Lateral Viscosity Research Articles

A possible mechanism for the initiation of the Yinggehai Basin: A visco-elasto-plastic model

The Yinggehai Basin, lying along the trace of the Red River fault zone in the South China Sea, has been related to the movements of the Red River fault zone and the rotation of the Indochina block. However, the tectonic origin of the Yinggehai Basin remains unclear. This paper explores the initiation of the Yinggehai Basin using a visco-elasto-plastic model. This model concentrates on two main aspects: lateral viscosity variations (mainly controlled by temperature) in the lithosphere and internal friction angles of the pre-existing fault zone. Modeling results show that: (1) for a layered viscosity, en echelon faults would likely form when there is no pre-existing strike–slip fault; whereas, the basin would be narrow and deep, if any, when there is a pre-existing fault; (2) for moderate lateral viscosity variations, a large basin forms, even without a pre-existing fault zone; and (3) for strong lateral viscosity variations, a major rift could form over geologic time. Our results indicate that the initiation of the Yinggehai Basin requires moderate lateral viscosity variations (i.e., a pre-existing gentle upwelling of the Mohorovicic discontinuity) but no pre-existing fault. In addition, the initial extension predicted is NE–SW and is generated by the rotation of the Indochina block. This differs from the NW–SE extension that resulted from the movements of the Red River fault zone. This indicates that the left-lateral displacements of the Red River fault zone and the spreading of the South China Sea only influence the basin evolution after its initiation.

Effect of subgrid-scale mixing on the evolution of forced submesoscale instabilities

We study the effect of subgrid-scale (SGS) mixing on the evolution of a density front initially in thermal-wind balance with a meridional density gradient and forced by downfront surface winds. The horizontal size of the model domain (O(100km)) is large enough to contain mesoscale eddies while the horizontal grid resolution (500m) is fine enough to resolve submesoscale eddies. The twin goals of this study are: (i) to determine what is a realistic level of SGS dissipation; and (ii) to explore the sensitivity of the resolved-scale dynamics to the SGS dissipation. To this end, we effect different levels of SGS dissipation using two SGS models: (i) constant lateral SGS viscosities (1m2s−1 and 5m2s−1) and an analytically prescribed vertical SGS viscosity; and (ii) an existing anisotropic Smagorinsky model (ASM) developed for anisotropic grids with large aspect ratios between the horizontal and the vertical directions. An analysis of the eddy kinetic energy (EKE) budgets shows the surface stress boundary condition constrains all simulations to yield realistic values of SGS dissipation in a near-surface layer that is shear-driven and similar to the traditional Monin–Obukhov layer. Deeper down within the mixed layer, the EKE budget is buoyancy-driven with a more complicated balance that varies considerably among the different simulations. The simulations with constant Kx predict the buoyant generation of EKE is balanced almost solely by pressure transport with negligible local destruction, which gives rise to waves near the front. Recent observations near fronts show enhanced levels of irreversible destruction. The simulations with the ASM predict EKE budgets where both local destruction—through SGS dissipation—and pressure transport are part of the EKE balance.The results obtained using the constant-Kx simulations suggest both horizontal and vertical SGS parameterizations have important effects on the resolved-scale dynamics. The simulations with Kx=5m2s−1 yield the most unrealistic results partly because the lateral viscosity is high enough to directly influence the instability scale. Yet the observed differences among the constant-Kx simulations are sometimes subtle and cannot be explained trivially by comparing Kx alone. For fixed Kx, we find simulations can exhibit higher spectral levels and stronger cascades (forward and inverse) upon increasing the vertical SGS viscosity. This suggests the sensitivity of submesoscale-resolving simulations to the vertical SGS parameterization needs to be better explored.

Interpretation of interseismic deformations and the seismic cycle associated with large subduction earthquakes

The deformations of the overriding and subducting plates during the seismic cycle associated with large subduction earthquakes are modelled using 2D and 3D finite element techniques. A particular emphasis is put on the interseismic velocities and on the impact of the rheology of the asthenosphere.The distance over which the seismic cycle perturbs significantly the velocities depends upon the ratio of the viscosity in the asthenosphere to the period of the seismic cycle and can reach several thousand km for rheological parameters deduced from the first years of deformation after the Aceh earthquake. For a same early postseismic velocity, a Burger rheology of the asthenosphere implies a smaller duration of the postseismic phase and thus smaller interseismic velocities than a Maxwell rheology. A low viscosity wedge (LVW) modifies very significantly the predicted horizontal and vertical motions in the near and middle fields. In particular, with a LVW, the peak in vertical velocity at the end of the cycle is predicted to be no longer above the deep end of the locked section of the fault but further away, above the continentward limit of the LVW.The lateral viscosity variations linked to the presence at depth of the subducting slab affect substantially the results.The north–south interseismic compression predicted by this preliminary 2D model over more than 1500km within the Sunda block is in good agreement with the pre-2004 velocities with respect to South-China inferred from GPS observations in Thailand, Malaysia and Indonesia. In Japan, before the Tohoku earthquake, the eastern part of northern Honshu was subsiding while the western part was uplifting. This transition from subsidence to uplift so far away from the trench is well fitted by the predictions from our models involving a LVW.Most of the results obtained here in a 2D geometry are shown to provide a good estimate of the displacements for fault segments of finite lateral extent, with a 3D spherical geometry, as long as the displacements during the seismic cycle are normalised by the coseismic displacement.

Predicting the lithospheric stress field and plate motions by joint modeling of lithosphere and mantle dynamics

The way in which basal tractions, associated with mantle convection, couples with the lithosphere is a fundamental problem in geodynamics. A successful lithosphere‐mantle coupling model for the Earth will satisfy observations of plate motions, intraplate stresses, and the plate boundary zone deformation. We solve the depth integrated three‐dimensional force balance equations in a global finite element model that takes into account effects of both topography and shallow lithosphere structure as well as tractions originating from deeper mantle convection. The contribution from topography and lithosphere structure is estimated by calculating gravitational potential energy differences. The basal tractions are derived from a fully dynamic flow model with both radial and lateral viscosity variations. We simultaneously fit stresses and plate motions in order to delineate a best‐fit lithosphere‐mantle coupling model. We use both the World Stress Map and the Global Strain Rate Model to constrain the models. We find that a strongly coupled model with a stiff lithosphere and 3–4 orders of lateral viscosity variations in the lithosphere are best able to match the observational constraints. Our predicted deviatoric stresses, which are dominated by contribution from mantle tractions, range between 20–70 MPa. The best‐fitting coupled models predict strain rates that are consistent with observations. That is, the intraplate areas are nearly rigid whereas plate boundaries and some other continental deformation zones display high strain rates. Comparison of mantle tractions and surface velocities indicate that in most areas tractions are driving, although in a few regions, including western North America, tractions are resistive.

On the Width of the Equatorial Deep Jets

Abstract The equatorial deep jets (EDJ) are a striking feature of the equatorial ocean circulation. In the Atlantic Ocean, the EDJ are associated with a vertical scale of between 300 and 700 m, a time scale of roughly 4.5 years, and upward energy propagation to the surface. It has been found that the meridional width of the EDJ is roughly 1.5 times larger than expected based on their vertical scale. Here, the authors use a shallow-water model for a high-order baroclinic vertical normal mode to argue that mixing of momentum along isopycnals can explain the enhanced width. A lateral eddy viscosity of 300 m2 s−1 is found to be sufficient to account for the width implied by observations.

Optimal locations for absolute gravity measurements and sensitivity of GRACE observations for constraining glacial isostatic adjustment on the northern hemisphere

SUMMARY Gravity rate of change is an important quantity in the investigation of glacial isostatic adjustment (GIA). However, measurements with absolute and relative gravimeters are laborious and time-consuming, especially in the vast GIA-affected regions of high latitudes with insufficient infrastructure. Results of the Gravity Recovery And Climate Experiment (GRACE) satellite mission have thus provided tremendous new insight as they fully cover those areas. To better constrain the GIA model (i.e. improve the glaciation history and Earth parameters) with new gravity data, we analyse the currently determined errors in gravity rate of change from absolute gravity (AG) and GRACE measurements in North America and Fennoscandia to test their sensitivity for different ice models, lithospheric thickness, background viscosity and lateral mantle viscosity variations. We provide detailed sensitivity maps for these four parameters and highlight areas that need more AG measurements to further improve our understanding of GIA. The best detectable parameter with both methods in both regions is the sensitivity to ice model changes, which covers large areas in the sensitivity maps. Also, most of these areas are isolated from sensitive areas of the other three parameters. The latter mainly overlap with ice model sensitivity and each other. Regarding existing AG stations, more stations are strongly needed in northwestern and Arctic Canada. In contrast, a quite dense network of stations already exists in Fennoscandia. With an extension to a few sites in northwestern Russia, a complete station network is provided to study the GIA parameters. The data of dense networks would yield a comprehensive picture of gravity change, which can be further used for studies of the Earth’s interior and geodynamic processes.

Lithosphere-mantle coupling and the dynamics of the Eurasian Plate

Mechanical equilibrium of tectonic plates implies that lithospheric edge and body forces are balanced by forces arising from interaction with the underlying mantle. We use this quantitative physical relation to integrate existing modelling approaches of lithosphere dynamics and mantle flow into a new combined approach applied to the Eurasian Plate. By combining a thorough analysis of lithospheric forces with the requirement of torque balance we constrain the orientation of the torque on Eurasia arising from mantle tractions. We use this constraint to evaluate convective mantle flow models driven by tomographic or subduction history model anomalies and observed plate motion. Mantle forcing is considered through both shear at the bottom of the plate and induced dynamic topography. We find that instantaneous semi-analytic flow models without lateral viscosity variations generate tractions that meet the constraint from Eurasian lithosphere dynamics, but only for specific ranges of mantle flow parameters. Of the explored set of mantle anomaly models, only mantle flow models based on S-wave tomography anomalies can balance Eurasia for realistic viscosity profiles and velocity–density scaling. Choices in mantle density forcing and viscosity are crucial in that they govern the relative magnitude of tractions due to convective mantle flow (‘active tractions’) and resistive tractions due to plate motion (‘passive tractions’). We find mechanical balance is only achieved for similar torque magnitudes of active and passive shear. The two shear contributions do however in no case balance each other and a considerable, dominant, net torque from edge forces is required to balance total mantle tractions and lithospheric body forces (LBFs). Our analysis provides a range of mechanically consistent total force sets acting on the Eurasian Plate. Using this result we find that mantle buoyancy forces and LBFs acting on Eurasia itself are important driving forces but do not drive Eurasia in the observed direction. Continental collision at Eurasia’s southern boundary significantly deviates Eurasia northwards. Our combined torque balance approach, in which mantle tractions from convective mantle flow modelling are combined with explicitly applied edge forces, thus emphasizes the role of plate interactions to the dynamics of tectonic plates

Geoid and topography of Venus in various thermal convection models

Important though indirect information about the internal structure of Venus is provided by its topography and geoid. In the last decades this information has been used to constrain the Venus mantle viscosity structure and its dynamic regime. Recently, the geodynamic inversion of the Venus’ geoid and topography resulted in a group of best fitting viscosity profiles. We use these viscosity models here as an input to our mantle convection code. We carry out simulations of the Venus’ mantle evolution in a 3D spherical shell with depth dependent viscosity and check whether the character of the dynamic topography and the geoid represented by their power spectra fits the observed quantities. We compare the results with several other models obtained for different viscosity stratifications (constant, constant with highly viscous lithosphere, linear increase of viscosity). Further, we estimate the effect of other factors such as internal heating and varying Rayleigh number. We use a 2D spherical axisymmetric convection code to study the effect of lateral viscosity variations. In these 2D models we monitor the topography and the geoid developing above the axisymmetric plume and compare them with the observed elevations of Venus’ geoid and topography in several Regia. Though none of the models fits observed data perfectly, we can generally conclude, that the best fit between the observed and predicted quantities is reached for viscosity profiles with 200 km thick lithosphere followed by a gradual increase of viscosity with depth and with the upper mantle viscosity of 2 × 10 21 Pa s. For all viscosity profiles the predicted geoid and topography spectra match the observed ones only up to the degree 40, thus indicating other than dynamic origin of these quantities for higher degrees.

Can Lateral Viscosity Contrasts Explain Asymmetric Interseismic Deformation around Strike-Slip Faults?

Abstract Geodetic studies have shown that deformation rates around several major strike‐slip faults are asymmetric. This asymmetry is often explained in terms of a crustal‐scale contrast in elastic properties across the fault. Motivated by the fact that elasticity variations for different rock types under similar ambient conditions are generally modest, whereas effective viscosity may vary over orders of magnitude, we have developed earthquake‐cycle models to evaluate whether contrasts in viscosity structure and effective plate thickness can explain observed asymmetric surface deformation. We find that an increased plate‐thickness contrast results in a more asymmetric surface‐velocity profile. Furthermore, for the same plate‐thickness contrast, asymmetry of the surface deformation is most pronounced in models with a low‐viscosity substrate. Initially, velocities relative to a point on the fault are higher on the side with the thin plate; however, late in the interseismic interval this reverses, and these velocities are higher on the side with the thick plate. Models with a contrast in the substrate viscosity on either side of the fault and a uniform plate thickness show behavior similar to that of the variable‐thickness plate models. In both suites of asymmetric models, the surface velocity at the fault varies through the earthquake cycle. This is necessary to reconcile symmetric coseismic deformation, asymmetric interseismic deformation, and the requirement of zero strain in the blocks on either side of the fault over an earthquake cycle. Given the modest asymmetry in surface deformation for models capable of producing localized interseismic deformation around the fault (i.e., models with high substrate viscosities), we conclude that lateral contrasts in viscosity or effective plate thickness cannot produce dramatic asymmetries in Global Positioning System surface‐velocity profiles across major strike‐slip faults.

Intrinsic low-frequency variability of the Gulf Stream

Abstract. In this paper a process study aimed at analyzing the low-frequency variability of intrinsically oceanic origin of the Gulf Stream (GS) and GS extension (GSE) is presented. An eddy-permitting reduced-gravity nonlinear shallow water model is implemented in an idealized North Atlantic Ocean, with schematic boundaries including the essential geometric features of the coastline and a realistic zonal basin width at all latitudes. The forcing is provided by a time-independent climatological surface wind stress obtained from 41 years of monthly ECMWF fields. The model response yields strong intrinsic low-frequency fluctuations on the interannual to decadal time scales. The modelled time-averaged GS/GSE flows are found to exhibit several features that can also be deduced from satellite altimeter data, such as the Florida Current seaward deflection, the GS separation at Cape Hatteras, and the overall structure of the GSE. The intrinsic low-frequency variability yields two preferred states of the GSE differing in latitudinal location that also have their counterpart in the altimeter data. A preliminary analysis of the variability in terms of dynamical systems theory is carried out by using the lateral eddy viscosity as the control parameter. A complex transition sequence from a steady state to irregular low-frequency variability emerges, in which Hopf and global bifurcations can be identified.

Dipole evolution in rotating two-dimensional flow with bottom friction

The evolution of a dipolar vortex in a quasi-two-dimensional rotating flow of barotropic fluid over a flat surface with a no-slip condition in the Ekman bottom layer is considered. The vorticity equation in this case becomes nonlinear. An effect of bottom friction is displayed mainly in cyclone-anticyclone asymmetry, which results in the expansion (diminution) of cyclonic (anticyclonic) local structures and in the stronger decay of positive vorticity. When a lateral viscosity is omitted, the vorticity equation has a solution in the form of vortex patches and hence a contour dynamics method may be used for numerical simulation. An approach of point vortices with decaying strengths is also discussed. In an approximation of two patches of opposite uniform vorticity, a three-parameter family of stationary (in an ideal fluid) orbital dipoles [V. G. Makarov and Z. Kizner, “Stability and evolution of uniform-vorticity dipoles,” J. Fluid Mech. 672, 307 (2011)] https://doi.org/10.1017/S0022112010006026 consisting of patches with unequal areas and absolute values of vorticity is considered. A three-dimensional domain of instability for this family is numerically constructed. It is shown that the evolution of such dipoles in a flow with bottom friction is described with good accuracy by a properly matched trajectory in parameter space of ideal-fluid steady states. Explicit time-dependent formulae for this phase trajectory are obtained. All characteristics (including the patch's shapes) of the evolutionary dipole, and the same characteristics for the corresponding ideal-fluid stationary dipole, almost completely coincide, at least while the phase trajectory remains in the stability region. The evolution of stationary translating dipoles (with zero net circulation) that have continuously distributed vorticity inside a quasi-elliptical finite region is also examined. When the circular Lamb dipole is used as the initial condition, good qualitative agreement is observed between the asymmetric dipoles obtained during evolution and the chain of the exact solutions from the known one-parameter family of non-symmetrical Chaplygin dipoles.

Numerical prediction of morphological change of straight trapezoidal open-channel

This paper presents a numerical model that is capable of simulating the morphological process of the trapezoidal open-channel. The fact that particles on the sloped side bank do not move in the same direction as the flow adds the difficulty in modeling. The lateral distribution model, which distributes the unit discharge over the width, is used for the flow analysis. Vectorial bedload formula by Kovacs and Parker (1994) is used and channel topography is calculated by solving Exner’s equation with the help of sliding algorithm proposed by Menendez et al. (2008). Values of roughness coefficient and lateral eddy viscosity are calibrated using laboratory data sets. Finally, the proposed model is applied to laboratory experiments of Ikeda (1981) and Izumi et al. (1991). Time evolutions of channel cross section and dimensionless shear stress are provided and discussed.

Spurious dianeutral mixing and the role of momentum closure

This paper examines spurious dianeutral transport within a suite of ocean models (GOLD, MITgcm, MOM, and ROMS). We quantify such transport through a global diagnostic that computes the reference potential energy, whose evolution arises solely through transport between density classes. Previous studies have focused on the importance of accurate tracer advection schemes in reducing the spurious transport and closure. The present study highlights complementary issues associated with momentum transport. Spurious dianeutral transport is shown to be directly proportional to the lateral grid Reynolds number (ReΔ), with such transport significantly reduced when ReΔ<10.Simulations with the isopycnal model GOLD provide a benchmark for the smallest level of spurious dianeutral transport realizable in our model suite. For idealized simulations with a linear equation of state, GOLD exhibits identically zero spurious dianeutral mixing, and thus maintains a constant reference potential energy when all physical mixing processes are omitted. Amongst the non-isopycnal models tested in idealized simulations, ROMS generally produces smaller spurious dianeutral mixing than MITgcm or MOM, since ROMS makes use of a higher order upwind-biased scheme for momentum transport that enforces a small ReΔ. In contrast, MITgcm and MOM both employ unbiased (centered) discretizations of momentum transport, and therefore rely on lateral friction operators to control the grid Reynolds number. We find that a lateral shear-dependent Smagorinsky viscosity provides an effective means to locally reduce ReΔ, and thus to reduce spurious dianeutral transport in MITgcm and MOM.In addition to four idealized simulations, we quantify spurious dianeutral transport in realistic global ocean climate simulations using GOLD and MOM with a realistic equation of state for seawater, both with and without mesoscale eddies in the resolved flow field. The GOLD simulations have detectable levels of spurious cabbeling from along isopycnal advective truncation errors. Significantly larger spurious dianeutral transport arises in a non-eddying MOM simulation. In an eddying MOM simulation, spurious dianeutral transport is larger still but is reduced by increasing momentum friction.

Time dependence of intraplate volcanism caused by shear-driven upwelling of low-viscosity regions within the asthenosphere

[1] Although volcanism far from tectonic boundaries is likely due to upwelling near the lithospheric base, the convective processes that induce upwelling are unclear. Numerical models show that asthenospheric shear can be deflected upward by lateral viscosity variations within the asthenosphere, producing “shear-driven upwelling” (SDU). To constrain the rate, duration, and surface expression of intraplate volcanism caused by SDU, we simulate 2-D flow and peridotite melting in the upper 200 km of the mantle. Asthenospheric shear is driven by lithospheric plates with different thicknesses moving at 3–9 cm/yr, and the initial low-viscosity region is a rectangularly shaped pocket with an imposed viscosity that is 2 orders of magnitude smaller than the surrounding asthenosphere. Melting decreases as the pocket deforms and reaches steady state after 3–12 Myr. The age progression of surface volcanism is nearly stationary in the reference frame of the plate, which distinguishes SDU from hot spot volcanism. Similar behavior occurs if the viscosity heterogeneity is induced by variations in the water content of mantle peridotite. If the pocket's low viscosity is caused by excess temperature, buoyant upwelling of the entire pocket dominates volcanism. Differences in the time dependence of volcanism associated with damp and warm pockets may help identify which type of mantle heterogeneity and associated dynamic process best explains weak, intermittent, intraplate volcanism with no obvious age progression. We suggest that asthenospheric shear induced by plate motions and global mantle flow, by exciting SDU, drives some of the non–hot spot small-scale volcanism that occurs away from plate boundaries.

Internal Kelvin waves in a two-layer liquid model

The subinertial internal Kelvin wave solutions of a linearized system of the ocean dynamics equations for a semi-infinite two-layer f-plane model basin of constant depth bordering a straight, vertical coast are imposed. A rigid lid surface condition and no-slip wall boundary condition are imposed. Some trapped wave equations are presented and approximate solutions using an asymptotic method are constructed. In the absence of bottom friction, the solution consists of a frictionally modified Kelvin wave and a vertical viscous boundary layer. With a no-slip bottom boundary condition, the solution consists of a modified Kelvin wave, two vertical viscous boundary layers, and a large cross-section scale component. The numerical solutions for Kelvin waves are obtained for model parameters that take account of a joint effect of lateral viscosity, bottom friction, and friction between the layers.

Lateral, radial, and temporal variations in upper mantle viscosity and rheology under Scandinavia

[1] The viscosity of the upper mantle has a large control on the dynamics of plate tectonic processes or the response of the Earth's crust after a period of glaciation. Temperature variations within the upper mantle, time-dependent stress changes due to glaciations, and/or variations in the microstructural characteristics of upper mantle rocks (grain size, water content) will result in orders of magnitude variations in upper mantle viscosity. In this study we have taken a microphysical approach to determine variations in viscosity under Scandinavia. We combined experimentally determined flow laws for olivine, data on olivine grain size from Scandinavian xenoliths and peridotites, and stress changes within the upper mantle over the last glaciation period (30 kyr B.P.–present) with two data sets of the temperature distribution within the upper mantle under Europe from Goes et al. (2000) derived from seismic tomography and Artemieva (2006) derived from heat flow measurements. Modeling of olivine viscosity under Scandinavia shows large lateral, radial, and temporal variations in upper mantle viscosity. Lateral temperature variations cause up to four orders of magnitude lateral viscosity variations. Glaciation-induced time-dependent changes of stress cause two orders of magnitude variations in viscosity. Mean viscosity values for a dry upper mantle under Scandinavia are expected to be larger than 1022 Pa s, whereas for a wet upper mantle lower viscosity values in the range of 1019–1022 Pa s are predicted. Estimates of the upper mantle viscosity under Scandinavia from glacial isostatic adjustment studies (1020–1021 Pa s) would indicate that a wet upper mantle below Scandinavia is most likely present. The viscosity modeling furthermore shows that the type of rheology of the upper mantle (linear versus nonlinear rheology) is very sensitive to the microstructural state of the upper mantle. Both mechanisms are active in the upper mantle under Scandinavia on timescales of glacial isostatic adjustment, and their relative contribution varies radially, laterally, and temporally.

Global slab deformation and centroid moment tensor constraints on viscosity

We analyze moment tensor solutions from deep subduction zone earthquakes to determine global slab deformation patterns. Inferred strain rates are compared to predicted deformation patterns from fluid models to help constrain the first‐order radial and lateral viscosity structure of the Earth. While all slabs that reach the lower mantle are compressed at their tip, intermediate depth patterns are more complex. We compute 3‐D spherical flow with various slab rheologies and compare the angular misfit between the compressive eigenvectors of the resultant stress field and global centroid moment tensor (gCMT) solutions. We find that upper mantle slab viscosities of ∼10–100 and lower mantle viscosities of ∼30–100 times the upper mantle produce the best match to gCMTs. A 0.1 viscosity reduction in the asthenosphere seems preferred. Slab geometry and lower mantle viscosity exert significant control on deformation. Inclusion of the phase changes at 410 km and 660 km increases extensional deformation at intermediate depth and compressional deformation at the lower mantle, improving the match to gCMTs for strong slabs. Our conclusions are fairly insensitive to surface boundary conditions. However, models which include net rotations of the surface with respect to the lower mantle produce compression at intermediate depths for west directed slabs and extension for east directed slabs. Without allowing for regional variations, these models yield the best match to gCMTs. While significant deviations between model and seismicity remain, our results show that seismicity provides an underutilized constraint for slab dynamics.

Heat flow and deep temperatures in the Southeast Basin of France: Implications for local rheological contrasts

Abstract Triassic salt at 5–10 km depth may drive some of the recent tectonic features in southeastern France. We estimate the likely temperature range of the salt using two different approaches. The first of these, based on the extrapolation of deep temperatures obtained in oil exploration wells, predicts temperatures at a depth of 8 km to be in the range of 230–300°C. However, this prediction could be biased by a lack of deep measurements and problems related to lateral heat transfer caused by thermal conductivity contrasts. The second approach can overcome these problems by modelling the actual heat transfer for appropriate basin geometry, including temperature-dependent thermal properties, and a mantle heat flow of 35 mW.m−2. This latter value enables us to reproduce available temperature measurements and surface heat flow data. Here we evaluate the stationary temperature field along two sections constrained by seismic profiles, one at a local scale across the Vistrenque graben and the other at a more regional scale across the Southeast Basin. Our findings suggest that the temperatures in the deepest parts of the evaporitic layer (11 km depth) can reach up to 398°C, but can be as low as 150°C on the edge of the basin at the top of the salty layer. This temperature difference leads to important changes in salt viscosity. Results indicate that at a depth of 8 km, lateral viscosity contrasts within the evaporitic layer may reach 40. Such rheological contrasts might favour and amplify local subsidence, as seems to have been the case near the two Palaeogene half-grabens of Vistrenque and Valence, where deep hot zones are identified.

Preliminary three-dimensional model of mantle convection with deformable, mobile continental lithosphere

Characteristic tectonic structures such as young orogenic belts and suture zones in a continent are expected to be mechanically weaker than the stable part of the continental lithosphere with the cratonic root (or cratonic lithosphere) and yield lateral viscosity variations in the continental lithosphere. In the present-day Earth's lithosphere, the pre-existing, mechanically weak zones emerge as a diffuse plate boundary. However, the dynamic role of a weak (low-viscosity) continental margin (WCM) in the stability of continental lithosphere has not been understood in terms of geophysics. Here, a new numerical simulation model of mantle convection with a compositionally and rheologically heterogeneous, deformable, mobile continental lithosphere is presented for the first time by using three-dimensional regional spherical-shell geometry. A compositionally buoyant and highly viscous continental assemblage with pre-existing WCMs, analogous to the past supercontinent, is modeled and imposed on well-developed mantle convection whose vigor of convection, internal heating rate, and rheological parameters are appropriate for the Earth's mantle. The visco-plastic oceanic lithosphere and the associated subduction of oceanic plates are incorporated. The time integration of the advection of continental materials with zero chemical diffusion is performed by a tracer particle method. The time evolution of mantle convection after setting the model supercontinent is followed over 800Myr. Earth-like continental drift is successfully reproduced, and the characteristic thermal interaction between the mantle and the continent/supercontinent is observed in my new numerical model. Results reveal that the WCM protects the cratonic lithosphere from being stretched by the convecting mantle and may play a significant role in the stability of the cratonic lithosphere during the geological timescale because it acts as a buffer that prevents the cratonic lithosphere from undergoing global deformation. From geological evidence that a cratonic root survives at the surface for billions of years, the WCM may have existed in the past supercontinent throughout the Earth's geologic history. The preliminary model presented here should represent an important step toward realizing a more realistic model that could be used to address many outstanding geodynamic problems about the thermal and mechanical feedbacks between the mantle and continents and the temporal evolution of the Earth's mantle structure.

Tectonic and dynamic controls on the topography and subsidence of the Argentine Pampas: The role of the flat slab

We analyze the Pampean foreland (the Pampas) along the modern flat-slab segment of the south-central Andes between 31° and 33° South latitude and to the east of the Argentine “flat-slab” province, using flexural and gravity studies and computations of dynamic topography. Bouguer anomalies and flexural analysis predict a foredeep of ∼250km width and a peripheral bulge amplitude of ∼25m, which match the regional morphologies of the modern Argentine Pampean Plain. However, these studies do not account for the subsurface Miocene–Quaternary basin preservation, represented by sedimentary thicknesses >400m and with depocenters >400km eastward with respect to flexural models. The discrepancy suggests that two mechanisms, acting at different wavelengths, influence the Argentine Pampas. The basin preservation is likely the result of a large-scale geodynamic forcing. Models of mantle flow, driven by realistic flat-slab subduction geometry and density contrasts, reproduce the depocenter location and the wavelength of subsidence as well as most of the remaining amplitude. Nonetheless, more sophisticated studies (e.g. considering lateral viscosity variations in the mantle wedge) might help reduce the dynamic amplitudes and better reproduce the observed geological record.