled mantle regions we need to modify model S2P to further improve the fit to the traveltime data. Anomalies in P12 s 2 p exist throughout the mantle. It is, therefore, not obvious that compositional heterogeneity is prominent in the lower 1000 km of the mantle only, as suggested previously. Low P-wave velocities in the upper mantle beneath oceans are the strongest anomalies in P12 s 2 p and explain better the delayed traveltimes of PP-wave phases with oceanic surface refection points. Lower mantle anomalies include high and low P-velocity structures beneath eastern Asia and North America, respectively. The high P-velocity anomaly in the lower mantle beneath the central Pacific is consistent with the assertion made by other researchers that large-scale lower mantle upwellings are not purely thermal in origin.

Predicting plate velocities with mantle circulation models

We predict plate motions from a comprehensive inversion of theoretical estimates of tectonic forces in order to evaluate the relative importance of these and the uncertainties of such models. Plate‐driving forces from the mantle are calculated using global flow models that are driven by tomography and subduction‐derived density fields. Observed and predicted plate velocities agree well for a variety of models, leading to varied conclusions about the relative importance of forces. The dominance of the subduction related density pattern in the mantle is confirmed; it appears that P wave models do not satisfactorily image all of the slab‐associated anomalies in the upper mantle. Furthermore, lower mantle structure always improves the plate motion fit with respect to models that are based on upper mantle anomalies and lithospheric thickening only. We show that the average torques from the lower mantle scale with the radial flow through the 660‐km phase transition; the amplitude of the lower mantle torques will be significant for a range of models if there is mass flux through 660 km. We also evaluate parameterized edge forces and find that the additional inclusion of such torques does not significantly improve the model fit. The main reason for the nonuniqueness of the inversions is plate boundary geometry since all plate motions are dominated by the trench‐ridge system, and plates move from ridges to trenches.

Lithosphere structure of European sedimentary basins from regional three-dimensional gravity modelling

A three-dimensional (3D) density model, approximated by two regional layers—the sedimentary cover and the crystalline crust (offshore, a sea-water layer was added), has been constructed in 1° averaging for the whole European continent. The crustal model is based on simplified velocity model represented by structure maps for main seismic horizons—the “seismic” basement and the Moho boundary. Laterally varying average density is assumed inside the model layers. Residual gravity anomalies, obtained by subtraction of the crustal gravity effect from the observed field, characterize the density heterogeneities in the upper mantle. Mantle anomalies are shown to correlate with the upper mantle velocity inhomogeneities revealed from seismic tomography data and geothermal data. Considering the type of mantle anomaly, specific features of the evolution and type of isostatic compensation, the sedimentary basins in Europe may be related into some groups: deep sedimentary basins located in the East European Platform and its northern and eastern margins (Peri-Caspian, Dnieper–Donets, Barents Sea Basins, Fore–Ural Trough) with no significant mantle anomalies; basins located on the activated thin crust of Variscan Western Europe and Mediterranean area with negative mantle anomalies of −150 to −200×10 −5 ms −2 amplitude and the basins associated with suture zones at the western and southern margins of the East European Platform (Polish Trough, South Caspian Basin) characterized by positive mantle anomalies of 50–150×10 −5 ms −2 magnitude. An analysis of the main features of the lithosphere structure of the basins in Europe and type of the compensation has been carried out.