Viscosity Structure Research Articles

Modelling post-glacial rebound with lateral viscosity variations

SUMMARY Observations of isostatic adjustment of the Earth's surface due to glacial loading provide important constraints on mantle viscosity structure. We solve the forward problem of glacial isostatic adjustment in two complementary ways: a spectral method for strictly 1-D (spherically symmetric) earth models, and a finite element method that can accommodate 3-D viscosity structure. We discuss how each method may be augmented in three ways: to accommodate motion of the centre of mass, to implement a gravitationally self-consistent ocean load via the sea level equation and to include the influence of polar wander. With all these effects implemented, the two methods are benchmarked against each other. We also study the influence of lateral viscosity variations upon measurements of post-glacial rebound (PGR) in two ways: first by observing the effect of viscosity perturbations in an idealized model and second by developing a realistic 3-D viscosity model and comparing it with results of related 1-D (spherically symmetric) models. The 3-D viscosity structure is derived starting from seismic tomography models. We conclude from both approaches that PGR observations are sensitive to both the local viscosity structure and to the viscosity structure beneath the loaded region, even if it is removed from where the observations are made. In particular, PGR measurements made at Hudson Bay tend to reflect the local viscosity structure beneath Hudson Bay; PGR measurements made along the east coast of North America, being sensitive to both the local (east coast) viscosity structure as well as the loaded (continental) viscosity structure, are not reproducible with a 1-D viscosity model.

Sublithospheric small‐scale convection and its implications for the residual topography at old ocean basins and the plate model

The seafloor topography and heat flux differ significantly from the predictions of the half‐space cooling (HSC) model at old ocean basins. Understanding the deviations has important implications for thermal evolution of oceanic lithosphere and large‐scale mantle dynamics. A widely used model that explains significant fraction of the deviations is the plate model, but the dynamical feasibility of the plate model has never been demonstrated. In this study, we investigated the effects of sublithospheric small‐scale convection (SSC) and of internal heating on seafloor heat flux and topography and mantle thermal structure, and we examined the dynamic feasibility of the plate model by formulating high‐ resolution two‐dimensional numerical models of mantle convection with strongly temperature‐ and depth‐dependent rheology. We found that mantle convection with tectonic plates often leads to formation of a broad thermal anomaly below old lithosphere where the mantle is not cooled by subducted slabs and heat transfer is less efficient because of thick lithosphere, especially when significant internal heating is present. This trapped heat may exist in the middle mantle when the SSC is absent, and it may also be redistributed by the SSC to shallow depths to reheat the lithosphere and to homogenize mantle temperature. When internal heating accounts for >∼60% of the total heat output, the trapped heat may provide sufficient heat supply to preferentially reheat old lithosphere via SSC while maintaining uniform mantle temperature. We suggest that the trapped heat and the SSC are responsible for the residual heat flux and topography at old ocean basins relative to the HSC model predictions. Our models also show that for the plate model to be dynamically viable, both the SSC and significant internal heating (>60%) are necessary. This is because only the SSC in a mantle with significant internal heating can erode and reheat the lithosphere while maintaining a nearly constant mantle temperature below lithosphere, which is the basic assumption of the plate model. With the viscosity structure and internal heating rate for the present‐day mantle, we think that the plate model is dynamically viable.

Glacial isostatic adjustment on 3-D Earth models: a finite-volume formulation

SUMMARY We describe and present results from a finite-volume (FV) parallel computer code for forward modelling the Maxwell viscoelastic response of a 3-D, self-gravitating, elastically compressible Earth to an arbitrary surface load. We implement a conservative, control volume discretization of the governing equations using a tetrahedral grid in Cartesian geometry and a low-order, linear interpolation. The basic starting grid honours all major radial discontinuities in the Preliminary Reference Earth Model (PREM), and the models are permitted arbitrary spatial variations in viscosity and elastic parameters. These variations may be either continuous or discontinuous at a set of grid nodes forming a 3-D surface within the (regional or global) modelling domain. In the second part of the paper, we adopt the FV methodology and a spherically symmetric Earth model to generate a suite of predictions sampling a broad class of glacial isostatic adjustment (GIA) data types (3-D crustal motions, long-wavelength gravity anomalies). These calculations, based on either a simple disc load history or a global Late Pleistocene ice load reconstruction (ICE-3G), are benchmarked against predictions generated using the traditional normal-mode approach to GIA. The detailed comparison provides a guide for future analyses (e.g. what grid resolution is required to obtain a specific accuracy?) and it indicates that discrepancies in predictions of 3-D crustal velocities less than 0.1 mm yr−1 are generally obtainable for global grids with ∼3 × 106 nodes; however, grids of higher resolution are required to predict large-amplitude (>1 cm yr−1) radial velocities in zones of peak post-glacial uplift (e.g. James bay) to the same level of absolute accuracy. We conclude the paper with a first application of the new formulation to a 3-D problem. Specifically, we consider the impact of mantle viscosity heterogeneity on predictions of present-day 3-D crustal motions in North America. In these tests, the lateral viscosity variation is constructed, with suitable scaling, from tomographic images of seismic S-wave heterogeneity, and it is characterized by approximately 2 orders of magnitude (peak-to-peak) lateral variations within the lower mantle and 1 order of magnitude variations in the bulk of the upper mantle (below the asthenosphere). We find that the introduction of 3-D viscosity structure has a profound impact on horizontal velocities; indeed, the magnitude of the perturbation (of order 1 mm yr−1) is as large as the prediction generated from the underlying (1-D) radial reference model and it far exceeds observational uncertainties currently being obtained from space-geodetic surveying. The relative impact of lateral viscosity variations on predicted radial motions is significantly smaller.

Study on displacement field generated by aftershocks in Landers earthquake fault zone and its adjacent areas

The displacement field generated by aftershocks in Landers earthquake fault zone and its adjacent areas is calculated in this study. The result is compared with the displacement field of the main shock calculated by co-seismic slip model of Wald and Heaton (1994). The result shows that the direction of displacement generated by aftershocks in Landers seismic fault plane and its adjacent areas is consistent with that generated by main shock. The rupture of aftershock is generally inherited from main shock. The displacement generated by aftershocks is up to an order of centimeter and can be measured by GPS sites nearby. So when we use geodetic data measured after earthquake to study the geophysical problems such as crustal viscosity structure, afterslip distribution, etc., only the displacement field generated by aftershocks considered, can uncertainty be reduced to minimum and realistic result be obtained.

Possible effects of lateral viscosity variations induced by plate-tectonic mechanism on geoid inferred from numerical models of mantle convection

In a traditional analytical method, the convective features of Earth’s mantle have been inferred from surface signatures obtained by the geodynamic model only with depth-dependent viscosity structure. The moving and subducting plates, however, bring lateral viscosity variations in the mantle. To clarify the effects of lateral viscosity variations caused by the plate-tectonic mechanism, I have first studied systematically instantaneous dynamic flow calculations using new density–viscosity models only with vertical viscosity variations in a three-dimensional spherical shell. I find that the geoid high arises over subduction zones only when the vertical viscosity contrast between the upper mantle and the lower mantle is O ( 1 0 3 ) to O ( 1 0 4 ) , which seems to be much larger than the viscosity contrast suggested by other studies. I next show that this discrepancy may be removed when I consider the lateral viscosity variation caused by the plate-tectonic mechanism using two-dimensional numerical models of mantle convection with self-consistently moving and subducting plates, and suggest that the observed geoid anomaly on the Earth’s surface is significantly affected by plate-tectonic mechanism as a first-order effect.

Lithospheric instability in obliquely convergent margins: San Gabriel Mountains, southern California

Oblique convergence across the San Andreas Fault in southern California has lead to rapid uplift of the San Gabriel Mountains since 5 Ma. Tomographic models of this region include a sheet‐like, high velocity anomaly, extending to depths >200 km. One proposed mechanism for the formation of this feature is gravitational instability of the lithosphere, resulting in a cold thermal downwelling. We present a systematic study of the sensitivity of lithospheric deformation (lithospheric downwelling, topography, deflection of the Moho, convergent velocity profile) to the viscosity structure in 2‐D numerical models of lithospheric instability, including a pre‐existing zone of weakness. Strike‐parallel strain within a convergent region should create a local zone of weakness due to the non‐Newtonian response of the lithosphere. Such a weak zone can focus convergent motion, increasing and localizing the growth‐rate of a lithospheric instability. We find that the observed characteristics of deformation depend on the width (xw) and relative viscous weakening factor (fw), the ratio of crust to mantle‐lithospheric viscosity (ηc/ηm), and the absolute viscosity of the lithosphere (ηm). For the San Gabriel Mountains, we find that a range of models is capable of reproducing the observed Moho deflection and depth extent of the downwelling within the short time since the onset of convergence. However, only a small subset of models, (xw = 20 km, fw = 50–100, ηm = 7.5 × 1020–1.5 × 1021 Pa s, ηc/ηm > 100) reproduce both the narrow, high topography and horizontal shortening profile.

Origin of the lithospheric stress field

An understanding of the tectonic stress field is geologically important because it is the agent that preserves in the crust a memory of dynamical processes. In an effort to elucidate the origin of the present state of stress of the lithosphere we use a finite element model of the Earth's lithosphere to calculate stresses induced by mantle flow, crustal heterogeneity, and topography and compare these to observations of intraplate stresses as given by the World Stress Map. We explore two models of lithospheric heterogeneity, one based directly on seismic and other observational constraints (Crust 2.0), and another that assumes isostatic compensation. Mantle tractions are computed from two models of mantle density heterogeneity: a model based on the history of subduction of the last 180 Myr, which has proved successful at accurately reproducing the present‐day geoid and Cenozoic plate velocities, and a model inferred from seismic tomography. We explore the effects of varying assumptions for the viscosity structure of the mantle, and the effects of lateral variations in viscosity in the form of weak plate boundaries. We find that a combined model that includes both mantle and lithospheric sources of stress yields the best match to the observed stress field (∼60% variance reduction), although there are many regions where agreement between observed and predicted stresses is poor. The stress field produced by mantle tractions alone shows a greater degree of long‐wavelength structure than is apparent in the stress observations but agrees very well with observations in some areas where radial mantle tractions are particularly strong such as in southeast Asia and the western Pacific. The stress field produced by lithospheric heterogeneity alone depends strongly on the assumed crustal model: Whereas the isostatically compensated model yields very poor agreement with observations, the model based on Crust 2.0 matches the observations about as well as mantle tractions alone and matches very well in certain areas where the influence of high topography is very important (e.g., Andes, East Africa). A possible interpretation of our results is that the stress field is significantly influenced by lateral variations in the viscosity of the mantle, which leads to variable amounts of decoupling between lithosphere and mantle, allowing the mantle signature to dominate in some areas and the crustal signature to dominate in others. The poor fit between the isostatically compensated crustal model and observations and the large differences between the two crustal models point toward the importance of dynamic topography and remaining uncertainties in crustal structure and rheology. We also consider the possibility that observations of stress from the shallow crust may not reflect the state of stress of the entire plate; stresses in the upper plate may be at least partially decoupled from broader‐scale plate driving forces by lateral and vertical variations in lithospheric rheology.

Crystal orientation fabrics within the Antarctic ice sheet revealed by a multipolarization plane and dual‐frequency radar survey

To investigate the viscosity structure of ice sheets induced by crystal orientation fabric (COF), we carried out a multipolarization plane and dual‐frequency radar survey in East Antarctica. Radar surveys were done along a 670‐km‐long flow line from Dome Fuji toward the coast and two transverse lines of 300‐km and 20‐km length, respectively. The radar echoes were highly dependent on the polarization plane for ice depths between about 40 and 60% of the ice thickness in the lower reaches of the convergent ice flow sector approaching the outlet glacier. When the polarization was perpendicular to the ice flow, echoes were about 10 dB stronger than when the polarization was parallel to the ice flow. This feature was not clear in the upper part of this convergent flow sector. Farther inland, where ice flow is divergent or parallel, the radar echo varied by several decibels because of changes of the radar polarization and had maxima in two orientations. Dual‐frequency data showed that the cause of the reflections was changes in COF. Multipolarization data identified anisotropic reflectivities and birefringence as causes of the anisotropic radar echoes in the lower and upper reaches, respectively. With the aid of ice‐core‐based studies on COF, we show that ice is composed of stacked layers of single‐pole and vertical girdle fabrics in the lower reaches. In contrast, we argue that changes of single‐pole clustering cause isotropic reflectivities in the upper reaches. We also discuss on the development of COF along ice flow and its implication to ice sheet dynamics.

Plume‐transform interactions at ultra‐slow spreading ridges: Implications for the Southwest Indian Ridge

We explore a potentially important variable in controlling ridge‐hot spot interaction, the effect of transform offsets in limiting along‐axis flow of plume material. We focus on the Southwest Indian Ridge (SWIR), where the “transform damming” effect is likely to be pronounced because of both long offset lengths and large contrasts in lithospheric thickness across the transform faults due to the ultra‐slow spreading rate. We investigate the degree to which transform faults affect axial asthenospheric flow by performing a series of three‐dimensional (3‐D) numerical experiments with simplified channel‐flow geometry and extrapolating their results to the SWIR. 3‐D mantle viscosity structure for a ridge‐transform‐ridge system is determined based on temperature‐ and pressure‐dependent viscosity laws. We consider six transform lengths, spanning 0 to 250 km in increments of 50 km. We then calculate the 3‐D viscous flow in response to an along‐axis pressure gradient corresponding to a ridge‐centered hot spot. Modeling results predict that transform faults affect along‐axis asthenospheric flow in two important ways. First, transforms reduce along‐axis flux. The longer the transform offset, the greater the reduction in across‐transform flux relative to the zero‐offset case. Second, transforms deflect shallow asthenospheric along‐axis flow. The predicted transform damming effect is most pronounced for a viscosity structure that is strictly pressure‐ and temperature‐dependent. Flux reduction effects could be less significant for viscosity laws that additionally consider dehydration, melting, and change in deformation mechanism. This model predicts that the waist width of an on‐axis plume is dependent not only on such previously explored factors as buoyancy and spreading rate, but also on the geometry of ridge segmentation. Along the SWIR, axial flow driven by the Marion plume is likely curtailed by the long‐offset Andrew Bain and Discovery II fracture zones, severely limiting its lateral extent.

Multiscale dynamics of the Tonga-Kermadec subduction zone

Our understanding of mantle convection and the motion of plates depends intimately on our understanding of the viscosity structure of the mantle. While geoid and gravity observations have provided fundamental constraints on the radial viscosity structure of the mantle, the influence of short-wavelength variations in viscosity is still poorly understood. We present 2-D and 3-D finite-element models of mantle flow, including strong lateral viscosity variations and local sources of buoyancy, owing to both thermal and compositional effects. We first use generic 2-D models of a subduction zone to investigate how different observations depend on various aspects of the viscosity structure, in particular, the slab and lower-mantle viscosity and the presence of a low-viscosity region in the mantle wedge above the slab. We find that: (1) the strain rate provides a strong constraint on the absolute viscosity of the slab (1023 Pa s); (2) stress orientation within the slab is sensitive to the relative viscosity of the slab, lower mantle and the wedge; and (3) the stress state and topography of the overriding plate depend on the wedge viscosity and local sources of buoyancy. In particular, the state of stress in the overriding plate changes from compression to extension with the addition of a low-viscosity wedge. We then use observations of strain rate, stress orientation, dynamic topography and the geoid for the Tonga—Kermadec subduction zone as simultaneous constraints on the viscosity and buoyancy in a 3-D regional dynamic model. Together these observations are used to develop a self-consistent model of the viscosity and buoyancy by taking advantage of the sensitivity of each observation to different aspects of the dynamics, over a broad range of length-scales. The presence of a low-viscosity wedge makes it possible to match observations of shallow dynamic topography and horizontal extension within the backarc, and down-dip compression in the shallow portion of the slab. These results suggest that a low-viscosity wedge plays an important role in controlling the presence of backarc spreading. However, for a model with a low-viscosity and low-density region that provides a good fit to the observed topography, we find that a reduction of the slab density by a factor of 1.3 relative to the reference density model, is required to match the observed geoid. These results suggest that compensation of the slab by dynamic topography may be a much smaller effect at short to intermediate wavelengths than predicted by long-wavelength modelling of the geoid.

Comparison of dynamic flow models for the Central Aleutian and Tonga‐Kermadec subduction zones

Using three‐dimensional (3‐D) instantaneous dynamic models of the Central Aleutian subduction zone, we show that a low viscosity, low density (δρ = 10 kg/m3) region in the wedge is needed to match observations of topography, the geoid and the stress‐state in the slab and overriding plate. Previous models of the Tonga‐Kermadec subduction zone also require a low viscosity, low density (δρ = 20 kg/m3) region in the wedge. A low viscosity region in the wedge decouples the overriding plate from the downward flow in the wedge caused by the sinking slab, reducing the magnitude of negative dynamic topography on the overriding plate and changing the state of stress in the slab from down‐dip tension to down‐dip compression. If the low viscosity region is large, as in the Tonga‐Kermadec models, then the decoupling may also cause the overriding plate to go into extension. These are the first dynamic models to simultaneously match the state of stress in the slab and overriding plate and suggest that the combination of stress state within the slab and overriding plate may provide an additional constraint on the viscosity structure within subduction zones.

Postglacial rebound and Earth's viscosity structure from GRACE

The Gravity Recovery and Climate Experiment (GRACE) satellite mission was launched in March 2002 and has an expected 5‐year lifetime. One potential application of GRACE measurements of time‐variable gravity will be to isolate the postglacial rebound signal, which can then be used to estimate the Earth's viscosity structure. In this paper we present a sensitivity analysis of simulated GRACE data, designed to assess the accuracy with which those data can be used to recover a simple model of Earth viscosity. We find that without combining with any other data type, but ignoring complications caused by uncertainties in the global ice loading history, GRACE data alone would allow us to determine the viscosity of a uniform lower mantle layer and an upper mantle/transition zone layer to within ±30–40% and to estimate lithospheric thickness to within ±15–20%. GRACE will have a harder time differentiating between the separate viscosities of the transition zone and upper mantle, but accuracies of within a factor of 2 might still be achievable for those parameters. Errors in the ice loading history could significantly degrade these viscosity estimates, particularly for the transition zone and upper mantle. The accuracy of recovery of the true Earth viscosity will depend in part on how well the model parameterization used for the grid search can represent the true Earth structure. However, combining GRACE data with data from other more traditional measurements of postglacial rebound has the potential of dramatically improving viscosity estimates throughout the Earth, particularly in the lower mantle.

Mantle viscosity beneath the Hudson Bay: An inversion based on the Metropolis algorithm

We revisit the problem of inferring mantle viscosity from postglacial relative sea level (RSL) data across the Hudson Bay. We invert a recently revised data set using the Metropolis algorithm together with an annealing schedule: this method, which is well established in geophysics, is applied here for the first time to the glacial isostatic adjustment problem. The Metropolis algorithm performs a search, which is not limited to downhill moves in the model space and thus is less influenced by local minima of the misfit than traditional inverse approaches. Furthermore, its CPU requirements are far superior to Monte Carlo methods. The major drawbacks include slow convergence and the need for careful tuning of crucial variables such as the temperature schedule and the increment in the model space. When all the Hudson Bay RSL data are considered, and the viscosity of the upper mantle above the 670 km discontinuity is inverted, the best fitting solution is characterized by a viscosity close to 2 × 1020 Pa s. However, when the shallow upper mantle and transition zone viscosity are separately inverted, other less traditional solutions with a more complex viscosity structure are found to be equally possible. A stable feature is the lower mantle viscosity, which is generally found to be close to the value of 1021 Pa s in all of the stochastic inversions we have performed. The solutions agree with previous findings concerning both postglacial rebound observables and global geodynamics signatures.

An explanation for the shape of Earth's gravity spectrum based on viscous mantle flow models

The Earth's gravity spectrum can be used as an observational constraint on geophysical modelling. Here we show how the spectrum up to degree l = 31 can be explained to a large part by viscous mantle flow in combination with a very simple model of random mantle density anomalies. Efforts to make the calculation more “realistic” by considering effects of thermal boundary layers, or using density anomalies based on tomography, or geodynamic modelling, tend to worsen the fit. Results are rather sensitive to assumptions on density anomalies in the upper thermal boundary layer. We suggest that, in combination with other observations, the shape of the Earth's gravity spectrum can serve to better constrain radial viscosity structure, density anomalies and flow in the Earth's mantle. Appropriate treatment of the lithosphere and of lateral viscosity variations will be the main challenges in modelling this spectrum.

Influence of melt on the creep behavior of olivine–basalt aggregates under hydrous conditions

The influence of melt on the creep behavior of olivine–basalt aggregates under hydrous conditions has been investigated by performing a series of high-temperature triaxial compression experiments. Samples with melt fractions of 0.02≤ φ≤0.12 were deformed under water-saturated conditions at temperatures between 1373 and 1473 K and a confining pressure of 300 MPa in a gas-medium apparatus. At constant differential stress and temperature, the rate of deformation increased rapidly but systematically with increasing melt fraction. In the diffusion creep regime, at a given differential stress, samples with melt fractions of 0.02 and 0.12 deformed a factor of ∼2 and ∼20, respectively, faster than a melt-free sample. In the dislocation creep regime, a sample with a melt fraction of 0.12 deformed a factor of ∼40 faster than a melt-free sample. For partially molten olivine–basalt aggregates deformed under hydrous conditions, the dependence of creep rate on melt fraction can be expressed in the form ϵ( φ)= ϵ(0) exp( αφ), where α≈26 for diffusion creep and α≈31 for dislocation creep. The results of this study, combined with reasonable estimates for the spatial variation in the concentrations of water and melt (as well as for the geotherm and the activation volume for creep), provide constraints on the viscosity structure of Earth’s upper mantle. As an example, we present a viscosity profile for the mantle wedge above a subducting plate, demonstrating that the viscosity in that region can vary by ∼3 orders of magnitude over a depth of ∼60 km due to the combined effects of water and melt weakening.

Spectral, initial value approach for viscoelastic relaxation of a spherical earth with a three-dimensional viscosity-I. Theory

Summary We present a spectral, initial value approach to the forward modelling of the viscoelastic response of a spherical earth with a 3-D viscosity structure. It represents an alternative to a variety of numerical methods for 2-D and 3-D postglacial rebound modelling used recently (the finite element method, the perturbation method and the semi-analytical approach). We employ surface spherical harmonics up to second rank to parametrize the spatial dependence of material and field quantities. The spectral parametrization converts the balance momentum equation, Poisson’s equation and the constitutive equation to a system of eight simultaneous ordinary differential equations in radial variables that may be solved by a method of numerical integration. In contrast with the spherically symmetric problem, the presence of lateral viscosity variations causes the spectral equations to be coupled and the system cannot be solved separately for individual angular degree and order. The time dependence of the problem is treated directly in a time domain (not in the Laplace domain) as a time evolution problem. Approximating time derivatives by forward Euler differencing leads to an explicit time differencing scheme with time splitting of the div operator in the balance momentum equation. The central point of this paper is to present the theory as transparently as possible. We hope to report on numerical results soon.

Glacial isostatic adjustment in Fennoscandia with a three-dimensional viscosity structure as an inverse problem

Glacial isostatic adjustment data are commonly used to invert for the radial viscosity structure of the mantle. However, the effects of lateral variations in mantle viscosity in such inversions are not yet accounted for. Here we analysed synthetic sea-level data for the Fennoscandian region, which are derived from a three-dimensional (3D) earth model with realistic lateral and vertical viscosity variations deduced from seismological and geological information. The inversion of the 3D synthetic data for a best-fitting 1D viscosity profile reveals that (i) lateral lithospheric thickness variations can be detected with 1D model predictions, if the data are grouped into regional subsets, (ii) combined lateral variations in lithospheric thickness and asthenospheric viscosity are not properly resolved with 1D model predictions, and (iii) the spatial and temporal distribution of observational data strongly affects the resulting 1D viscosity profile.

Effects of water load on geophysical signals due to glacial rebound and implications for mantle viscosity

We investigate the effects of the ocean function on predictions of the sea-level changes and other geophysical signals due to glacial rebound. To precisely predict these signals, a realistic ocean function including the effects of the palaeotopography, the distribution of ice sheet and meltwater influx is required. The adoption of a precise ocean function is very important in simulating the observables in Hudson Bay for an earth model with a low lower mantle viscosity of ∼1021 Pa s. In this case, the contribution from water loads can be comparable to that from ice loads. In the Fennoscandian region, however, the predictions are less sensitive to the details of the ocean function, because the width of the Gulf of Bothnia is very small compared with that of Hudson Bay. With an assumption that the ice model is represented by ARC3+ANT4b, we have examined the viscosity structure using relative sea-levels, gravity anomaly and solid surface gravity changes in North America and northern Europe. This study suggests a lower mantle viscosity of greater than 1022 Pa s and a upper mantle viscosity of (4 ∼ 10) × 1020 Pa s.

Can the 1D viscosity profiles inferred from postglacial rebound data be affected by lateral viscosity variations in the tectosphere?

We have tested the impact of lateral viscosity variations (LVV) in the top 200 km of the mantle (tectosphere) on the interpretation of Fennoscandian postglacial uplift data in terms of 1D viscosity models. Our test consists of two steps. In the first step, we simulate the postglacial uplift for a given 2D viscosity structure involving a cratonic lithospheric root below the ice sheet and an asthenosphere below oceanic areas. In the second step, we interpret the result of this simulation in terms of a 1D viscosity model. The resultant 1D viscosity model shows a lower viscosity in the lower mantle, a higher viscosity in the shallow sublithospheric mantle and a smaller thickness of the lithosphere than the synthetic model of viscosity.

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.