While the bulk of topography on Earth is generated and maintained by variations in the thickness and density of crust and lithosphere, a significant time-variable contribution is expected as a result of convective flow in the underlying mantle. For over three decades, this dynamic topography has been calculated numerically from inferred density structure and radial viscosity profiles. Resulting models predict ±2 km of long wavelength (i.e., ~ 20,000 km) dynamic topography with minor contributions at wavelengths shorter than ~ 5,000 km. Recently, observational studies have revealed that, at the longest wavelengths, dynamic topography variation is ~ 30% that predicted, with ±1 km amplitudes recovered at shorter wavelengths. Here, the existing database of water-loaded basement depths is streamlined, revised and augmented. By fitting increasingly sophisticated thermal models to a combined database of these oceanic basement depths and corrected heat flow measurements, the average thermal structure of oceanic lithosphere is constrained. Significantly, optimal models are consistent with invariable geochemical and seismological constraints whilst yielding similar values of mantle potential temperature and plate thickness, irrespective of whether heat flow, subsidence or both are fit. After recalculating residual depth anomalies relative to optimal age-depth subsidence and combining them with continental constraints from gravity anomalies, a global spherical harmonic representation is generated. Although, long wavelength dynamic topography increases by ~ 40% in the revised observation-based model, spectral analysis confirms that a fundamental discrepancy between observations and predictions remains. Significantly, residual depth anomalies reveal a ~4,000 km-scale eastward tilt across the Indian Peninsula. This asymmetry extends onshore from the high-elevation Western Ghats in the west to the Krishna-Godavari floodplains in the east. Calibrated inverse modelling of drainage networks suggest that the tilt of the peninsula grew principally in Neogene times with vertical motions linked to asthenospheric temperature anomalies. Uplift rates of up to 0.1 mm a⁻¹ place important constraints on the spatio-temporal evolution of dynamic topography and suggest that rates of transient vertical motion exceed those predicted by many modelling studies. Most numerical models excise the upper ~ 300 km of Earth's mantle and are unable to reconstruct the wavelength and rate of uplift observed across Peninsular India. By contrast, through conversion of upper mantle shear wave velocities to density using a calibrated anelastic parameterisation, it is shown that shorter wavelength (i.e., ≤ 5,000 km) dynamic topography, can mostly be explained by ±150°C asthenospheric temperature anomalies. Inclusion of anelastically corrected density structure in whole-mantle instantaneous flow models also serves to reduce discrepancy between predictions and observations of dynamic topography at long wavelengths. Residual mismatch between observations and predictions is further improved if the basal 300-600 km of large low shear wave velocity regions in the deep mantle are geochemically distinct and negatively buoyant. Finally, inverse modelling of geoid, dynamic topography, gravity and core-mantle boundary topography observations using adapted density structure suggests that geodynamic constraints can be acceptably fit using plausible radial viscosity profiles, contradicting a long-standing assertion that modest long wavelength dynamic topography is incompatible with geoid observations.

No description available.

The mean dynamic topography (MDT) of the ocean is closely related to the mean surface circulation. The objective of this thesis is to produce estimates of the MDT for the North Atlantic using newly available data from ocean and gravity observing systems, to evaluate these new estimates and so improve our understanding of the circulation. Oceanographic estimates of the MDT are based on the mean temperature and salinity (TS) fields of the ocean. These are typically averages from sparse observations collected over many decades. The ocean is a non-stationary system so it is more appropriate to define the mean for shorter, specific periods. Whilst the Argo observing system has increased the frequency and resolution of in situ oceanographic measurements, high frequency (eddy) variability remains. A new technique is described for removing this variability using satellite altimeter measurements of the sea surface height. A new TS mean is produced, relating to the period from 2000 to 2007, and this is used to map a new oceanographic estimate of the MDT using an ocean circulation model. New geodetic estimates of the MDT are produced using geoid models that incorporate gravity measurements from the ongoing GRACE and GOCE satellite missions. These are compared with the new oceanographic estimate and validated against independent observations such as drifter speeds. The geodetic method produces realistic estimates of the mean surface circulation, thereby realizing the long time dream of oceanographers to observe the ocean circulation from space. The new oceanographic estimates are not as accurate, but the new TS mean contributes to improvements in the performance of ocean models, a necessary step in understanding and predicting the oceans. Coastal tide gauges can provide an accurate estimate of the alongshore tilt of the coastal MDT and this has been used to evaluate the above estimates. Temporal variability of the tilt along the coast of the South Atlantic Bight is used, with statistical methods and an ocean circulation model, to identify the processes contributing to the tilt. A new opportunity to use tide gauges as part of an observing system for the ocean circulation is discussed.

Isostasy, flexure and dynamic processes all influence the shape of the Earth’s surface. While the first two processes are well understood, dynamic topography remains controversial. On the continents, dynamic uplift is often expressed by positive long-wavelength gravity anomalies, radial drainage patterns, and slow seismic velocity anomalies within the upper mantle. Volcanic activity and elevated heat flow are also often observed. The aim of this study is to investigate the link between geochemical compositions of intracontinental magmatism and geophysical, geomorphological and geodetic observations of dynamic uplift. Three volcanic regions are considered in detail: western North America, northeast Brazil and Madagascar. The combined database includes 348 new whole-rock geochemical analyses. Rare earth element concentrations of mafic, asthenospheric-derived volcanic samples are exploited to calculate the depth and temperature of melt generation by inverse modelling. A sensitivity test of this modelling scheme is carried out. Lithospheric thickness and mantle temperature are independently determined from shear wave velocity models. Beneath western North America, a negative correlation between shear wave velocities at depths of 70–150 km and degree of melting is observed. Temperatures obtained from igneous compositions and from shear wave velocity profiles beneath volcanic fields closely agree. Melts are produced within, or close to, the spinel-garnet transition zone at depths shallower than $\sim$70 km, yielding mantle potential temperatures of up to 1380$^{\circ}$C. Calculated uplift and heat flow based upon these results match observed surface elevation and heat flow measurements. In northeast Brazil, Jurassic, Cretaceous and Cenozoic phases of mafic igneous activity are recognised. Jurassic magmatic activity probably resulted from spinel-field melting at potential temperatures of $\sim$1380$^{\circ}$C. This episode is associated with regional magmatism during break-up of the Central Atlantic Ocean. Cretaceous compositions record melting at potential temperatures of 1330–70$^{\circ}$C at similar depths. This activity is linked to extension at the time of break-up of the equatorial and South Atlantic Ocean. Cenozoic volcanism comprises low-degree melts within the spinel-garnet transition zone at ambient potential temperature. Shear wave velocity models support these results. Cenozoic volcanism in Madagascar is predominantly alkaline and records small-degree melting with minor temperature anomalies within the spinel-garnet transition zone. Rare tholeiitic basalts record temperatures up to 1360$^{\circ}$C. Analysis of global and regional shear wave velocity models closely matches these results. The principal control on continental magmatism appears to be temperature anomalies within the upper mantle beneath thin lithosphere. Highest mantle potential temperatures correlate with largest dynamic uplift. Mantle potential temperatures $ < $1350$^{\circ}$C are matched with minimal or negative dynamic topography.

The Icelandic mantle plume has had a profound influence on the development of the North Atlantic region over its 64 Myr existence. Long-wavelength free-air gravity anomalies and full waveform tomographic studies suggest that the planform of the plume is highly irregular, with up to five fingers of hot asthenosphere radiating away from Iceland beneath the lithospheric plates. Two of these fingers extend beneath the British Isles and southern Scandinavia, where departures from crustal isostatic equilibrium and anomalous uplift have been identified. In this study, the spatial extent of present-day dynamic support associated with the Icelandic plume is investigated using receiver function analysis. Teleseismic events recorded at nine temporary and 59 permanent broadband, three-component seismometer stations are used to calculate 3864 P-to-S crustal receiver functions. The amplitude and arrival time of particular converted phases are assessed, and H-k stacking is applied to estimate bulk crustal properties. Sub-selections of receiver functions are jointly inverted with Rayleigh wave dispersion data to obtain crustal VS profiles at each station. Both inverse- and guided forward modelling techniques are employed, as well as a Bayesian, trans-dimensional algorithm. Moho depths thus obtained are combined with seismic wide-angle and deep reflection data to produce a comprehensive crustal thickness map of northwestern Europe. Moho depth is found to decrease from southeast (37 km) to northwest (26 km) in the British Isles and from northeast (46 km) to southwest (29 km) in Scandinavia, and does not positively correlate with surface elevation. Using an empirical relationship, crustal shear wave velocity profiles are converted to density profiles. Isostatic balances are then used to estimate residual topography at each station, taking into account these novel constraints on crustal density. Areas of significant residual topography are found in the northwestern British Isles (1400 m), southwestern Scandinavia (464 m) and Denmark (620 m), with convective support from the Icelandic plume as its most likely source. Finally, the irregular planform of the Icelandic plume is proposed to be a manifestation of radial viscous fingering due to a Saffman-Taylor instability. This fluid dynamical phenomenon occurs when less viscous fluid is injected into a layer of more viscous fluid. By comparing the thermal and convective characteristics of the plume with experimental and theoretical results, it is shown that viscous fingering could well explain the present-day distribution of plume material.

No description available.

It is recognised that some proportion of South American large-scale topography has been generated by convection within the Earth's mantle. Yet, spatial and temporal patterns of dynamic topography remain poorly understood. Variation of present-day dynamic topography can be mapped in the oceans by calculating residual depths with respect to the well-known age-depth relationship. Along the margins abutting South America, anomalies with amplitudes of $\pm1$ km and wavelengths of $\sim10^{3}$ km are observed. Onshore, dynamic topography is investigated by analysing a range of disparate datasets. Positive long-wavelength free-air gravity anomalies and slow shear-wave mantle velocities correlate with high plateaux of the Borborema Province and the Central Andean Altiplano. Admittance analyses of these regions are used to gauge dynamic support. Admittance of $ > 20$ mGal km$^{-1}$ at wavelengths $ > $ 500 km suggests partial dynamic support. In this context, inverse modelling of longitudinal river profiles is applied to retrieve a continental-scale uplift history. Erosional parameters are calibrated against an independently derived uplift history of the Borborema Province that reveals uplift in the last 30 Ma. Results suggest that the bulk of South American regional topography grew during Cenozoic times. In the Central Andean Altiplano and Southern Patagonia, most uplift occurred in the last 20 Ma. In both areas, widespread Cenozoic magmatism suggests that youthful uplift might be related to asthenospheric upwelling. Uplift histories are used to predict sediment flux to the Amazon Fan, which reveals that onset of the delta is a direct consequence of intensified Andean uplift. Analysis of the Parna\'iba cratonic basin of northeast Brazil is carried out to evaluate long-term evolution of vertical motions and to understand the mechanisms driving this basin's subsidence. Joint interpretation of a deep seismic reflection profile that traverses the basin and receiver function analyses reveal a 3 km thick basin underlain by three crustal blocks. Moho depths of 38--43 km are observed beneath the Amazon craton west of the basin, whereas depths of 35 km are found underneath the Borborema Province to the east. The Moho is located at 38--42 km depth beneath the basin. Stratigraphic architecture from shallow seismic reflection profiles reveals undisturbed deposition between Palaeozoic and Mesozoic times. Rift-type structures are locally imaged. Subsidence analysis reveals thermally-driven subsidence with thermal time constants of $\sim$ 70--80 Ma. Assessment of crustal thickness variations indicates that minimal extension of up to 80 km, with small stretching factors (up to 1.15), is plausible beneath Parna\'iba. One- and two-dimensional strain rate histories suggest that pre-Silurian rifting followed by thermal subsidence is possible if a minimum of 1 km of syn-rift deposition occurred. Basin-wide erosional unconformities are observed throughout the sedimentary section and correlate with departures from long-term subsidence trends. These steps are interpreted as transient uplift events that led to development of ephemeral landscapes, suggesting that dynamic topography could have played a role in the evolution of this Phanerozoic basin.

En défléchissant la Terre, la topographie dynamique module l'extension des zones inondées dans les régions où l'altitude est proche du niveau marin. Ce phénomène contribue ainsi à modifier la paléogéographie à grande échelle et ont un impact sur les sphères externes (atmo-, hydro- et bio-sphère) en altérant notamment les circulations atmosphériques et océaniques. Ces travaux de thèse, qui s’appuient sur une approche interdisciplinaire, illustrent la chaîne de connections entre dynamique mantellique et climat à travers l'étude de l'évolution Quaternaire du Continent Maritime. Le caractère insulaire de la région et la présence de mers peu profondes comme la mer de Java, permettent des modifications rapides de la répartition terre-océan à grande échelle, et en font un cas idéal pour étudier les connections entre géodynamique et climat. D’autre part, la dynamique mantellique, excitée par les nombreuses subductions, y est très active et contribue à déformer la surface et la dynamique climatique régionale est étroitement associée à la géographie particulière de l’archipel Indonésien.Les changements paléogéographiques sont d'abord révélés par la cartographie des morphologies côtières. Celle-ci souligne la répartition contrastée de la déformation Quaternaire en soulignant le soulèvement général de la région centrale (Wallacea), alors que les deux plateformes continentales localisées à l'Ouest et au Sud-Est subsident. L'utilisation combinée des observations et de la modélisation de la croissance des récifs coralliens est utilisée afin de quantifier la vitesse verticale de déformation. Notre méthode est basée sur la comparaison entre la morphologie des récifs observés sur la plateforme de la Sonde, à l'ouest de l'Asie du Sud-Est, et les morphologies récifales issues des simulations numériques et permet une quantification inédite de la vitesse de subsidence de la plateforme. Les résultats suggèrent que la Sonde était émergée de manière permanente avant 400 000 ans, formant une masse continentale entre les îles de l'Ouest Indonésien et le continent asiatique. Les causes de ces changements paléogéographiques sont appréhendées à l'aide de la modélisation mécanique de la géodynamique. Un modèle numérique en trois dimensions d'une zone de subduction a été utilisé afin de d'explorer les causes dynamiques de la déformation. L'analyse des simulations permet de décrire l'évolution spatio-temporelle de la déformation à l'aplomb d'une zone de subduction, lors d'une perturbation provoquée par l'arrivée dans la fosse d'un bloc continental ou d'un plateau océanique, un cas simplifié similaire à l'Asie du Sud-Est. Les résultats montrent que lors d'un épisode de collision, l'initiation d'une déchirure dans la plaque en subduction générée par l'entrée dans la fosse de matériel peu dense entraîne une modification de l'écoulement mantellique. Cette modification provoque un épisode de subsidence dynamique qui fait suite à un épisode de surrection provoquée par la collision. Les vitesses de déformations calculées ont un ordre de grandeur comparable aux vitesses de déformations enregistrées et modélisées à l'échelle régionale. Les conséquences des changements paléogéographiques sont appréhendées à l'aide d'un modèle du climat IPSL-CM5A2. Les résultats montrent que la présence d'une plateforme de la Sonde émergée provoque une augmentation saisonnière des précipitations sur le Continent Maritime. Cette augmentation est engendrée par une intensification de la convergence à l'échelle régionale contrôlée par le chauffage radiatif des surfaces continentales exposées. L'exposition de la plateforme de la Sonde engendre également une modification du transport dans le détroit de Makassar avec un impact local sur la salinité et les températures de surface de l'océan. Nos analyses montrent par ailleurs que l'augmentation de la saisonnalité des précipitations est indépendante de la paramétrisation de la convection et des nuages dans le modèle. / Dynamic topography modulates the extension of inundated areas, at places where elevation is near sea level, by deflecting the surface of the Earth. This phenomenon produces large-scale paleogeography changes, which in turn modify external spheres (atmo-, hydro- and biosphere) by subsequent alteration of atmospheric and oceanic circulations and biodiversity. This inter-disciplinary work illustrates the connection string between Earth mantle dynamics and climate through the study of Quaternary evolution of South East Asia. The insularity of the region and the presence of low bathymetry seas, as the Java sea, enable fast and efficient modifications of land-sea mask and make it an ideal case for studying the connection between geodynamics and climate. Mantle flow, excited by the numerous subduction zones, is vigorously stirred and contributes to surface deformation. In this region, climate dynamics is also tightly related to the peculiar geography of the Indonesian archipelago. Paleogeographic changes are first revealed by coastal morphologies. They show the contrasted pattern of large-scale Quaternary deformation that underlines general uplift within the central-eastern part of the region, namely Wallacea, whereas the continental shelves, to the West and Southeast, are more likely subsiding. The combination of field observations with numerical modeling of coral reef growth is used to quantify vertical deformation. Our method is based on reef morphology (terrace number, depth, modern reef length) that we observed on the Sunda shelf (Western South East Asia) and reef morphologies obtained by numerical modeling, and enable an original quantification of subsidence rates of the platform. The results imply that Sundaland region was entirely and permanently emerged before 400 000 yr and formed at this time a unique continental mass between West Indonesian islands and continental Asia. The causes of paleogeographic changes are explored using modeling of regional geodynamics. A three-dimension subduction numerical model was devised to simulate the dynamical origin of deformation. This model analysis enables us to describe the spatio-temporal evolution of the deformation above a subduction zone in case of perturbation induced by the arrival at the trench of a continental block or oceanic plateau, a simplified case that is similar to SE Asia. Our results show that during a collisional episode, slab tearing generated by the arrival of light material unable to subduct is responsible for changes in mantle convection. Those changes are responsible for dynamic subsidence that followed an uplift event related to the first stages of collision. Inferred deformation rates have an range of magnitude similar to both measured and modeled rates at regional scale. The consequences of paleogeographic changes are studied using general circulation model simulations. Results show that the presence of an emerged Sunda shelf leads to a seasonal increase in precipitation over the Maritime Continent. This increase is related to seasonal increase in large-scale convergence induced by thermal heating of exposed land surfaces, a situation that, as we show, occurred before 400 ka. Sunda shelf exposure is also responsible for changes in horizontal water transport within the Makassar strait that modify sea surface salinities and temperatures at local scale. Our analysis further shows that increased precipitation seasonality is independent on model convection and cloud parameterization

Constituant la couche limite supérieure de la convection mantellique, la lithosphère terrestre est à l'interface entre les enveloppes externes et internes de notre Planète. Les interactions multiples entre celle-ci et le manteau sont à l'origine de déformations latérales (tectonique des plaques) et verticales (topographie dynamique) de la surface terrestre. Comprendre comment la formation et l'évolution d'instabilités convectives mantelliques renouvellent sans-cesse la surface est donc primordial pour améliorer nos interprétations d'un grand nombre d'observations de surface, telles que la formation de bassins sédimentaires, le mouvement des continents, la localisation des points chauds, la formation d'anomalies gravimétriques ou encore les variations du niveau marin.Cette thèse propose de développer des modèles numériques de convection mantellique générant defaçon auto-organisée de la tectonique des plaques en surface an d'étudier la façon dont le développement et la dynamique d'instabilités convectives telles que les panneaux de subduction ou les panaches mantelliques modifient la surface, dans un contexte de tectonique de surface approchant le régime terrestre.Dans une première partie, je m'intéresse à l'influence du couplage des mouvements de convection mantellique et de tectonique des plaques sur le développement de topographie dynamique (i.e. les mouvements verticaux de la lithosphère induits par la convection mantellique) à différentes échelles spatio-temporelles. Mes résultats suggèrent que la surface terrestre peut se déformer à toutes les échelles spatiales, du fait de mouvements convectifs de grande ampleur faisant intervenir le manteau entier (> 104 km) ou encore de convection à petite échelle sub-lithosphérique (< 500 km). Les variations temporelles de topographie dynamique s‘étendent de cinq à plusieurs centaines de millions d'années selon la nature des processus convectifs dont elles dérivent. En particulier, la dynamique d'initiation ou d'arrêt des zones de subduction contrôle l'existence d'échelles intermédiaires de topographie dynamique (longueurs d'onde variant entre 500 et 104 km). Ces résultats montrent donc que les interactions entre la dynamique de la lithosphère et la convection mantellique génèrent des motifs spatio-temporels de topographie dynamique complexes et cohérents par rapport aux observations terrestres.Dans un deuxième temps, cette thèse se focalise sur la dynamique des panaches mantelliques, et leurs interactions avec la surface. Je caractérise d'abord précisement le comportement des panaches générés dans nos modèles de convection à la lumière d'observations de surface. Puis, j'étudie la façon dont leurs interactions avec la tectonique de surface et les différentes échelles convectives modifient leurs mouvements latéraux. Enfin, la compréhension de la signature thermique des interactions entre panaches et rides océaniques me permet de proposer une reconstitution des mouvements relatifs entre le panache des Açores et la ride médio-Atlantique. / Earth's lithosphere, which is the upper boundary layer of mantle convection, represents the interface between the external and internal envelopes of our Planet. The multiple interactions between the mantle and lithosphere generate lateral (plate tectonics) and vertical (dynamic topography) deformations of Earth's surface. Understanding the influence of the dynamics of mantle convective instabilities on the surface is fundamental to improve our interpretations of a large range of surface observations, such as the formation of sedimentary basins, continental motions, the location of hotspots, the presence of gravity anomalies or sea-level variations.This thesis aims at developing numerical models of whole-mantle convection self-generating plate-like tectonics in order to study the impacts of the development and the dynamics of mantle convective instabilities (such as slabs or mantle plumes) on the continuous reshaping of the surface.First, I focus on the influence of the coupling between mantle convective motions and plate tectonics on the development of dynamic topography (i.e. surface vertical deformations induced by mantle convection) at different spatial and temporal scales. The results suggest that Earth's surface can deform over large spatial scales (> 104 km) induced by whole-mantle convection to small-scales (< 500 km) arising from small-scale sub-lithospheric convection. The temporal variations of dynamic topography range between five and several hundreds of millions of years depending on the convective instabilities from which they originate. In particular, subduction initiation and slab break-off events control the existence of intermediate scales of dynamic topography (between 500 and 104 km). This reflects that the interplay between mantle convection and lithosphere dynamics generates complex spatial and temporal patterns of dynamic topography consistent with constraints for Earth.A second aim of this thesis is to understand the dynamics of mantle plumes and their interactionswith surface. I first characterize in detail the behaviour of mantle plumes arising in models ofwhole-mantle convection self-generating plate-like tectonics, in light of surface observations. Then, I study how the interactions between surface plate tectonics and mantle convection affect plume motions. Finally, I use observations of the thermal signature of plume/ridge interactions to propose a reconstruction of the relative motions between the Azores mantle plume and the Mid-Atlantic Ridge.