Gabriele Marquart

Large-scale lithospheric stress field and topography induced by global mantle circulation

Stresses in the lithosphere are one indication of processes in the Earth interior: here we present a calculation of largescale lithospheric stresses caused by global mantle circulation. The mantle flow field is calculated based on density structures inferred from global seismic tomography. Predicted principal stress directions are compared to interpolations based on observed stresses. Agreement between predictions and observations is often good in regions where lithospheric stresses and mantle tomography are well constrained. Predicted magnitudes of scalar stress anomalies vary more strongly than predicted stress directions for various tomographic models. Hotspots preferentially occur in regions where calculated stress anomalies are tensile or slightly compressive. Results do not strongly depend on radial mantle viscosity structure, lithospheric rheology (viscous or elastic) or plate motion model. The model also predicts the directions of motion well for most plates; misfits in the predicted magnitudes can be explained qualitatively. Stress anomalies due to causes within the lithosphere (oceanic cooling with age, variations in crustal thickness, topography isostatically compensated at subcrustal levels) are also computed. Predicted stress directions in the absence of mantle flow can explain observations almost as well as mantle flow. Nevertheless, current models of mantle flow are largely in accord with interpolations of observed principal stress directions and the observed plate motions. fl 2001 Elsevier Science B.V. All rights reserved.

Finite deformation in and around a fluid sphere moving through a viscous medium: implications for diapiric ascent

Abstract Different types of geodynamically active regions like mountain belts, hot spots, or areas of salt tectonics are characterized by diapirism. One key for the reconstruction of the dynamic history of such structures is the progressive strain, which can sometimes be determined from field observations. Successful interpretation of such observations requires a quantitative model of the finite strain in and around a rising diapir. A constant viscosity fluid sphere of radius R rising through another isoviscous fluid is assumed to approximate the buoyant motion of a diapir. Known analytical solutions for the velocity fields are used to numerically evaluate the finite deformation in and around the sphere. Regions of very high strains are found in a tube with a radius 1 2 R behind the sphere and in a shell of thickness of 0.1R around the sphere. The following three-dimensional strain regimes can be identified: In the half space above the sphere down to its equator finite strains are oblate. Behind the sphere they progressively change to plane strain. In the diapiric source region they are prolate. Viscous drag at the spheres surface leads to an internal circulation with one overturn after 10R of rise, if the sphere has a relatively low viscosity. Finite strains within the fluid sphere show a continuous increase with superimposed cyclic straining and unstraining. After several body radii of rise, the strains become highly inhomogeneous inside the sphere except along the vertical axis and just inside the spheres surface, where strong prolate and oblate strains are observed, respectively. Finite strain determinations in a falling ball experiment (Cruden, 1988) are compared with the theoretical results. At horizontal distances of a few body radii from the fall axis, the effect of confining container walls is clearly seen in the experimental strain data. The results are compared briefly with available strain data from the field which seem to be significantly lower than predicted. Proposed explanations include a short memory of the rock fabric, and a lack of recognition of the strain concentration which would be expected for a temperature-dependent rheology. It might also be possible that few natural diapirs rise more than a few radii in the solid state.

The influence of second-scale convection on the thickness of continental lithosphere and crust

Abstract The effect of sublithospheric convection on the thickness and structure of the continental lithosphere is studied in numerical models assuming different rheologies (Newtonian, non-Newtonian, and temperature and pressure dependent), heat fluxes, and heating modes (bottom versus internal heating). The stagnant regions near the top of the models are identified with the lithosphere. We distinguish a seismic and thermal lithosphere (controlled by temperature), an elastic lithosphere (controlled by viscosity), and a mechanical lithosphere (controlled by strain rate). Strong lateral thickness undulations with a thick lithosphere above the downwelling regions and a thin lithosphere above the upwelling regions develop. The thickness of the elastic lithosphere is about two-thirds of the mechanical lithosphere, while the ratio of the elastic to the thermal lithosphere varies between 0.4 and 0.8. The time dependence of some models (with internal heating and a surface heat flow of 20 mW/m 2 ) is characterized by long periods (of the order of 1 Ga) close to steady state and short periods (100 Ma) of changing cell patterns and lithospheric thicknesses. Models showing thick lithospheric roots suggest that the mantle beneath some old shields may be associated with cold, slowly downwelling convective flow rather than being firmly attached to the lithosphere. The missing gravity and topography signals from such regions can be explained by elastic bending of the lithosphere and by dynamic adjustment of the Moho. Observed lithospheric thickness variations in Europe and their minimum to maximum spacing agree well with the models, suggesting sublithospheric convection to be the cause. Compared to other observed features, thickness undulations of the lithosphere seem to provide the strongest indications for sublithospherical convection. Relatively low viscosities in the lower crust may lead to a long-term decoupling of surface topography from Moho topography and mantle dynamics. The Moho depth is capable of adjusting to mantle stresses within relaxation times of the order of 100 Ma, while surface topography may flatten out. Such a mechanism may be important in regions with a deep Moho such as Fennoscandia or with a shallow Moho such as the Pannonian Basin, neither region showing a pronounced surface topography.

Mantle flow and the evolution of the lithosphere

Abstract The evolution of the lithosphere is mainly controlled by time-dependent forces due to (1) plate tectonic processes and (2) sublithospheric mantle flow. Plate tectonic processes like continental collision may provide strong thermal disturbances and, after completion, may trigger secondary convection beneath the lithosphere. Without such mantle flows lateral variations of temperature (and associated variations of lithospheric thickness hL or seismic velocities) will equilibrate after time scales which are considerably shorter than the geologic ages of several provinces in Europe. We will discuss the role of sublithospheric mantle flows on the evolution of the lithosphere and the asthenosphere and compare the results with observations from the European lithosphere. Steady-state convection models with a rheology based on laboratory data on lherzolite show, that there exists a simple relationship between mantle heat flow and hL. However, steady state may be reached only after transition times of the order of 1 Ga. During such times, hL shows strong lateral variations. If such variations were inherited from plate tectonic events, the role of sublithospheric convection would be to prolong their lifetime considerably. At higher heat flows, sublithospheric mantle convection becomes strongly time-dependent with a time scale of 25–50 Ma. On this time scale, variations of hL are small. Altogether, we observe no strong direct correlation between upwelling mantle flows and thin lithosphere. In some cases the correlation is even reversed. On the other hand, the presence or absence of a partially molten asthenosphere may be strongly affected by sublithospheric convection, and may show strong lateral variations as well. These variations in space and time can be correlated with the laterally heterogeneous LVZ, with seismic tomography data and with intraplate volcanism in Europe. Mantle convection, mantle diapirs, and variations in thickness of the lithosphere may exert forces on the lithosphere and crust. They will lead to bending of the lithosphere, changes in surface topography and thinning or thickening of the crust, induced by lateral flows within the lower crust. Depending on the time scale of loading and unloading of the lithosphere and the relaxation time associated with lower crustal flows characteristic relations between Moho depth and surface topography are predicted. Comparison with observed correlations between Moho depth, surface topography and lithospheric thickness of European crustal age provinces show some trends in agreement with the proposed crustal deformation model.

Isostatic topography and crustal depth corrections for the fennoscandian geoid

Abstract The medium wavelength geoid (wavelengths The result of this study demonstrate that crustal thickness variations are important for the interpretation of the Fennoscandian geoid or gravity. However, submoho deviatoric stresses might become very high (about 200 MPa) if the entire anomaly is assumed to be supported by lateral density variations at less than 80 km depth. This implies that the pronounced negative anomaly is additionally maintained by dynamical flow inside the mantle.

Conditions for plumes to penetrate the mantle phase boundaries

At a depth of ∼660 km in the Earths mantle the spinel-perovskite phase boundary is a prominent barrier for mantle convection. This is due to the negative Clapeyron slope of the phase equilibrium curve which leads to an elevation of the phase boundary within hot upwellings causing negative buoyancy forces. We have investigated the conditions for rising plumes either to penetrate and pass the spinel-perovskite phase boundary or to stick and spread below it by studying the fundamental physics of this process. The plume heads were simply modeled as hot three-dimensional (3-D) spheres or 2-D cylinders. A simple calculation balancing the positive thermal and the negative phase bouyancy forces leads to a better parameterization using two dimensionless quantities. In addition to the phase buoyancy parameter, we defined a deflection parameter, relating the elevation of the phase boundary to the plume head radius to account for the geometrical shape of a plume head. This parameterization is further tested with numerical models that include the effects of thermal diffusion, latent heat, the olivine-spinel phase boundary at a depth of 410 km, and temperature and/or phase-dependent viscosity structure. For laboratory estimates of the slope (-3 MPa/K) and density increase at the spinel-perovskite phase boundary (250 kg/m 3 ) our models predict that plumes with excess temperatures of 50°-600°C will stick at the top of the lower mantle if their head radii are less than ∼100 km. Plumes will penetrate into the upper mantle if plume head radii exceed 100 km. While the style of plume penetration or spreading at the top of the lower mantle strongly depends on the viscosity structure, the conditions for penetration do not. All rising hot volumes with nonpenetrating conditions stick at the top of the lower mantle and spread laterally, independent of their viscosity structure. For weakly nonpenetrating conditions, heat diffusion increases the radius of the hot volume and leads to penetration during a secondary stage. For strongly nonpenetrating conditions, spreading at the top of the lower mantle drives a mechanically coupled counterflow in the upper mantle, which is stable for a very long time. For volumes with an excess temperature of more than 350°C heat diffusion across the phase boundary will eventually inhibit this counterflow and stabilize a thermally coupled flow which might entrain some material of the hot volume. However, our results suggest that the spinel-perovskite phase boundary is unlikely to inhibit the penetration of mantle plumes of the size thought to have generated many flood basalt provinces and hotspot chains.

Arctic geodynamics: A satellite altimeter experiment for the European Space Agency Earth Remote‐Sensing Satellite

The opening and evolution of the Arctic Ocean seafloor, and its morphology and dynamics, is an area of study that can be addressed by the use of satellite altimetry. Recent work indicates that satellite altimetric data can be successfully applied to the study of Arctic Ocean seafloor formation and in particular Arctic margin formation, evolution, and structure. A comparative study of such structures and their mechanism of formation is now underway at several institutes throughout the world. This investigation will develop satellite-determined detailed gravity field models for high-latitude and Arctic Ocean applications. The Earth Remote- Sensing Satellite (ERS-1) study will also test and improve ocean tidal models in high-latitude regions and in the Arctic Ocean. The investigation will address the question of the mechanism of tidal energy dissipation in the high-latitude margins and the Arctic Ocean. Special concern will also be given to studying the ice regions.

Iceland: The current picture of a ridge-centred mantle plume

Currently the North Atlantic ridge is overriding the Iceland plume. Due to several ridge jumps the plume has been virtually ridge-centred since 20–25 Ma giving rise to extensive melting and crust formation. This review gives an overview over the results of the geophysical and, to minor extent, the geochemical research on the general structure of the Icelandic crust and the mantle beneath Iceland. In the first part, results mostly from topography/bathymetry, gravity, seismics/seismology, magnetotellurics, and geodynamical numerical modelling are summarised. They support the main conclusion that the Icelandic crust is up to ca. 40 km thick, whereby the lower crust and the uppermost mantle have an anomalously small density contrast and a gradual transition rather than a well-defined Moho. The interpretation of a good electrical conductor at 10–15 km depth as a molten layer is irreconcilable with a thick crust, so that alternative explanations have to be sought for this still enigmatic feature. In the second part, results from different branches of seismology, geochemistry, and numerical modelling on the Iceland plume arc reviewed and discussed. For the upper mantle, combining seismological models, geodynamical models and crustal thickness data suggests that the plume has a radius of 100–120 km and an excess temperature of 150–200 K, while the structure of the plume head is less well known. The volume flux is likely to be 5–6 km3/a, and numerical modelling indicates that water and its loss upon melting have a substantial impact on melt production and on the dynamics and distribution of segregating melt. Geochemical studies indicate that the plume source is quite heterogeneous and very probably contains material from the lower mantle. An origin of the plume somewhere in the lower mantle is also supported by several seismological findings, but evidence is not unambiguous yet and has still to be improved.

Finite element modeling of lower crustal flow: A model for crustal thickness variations

Small-scale convection beneath continental lithosphere is likely to initiate viscous flow in the ductile lower crust. Positive lithospheric bending will develop above upwelling mantle flow leading to a lateral squeezing out of crustal material from elevated areas which results in significant crustal thickness variations in its final stage. This process was studied with a finite element approach for viscous material for a number of different viscosity contrasts between lower crust and lithospheric mantle. Significant Moho undulations of the order of at least 5 km within a time period of about 50 m.y. can only be reached if the lower crustal viscosity is less than 1021 Pa s or if the lithospheric mantle viscosity is less than 1024 Pa s. The maximum topography is a function of the viscosity contrast and is generally of the order of a few hundreds of meters with a relaxation time between 5 and 103 m.y., strongly dependent on the crustal viscosity. In conclusion, this process might be important to explain crustal thinning and thickening of the order of about 10 km in areas of enhanced thermal gradient.

Interaction of small mantle plumes with the spinel–perovskite phase boundary: implications for chemical mixing

Abstract Geochemical observations of ocean island basalts show a broad variety in the chemical composition of trace elements. These observations cannot be explained by different temperature and pressure conditions at the melt production region only but also demand differences in the parental material at the base of the lithosphere. Here, we add a new explanation for this finding based on the different interaction of small and large plumes with the spinel–perovskite phase boundary (SPB) in the Earth’s mantle. We assume that mantle plumes detaching from the core–mantle boundary (CMB) are of variable size and excess temperature. While rising through the lower mantle, they form an approximately spherical plume head. When arriving at the SPB, they are retained from further rise depending on their size and excess temperature. We have investigated the interaction of a plume with the SPB using a numerical approach for viscous flow. To focus particularly on this interaction, we simply modelled the initial plume as a hot volume of variable cross-section and excess temperature. Volumes with a radius exceeding about 150 km and an excess temperature of more than 100°C rapidly cross the SPB. With decreasing size and increasing excess temperature, volumes are retarded at the SPB and those with a radius smaller than about 80 km stagnate entirely below the SPB. These results are roughly independent of the particular rheology or two-dimensional versus three-dimensional geometry. In the present study, we investigated the degree of thermal entrainment of material at the SPB for strongly retarded plumes. We estimate the amount of chemical mixing by tracking passive markers from the original plume volume and the regions around the SPB to the base of the lithosphere. The amount of entrainment of material from close to the SPB depends on the retardation time and the thermal growth of the plume head at the SPB. Plume heads which reach the base of the lithosphere after a retardation time of more than 50 Ma might entrain (in the numerical model) large amounts of material from the SBP but they may not be characteristic for the Earth, since they might be distorted by large scale mantle flow. However, even if we limit the retardation time to reach the base of the lithosphere to 50 Ma, plumes, originally composed of material from the CMB and the adjacent lower mantle, might easily entrain up to 15% of their volume of material originating from the SPB.