Luce Fleitout

Mantle convection and stability of depleted and undepleted continental lithosphere

We address the question of how convective processes control the thicknesses of oceanic and continental lithospheres. The numerical convection model involves a Newtonian rheology which depends on temperature and pressure. A repeated plate tectonic cycle is modeled by imposing a time-dependent surface velocity. One part of the surface, representing a continent, never subducts. The asymptotic equilibrium thickness of the lithosphere varies with the viscosity at the base of the lithosphere, but is not directly sensitive to the pressure dependence of the viscosity law and to the plate velocity. For small activation volumes, and average upper mantle viscosities deduced from postglacial rebound, the equilibrium plate thickness is more than 400 km (regimes 1 and 2). The equilibrium thickness of the oceanic lithosphere (around 100 km) implies that the viscosity in the asthenosphere is less than 7×1018 Pa s. Only models with strongly pressure-dependent viscosity laws (activation volumes greater than 9×10−6 m3/mol) are able to reconcile this value with the average upper mantle viscosity (5×1020 Pa s). For these models, there are two lithospheric thicknesses such that the heat supplied by convection at the base of the lithosphere equals the surface conductive heat flow (regime 3). They could be that of an aged oceanic lithosphere and that of a shield lithospheric root. They indeed appear as points of prefered thickness in our numerical models. However, convection triggered by the lateral density jumps at the boundaries between the root and the thinner lithosphere slowly destabilizes the thick lithosphere. A plausible degree of chemical buoyancy in a depleted lithospheric root does not prevent convective erosion. In our simulations, long-term stability of a cratonic lithospheric root is best achieved when its material is both buoyant and more viscous than the surrounding mantle. Extensive devolatilization of the refractory rocks forming the root is invoked to explain this viscosity increase.

Thermal and mechanical evolution of shear zones

Physical models of geological shear zones are computed taking into account heating by deformation and consequent softening of the rock. The models show that initiation of a ductile shear zone proceeds by the rapid build up of a thermal peak and by concentration of the strain. After this the temperature levels off as the widths of the thermal anomaly and of the sheared volume slowly increase. For imposed velocities compatible with plate motion, shear heating softens the rock efficiently but does not produce melting. This is not true in stratified structures where a sheared hard layer can heat up sufficiently to melt a neighbouring layer. Other situations where melting might occur are also analysed.

Thermal evolution of the oceanic lithosphere: an alternative view

Abstract The most common model used for representing the evolution with age of the oceanic lithosphere is the ‘plate model’ where the temperature is set at a fixed depth, called the base of the plate. This ‘base of the plate’ has no physical meaning but this model provides a mathematical substitute for a system where small-scale convection occurs through instabilities growing at the base of the cooling lithosphere and becomes effective only below old ocean. Another possible view is that convection provides heat at the base of the lithosphere whatever the age of the overlying plate. This last process can be modeled by a Constant Heat flow Applied on the Bottom Lithospheric ISotherm (CHABLIS model). A good fit to the observables (bathymetry and geoid as function of age, and old age heat-flow) can be obtained both for plate and CHABLIS models in spite of an experimentally determined thermal expansion coefficient much larger than assumed in previous plate models. These models have important consequences for several geodynamic processes. The plate, at an age of 100 Ma is only 80 km thick for both models: melting above a hot-spot can then occur in the garnet-spinel transition field without much plate thinning. In the plate model the subsidence is stopped at an age of about 80 Ma while, according to the CHABLIS model, several hundred meters of subsidence are expected after 100 Ma. Thus the two models predict quite a different long-term pattern of subsidence in the sedimentary basins. Finally, in the CHABLIS model, the global cooling of the mantle coming from cold material eroded by secondary convection at the base of the plates is considerably larger than in plate models: it amounts to 40%, the remaining 60% being due to the subduction process.

Heat transport in stagnant lid convection with temperature- and pressure-dependent Newtonian or non-Newtonian rheology

A numerical model of two-dimensional Rayleigh-Benard convection is used to study the relationship between the surface heat flow (or Nusselt number) and the viscosity at the base of the lithosphere. Newtonian or non-Newtonian, temperature- and pressure-dependent rheologies are considered. In the high Rayleigh number time-dependent regime, calculations yield Nu ∝ RaBL1/3beff−4/3 where beff is the effective dependence of viscosity with temperature at the base of the upper thermal boundary layer and RaBL is the Rayleigh number calculated with the viscosity νBL (or the effective viscosity) at the base of the upper thermal boundary layer. The heat flow is the same for Newtonian and non-Newtonian rheologies if the activation energy in the non-Newtonian case is twice the activation energy in the Newtonian case. In this chaotic regime the heat transfer appears to be controlled by secondary instabilities developing in thermal boundary layers. These thermals are advected along the large-scale flow. The above relationship is not valid at low heat flow where a stationary regime prevails and for simulations forced into steady state. In these cases the Nusselt number follows a trend Nu ∝ RaBL1/5beff−1 for a Newtonian rheology, as predicted by the boundary layer theory. We argue that the equilibrium lithospheric thickness beneath old oceans or continents is controlled by the development of thermals detaching from the thermal boundary layers. Assuming this, we can estimate the viscosity at the base of the stable oceanic lithosphere. If the contribution of secondary convection to the surface heat flux amounts to 40 to 50 mW m−2, the asthenospheric viscosity is predicted to be between 1018 and 2×l019 Pa s.

Geoid anomalies and the structure of continental and oceanic lithospheres

Geoid anomalies in the wavelength range corresponding to spherical harmonies 6 to 30 are used to bring new constraints on the deep structure of the continental lithosphere, on the nature of upper mantle seismic anomalies, and on the thermal evolution of the oceanic lithosphere. For the shallow lithospheric density anomalies considered here, the geoid anomaly is shown to follow approximately the moment law derived from isostasy. By a procedure of least squares inversion, the geoid over old cratons is found to be equal to that over relatively young (50 Myr) oceanic lithosphere. The oceanic geoid as a function of age follows the expected linear trend with flattening at old ages. These results are independent of the degree at which the spherical harmonics are truncated. If the tectosphere consisted only of a thick thermal boundary layer, marked geoid lows, with an amplitude larger than 20 m, should be present over cratons, in disagreement with the present observation. The predicted geoid agrees with the observations if the negative density anomalies associated with a depleted peridotitic layer are present to the bottom of the lithosphere. This analysis of the geoid confirms the petrological inferences that the deep roots observed seismically in the top upper mantle form a chemically differentiate reservoir. The density of these cold roots is similar to that of mantle material elsewhere, because the increase in density resulting from their lower temperature is largely balanced by the decrease in density caused by their depletion. Analysis in wavelength range corresponding to harmonics 6 to 30 in the oceans confirms that the geoid flattens at old ages, and thus supports the view that an important amount of heat is transferred at the base of the lithosphere by small-scale convection.

Secondary convection and the growth of the oceanic lithosphere

Abstract This paper presents a study of the evolution of the oceanic lithosphere from a thermomechanical approach. We have investigated the finite-amplitude development of secondary convection cells beneath the oceanic plates by means of the single-mode mean-field equations and the fully 2-dimensional convection equations, using finite-element techniques. Both Newtonian and non-Newtonian rheologies with highly temperature- and pressure dependences, and an activation energy of 100 kcal mol−1 have been employed. The temperature at the base of the convecting medium governs the final thickness of the lithosphere. The mean interior temperature varies only slightly during the temporal evolution. Non-Newtonian rheology has the tendency to induce oscillatory time-dependent behavior of the flow. Heat flow, topography and gravity are influenced by secondary convection in two ways. Small scale perturbations with wavelengths of around 600 km arise from the lateral thermal differences between the uprising and descending convective limbs; large-scale features are also produced as a consequence of lithospheric growth. The calculated quantities of the heat-flow topography and gravity associated with small-scale convection are typically in the range of O(HFU), O(102m), and O(10 mgal). The horizontal mean-temperature profiles from the convection model are used to calculate long wavelength geophysical observables as a function of age. Convective processes are found to reduce the rate of lithospheric thickening. Predictions from our model can fit well the observed data of heat flow, ocean floor topography and geoid offsets along fracture zones, the last data base exhibiting the most sensitivity to thermal perturbations below the lithosphere. Our calculations show that the oceanic lithosphere is able to grow continuously up to O(109 y), long past the flattening of the seafloor. We report here that the thickness of a thermally equilibrated lithosphere could reach ∼ 250 km, which lies within the appropriate range of values for the continental lithosphere, as inferred from studies in seismology, flexure observations, and secular polar motions.

April 2012 intra-oceanic seismicity off Sumatra boosted by the Banda-Aceh megathrust

Large earthquakes nucleate at tectonic plate boundaries, and their occurrence within a plate’s interior remains rare and poorly documented, especially offshore. The two large earthquakes that struck the northeastern Indian Ocean on 11 April 2012 are an exception: they are the largest strike-slip events reported in historical times and triggered large aftershocks worldwide. Yet they occurred within an intra-oceanic setting along the fossil fabric of the extinct Wharton basin, rather than on a discrete plate boundary. Here we show that the 11 April 2012 twin earthquakes are part of a continuing boost of the intraplate deformation between India and Australia that followed the Aceh 2004 and Nias 2005 megathrust earthquakes, subsequent to a stress transfer process recognized at other subduction zones. Using Coulomb stress change calculations, we show that the coseismic slips of the Aceh and Nias earthquakes can promote oceanic left-lateral strike-slip earthquakes on pre-existing meridian-aligned fault planes. We further show that persistent viscous relaxation in the asthenospheric mantle several years after the Aceh megathrust explains the time lag between the 2004 megathrust and the 2012 intraplate events. On a short timescale, the 2012 events provide new evidence for the interplay between megathrusts at the subduction interface and intraplate deformation offshore. On a longer geological timescale, the Australian plate, driven by slab-pull forces at the Sunda trench, is detaching from the Indian plate, which is subjected to resisting forces at the Himalayan front.

A global geoid model with imposed plate velocities and partial layering

Most inversions of the long-wavelength geoid in conjunction with the seismic tomographic information have so far been carried out under the assumption of either purely whole mantle or perfectly layered circulation. Moreover, modeling the lithosphere as a spherical shell with a uniform low viscosity was found to yield the best fit to the observed geoid. We have tested whether a good prediction of the geoid can also be achieved by including two constraints: a semipermeable behavior of the 660-km discontinuity and surface plate velocities equal to the observed ones. The mass transfer between upper and lower mantle has been changed by imposing a surface density anomaly at a depth of 660 km, which is proportional to the mass anomaly needed to achieve perfectly layered circulation. On the top of the mantle we assume a stiff lithosphere that moves with a velocity corresponding to the observed plate motion. The viscosity only varies with depth. Considering a simple three-layer viscosity structure and changing the permeability of the 660-km interface, we have obtained a satisfactory variance reduction of the geoid data (∼75% for degrees 2–12). The best fit to the geoid is obtained if the mass transfer across the 660-km boundary is reduced to one third in comparison with the purely whole mantle model. The best fitting viscosity profile is characterized by a clearly defined asthenosphere and a viscosity increase by at least 2 orders of magnitude in the lower mantle. The amplitudes of dynamic topography predicted by our model are remarkably small (∼100 m), thus fully compatible with the observation.

Active lithospheric thinning

Abstract This paper starts with a discussion of the physical mechanisms capable of inducing a rapid upward migration of the lithosphere-asthenosphere boundary. Two models based on a thermomechanical destabilization of the lower lithosphere are proposed. First, we consider the enhancement of small scale sublithospheric convection modes by a hot thermal plume. A perturbation of a few hundred degrees is found to thin the lithosphere and generate uplift within a few tens of million years. This efficient erosion from below only stops when it reaches a layer of low density such as the crust or some depleted mantle material. Second, we show that passive lithospheric stretching leads to an unstable lithospheric configuration on the sides of the rift. The denser unstretched lithosphere is subject to a lateral convective penetration by the hotter asthenospheric mantle material coming from the rift zone. This lateral destabilization generates uplift of the shoulders. Natural examples can be found along passive continental margins. A good example is given by the uplift of the Scandinavian Caledonides following the break-up in the North-Atlantic.

Stability of the oceanic lithosphere with variable viscosity: an initial-value approach

Abstract We have studied the problem concerning the onset of convective instabilities below the oceanic lithosphere. A system of linear partial differential equations, in which the background temperature field is time-dependent, is integrated in time to monitor the evolution of incipient disturbances. Two types of rheologies have been examined. One depends strongly on temperature. The other involves a viscosity which is both temperature- and pressure-dependent. The results from this initial-value approach, in which the viscosity profiles migrate downward with time, reveal the importance of considering temperature- and pressure-dependent rheology in issues regarding the development of local instabilities in upper mantle convection. For temperature-dependent viscosity, viscosities of 0(1020P) are required to produce instabilities with growth-rates of 0(.1/Ma). In contrast, these same growth rates can be attained for a temperature- and pressure-dependent viscosity profile with a mean value close to 0(1020P) in the upper mantle, owing to the presence of a low viscosity zone, 0(1020P), existing right below the lithosphere. Unlike the results of temperature-dependent viscosity, whose growth-rates increase with time, the amplification of disturbances in a fluid medium with temperature- and pressure-dependent rheology reaches a maximum at an early age,