Hana Čížková

The influence of rheological weakening and yield stress on the interaction of slabs with the 670 km discontinuity

Results of high resolution seismic tomography showing subducting slabs deflected in the transition zone and thickened in the lower mantle seem to call for slab material weaker than inferred from mineral physics deformation mechanisms. A possible mechanism suggested by several authors could be the weakening due to grain size reduction, which should occur in the cold portion of fast slabs after an exothermic phase transition at a depth of 400 km. Since the amount of weakening as well as the rate of subsequent strengthening due to the grain growth are not precisely known, we present here a parametric study of slab behavior in the transition zone and upper part of the lower mantle. We simulate a subducting slab in a two-dimensional (2-D) Cartesian box in the numerical model with composite rheology including diffusion creep, dislocation creep and a general stress limiting rheology approximating Peierl’s creep. We concentrate on two rheologic effects: the dynamic effect of slab weakening due to grain size reduction at the phase boundary and the effect of yield stress of stress limiting rheology. The effect of trench migration on slab deformation is also included in our study. Results show that the slab ability to penetrate into the lower mantle is not significantly affected by a trench retreat in the absence of grain size weakening. However, in case of a 4 cm/yr trench retreat, grain size weakening provides a viable mechanism to deflect the slab in the transition zone, provided that stress limiting deformation mechanism would limit the effective viscosity outside the areas of grain size weakening to about 10 24 Pa s. < 2002 Elsevier Science B.V. All rights reserved.

Can long‐wavelength dynamical signatures be compatible with layered mantle convection?

Analyses of the long-wavelength geoid with seismic tomographic models have been providing for a long time important estimates of mantle viscosity. These estimates have nearly been derived under the assumption of whole mantle flow. It has been commonly held that a fully impermeable boundary at 660 km depth is incompatible with the long-wavelength gravity signal. On the other hand, models with whole mantle circulation, which can explain a large portion of the geoid signal, usually produce excessive amplitudes of the dynamical topography, especially for long wavelengths. Using recent tomographic models together with genetic algorithm we have successfully demonstrated that the layered convection model can also produce a reasonable fit to the geoid, which is comparable in quality with that obtained for the whole mantle model. The layered model can simultaneously yield realistic amplitudes of the dynamical topographies of the surface and the 660-km discontinuity.

Slope of the geoid spectrum and constraints on mantle viscosity stratification

Spectral analysis of a recently obtained high resolution tomographic model, describing the top 1200 km of the mantle, shows a power-law dependence on the degree, for degrees greater than around 10. The spectrum of recent geoid models is also found to decay in a linear fashion with degree on a log-log plot. We have employed the logarithmic slope of the geoid between degrees 10 and 25 as a constraint on the viscosity structure of the top 1200 km of the mantle. The constraint of fitting the geoid slope represents a new and independent approach to the determination of the upper-mantle viscosity structure. From conducting over one million runs in a Monte Carlo inversion, we have found that there are basically three families of viscosity which can fit the geoid slope. They are (1) with a viscosity hill between 660 and 1000 km, (2) with a weak viscosity increase at 660 km, and (3) with a significant viscosity increase at depths between 820 and 1000 km. Below 1000 km the viscosity of the lower-mantle for all 3 families is larger than that in the upper mantle. These results corroborate the complexity of the mantle viscosity profile between 660 and 1200 km, which would have important ramifications on flows between the upper and lower mantle.

The effects of rheological decoupling on slab deformation in the Earth’s upper mantle

Processes within subduction zones have a major influence on the plate dynamics and mantle convection. Subduction is controlled by a combination of many parameters and there is no simple global relationship between the resulting slab geometry and deformation and any specific subduction parameter. In the present work we perform a parametric study of slab dynamics in a two-dimensional model with composite rheology including diffusion creep, dislocation creep and stress limiter or Peierls creep. The mechanical decoupling of the subducting and overriding plates is facilitated by a low viscosity crust. We are particularly interested in the effect of the contact of subducting and overriding plates on the plate geometry in the upper mantle. We also study the influence of the surface boundary condition and of the rheological description (yield stress of stress-limiting rheology, additional viscosity contrast at 660-km discontinuity). Our results demonstrate that the slab morphology and deformation in the upper mantle and the transition zone is sensitive not only to the slab strength, but also to the decoupling mechanism at the contact of the subducting and overriding plates. Weak crust with a viscosity of 1020 Pa s effectively decouples the subducting and overriding plates and produces reasonable slab morphologies. The geometry of the slab in the upper mantle is strongly influenced by the initial geometry of the contact between the subducting and overriding plates. Further, a step-wise viscosity increase by about an order of magnitude at 660 km depth is necessary to limit the plate velocities to a reasonable value around 5 cm/yr.

New Perspectives on Mantle Dynamics from High-resolution Seismic Tomographic Model P1200

Recently a high-resolution tomographic model, the P1200, based on P-wave travel times was developed, which allowed for detailed imaging of the top 1200 km of the mantle. This model was used in diverse ways to study mantle viscosity structure and geodynamical processes. In the spatial domain there are lateral variations in the transition zone, suggesting interaction between the lower-mantle plumes and the region from 600 km to 1000 km. Some examples shown here include the continental region underneath Manchuria, Ukraine and South Africa, where horizontal structures lie above or below the 660 km discontinuity. The blockage of upwelling is observed under central Africa and the interaction between the upwelling and the transition zone under the slow Icelandic region appears to be complex. An expansion of the aspherical seismic velocities has been taken out to spherical harmonics of degree 60. For degrees exceeding around 10, the spectra at various depths decay with a power-law like dependence on the degree, with the logarithmic slopes in the asymptotic portion of the spectra containing values between 2 and 2.6. These spectral results may suggest the time-dependent nature of mantle convection. Details of the viscosity structure in the top 1200 km of the mantle have been inferred both from global and regional geoid data and from the high-resolution tomographic model. We have considered only the intermediate degrees (l= 12–25) in the nonlinear inversion with a genetic algorithm approach. Several families of acceptable viscosity profiles are found for both oceanic and global data. The families of solutions for the two data sets have different characteristics. Most of the solutions asociated with the global geoid data show the presence of asthenosphere below the lithosphere. In other families a low viscosity zone between 400 and 600 km depth is found to lie atop a viscosity jump. Other families evidence a viscosity decrease across the 660 km discontinuity. Solutions from oceanic geoid show basically two low viscosity zones: one lying right below the lithosphere; the other right under 660-km depth. All of these results bespeak clearly the plausible existence of strong vertical viscosity stratification in the top 1000 km of the mantle. The presence of the second asthenosphere may have important dynamical ramifications on issues pertaining to layered mantle convection. Numerical modelling of mantle convection with two phase transitions and a realistic temperature- and pressure-dependent viscosity demonstrates that a low viscosity region under the endothermic phase transition can indeed be generated self-consistently in time-dependent situations involving a partially layered configuration in an axisymmetric spherical-shell model.

Mantle viscosity inferred from geoid and seismic tomography by genetic algorithms: Results for layered mantle flow

Until recently, the long-wavelength geoid has almost been exclusivly analysed with whole mantle flow models. Such models may not reflect the actual flow situation across the transition zone, where the flow conditions may vary regionally from whole mantle flow models to partially layered conditions. We have investigated whether the observed geoid signal can be compatible with a layered flow model. By using the genetic algorithm, a non-linear global optimization technique, we demonstrate that it is indeed possible to find such parameters of the mantle, namely the viscosity and the seismic velocity-to-density scaling factor, which allow the long wavelength geoid to be fit with the same accuracy as for the whole mantle models. A more reliable estimate of the local flow situation can be obtained from analysis of the long-wavelength geoid together with observation of the surface dynamic topography at different wavelengths. Both the layered and the whole-mantle flow models produce similar dynamic topographies at intermediate and high degrees. At the lowest degrees, however, the topographies are strikingly different, showing opposite signs and very different amplitudes in many tectonic regions, in particular behind the subduction zones. We propose that comparing the sign and amplitudes of the observed topography in different portions of the spectrum would serve as a useful diagnostic tool for quantifying the local permeability across the 660-km boundary.

A recent deep earthquake doublet in light of long-term evolution of Nazca subduction

Earthquake faulting at ~600 km depth remains puzzling. Here we present a new kinematic interpretation of two Mw7.6 earthquakes of November 24, 2015. In contrast to teleseismic analysis of this doublet, we use regional seismic data providing robust two-point source models, further validated by regional back-projection and rupture-stop analysis. The doublet represents segmented rupture of a ∼30-year gap in a narrow, deep fault zone, fully consistent with the stress field derived from neighbouring 1976–2015 earthquakes. Seismic observations are interpreted using a geodynamic model of regional subduction, incorporating realistic rheology and major phase transitions, yielding a model slab that is nearly vertical in the deep-earthquake zone but stagnant below 660 km, consistent with tomographic imaging. Geodynamically modelled stresses match the seismically inferred stress field, where the steeply down-dip orientation of compressive stress axes at ∼600 km arises from combined viscous and buoyant forces resisting slab penetration into the lower mantle and deformation associated with slab buckling and stagnation. Observed fault-rupture geometry, demonstrated likelihood of seismic triggering, and high model temperatures in young subducted lithosphere, together favour nanometric crystallisation (and associated grain-boundary sliding) attending high-pressure dehydration as a likely seismogenic mechanism, unless a segment of much older lithosphere is present at depth.

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.

Effect of a Viscosity Interface at 1000 km Depth on Mantle Circulation

We present the preliminary results of axisymmetric numerical simulations of thermal convection in the mantle with a phase transition boundary at 660 km depth and a viscosity interface at 1000 km depth. The results, obtained for Ra = 2 × 106, are compared with the case when both the phase and the viscosity boundaries are located at the same depth of 660 km.

INFLUENCE OF THE LOAD WAVELENGTH ON THE PERMEABILITY OF A VISCOSITY INTERFACE IN THE MANTLE

Assuming a radially stratified Newtonian mantle in a steady-state approximation, we demonstrate that the permeability of a viscosity interface at 660-km depth strongly depends on the wavelength of buoyancy forces driving the flow. The flow induced by long-wavelength loads penetrates through the boundary freely even if the viscosity increases by two orders. In contrast, the boundary is practically impermeable for short-wavelength loads located in the upper mantle. Thus, a stepwise increase of viscosity is a significant obstacle for small descending features in the upper mantle, but huge upper mantle downwellings, or upwellings formed in the-lower mantle can overcome it easily. This indicates that certain care is necessary in interpreting the seismic structure of the mantle by means of flow models. The global tomographic image includes only the first few degrees of the harmonic series and, consequently, its interpretation in terms of a present-day flow field results in a predominantly whole-mantle circulation even for extreme viscosity contrasts.