We obtained likelihoods in the lower mantle for long-wavelength models of bulk sound and shear wave speed, density, and boundary topography, compatible with gravity constraints, from normal mode splitting functions and surface wave data. Taking into account the large uncertainties in Earth's thermodynamic reference state and the published range of mineral physics data, we converted the tomographic likelihoods into probability density functions for temperature, perovskite, and iron variations. Temperature and composition can be separated, showing that chemical variations contribute to the overall buoyancy and are dominant in the lower 1000 kilometers of the mantle.

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We report a systematic study on the conditions under which an endothermic phase transition can enforce layered convection. Two‐dimensional numerical calculations of convection in a domain containing a divariant phase change were performed in the framework of the “extended Boussinesq approximation,” i.e., considering the effects of adiabatic gradient, latent heat, and frictional heating in the energy equation. We find that the critical value of the negative Clapeyron slope, which must be surpassed in order to induce layered convection, decreases in magnitude with increasing Rayleigh number Ra in the range 104 ≤ Ra ≤ 2×106. Near the critical Clapeyron slope, vacillations between double‐ and single‐layer convection or strongly leaking double‐layer convection are possible. The breakdown into layers is influenced very little by the latent heat release but depends solely on the phase boundary deflection caused by lateral temperature differences. The value of the critical Clapeyron slope also seems little affected by the width of the transition zone or by its depth. A possible superplastic rheology within the transition zone would tend to favor layered convection. Scaling the model results to the 670‐km discontinuity in the earth's mantle as a possible endothermic phase boundary, we estimate the critical Clapeyron slope to be in the range of −4 to −8 MPa/K (−40 to −80 bar/K). The possibility that the spinel → perovskite + periclase transition is within this range appears to be remote but certainly cannot be neglected.

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We have used a finite element model of time‐dependent convection to determine the conditions for penetration of the subducted plate into the lower mantle. A temperature‐dependent and non‐Newtonian rheology is applied to achieve platelike behavior of the upper and sinking thermal boundary layer of convection. The 650‐km discontinuity is taken as either a chemical or phase boundary or as a combination of both. It is represented by a marker chain which effects additional buoyancy when distorted out of its equilibrium position. When the compositional density contrast is greater than about 5%, the descending slab is deflected sidewards at the boundary and two‐layer convection prevails. A resulting depression of the boundary in the range of 50–200 km should be detectable with seismic methods. Below 5% density difference the slab plunges several hundred kilometers into the lower mantle, and below 2% it will probably not stop before reaching the core‐mantle boundary and extensive mixing would be expected. With a pure phase change a negative Clapeyron slope of about −6 MPa/K (−60 bar/K) is required to establish a type of “leaky” double‐layer convection. A more moderate slope can aid a small compositional density difference to prevent slab penetration into the lower mantle. With the present uncertainties about the physical nature of the 650‐km discontinuity, a variety of convective styles appears possible on dynamical grounds.

Subduction of the lithosphere at convergent-plate boundaries takes place asymmetricallythe subducted slab sinks downward, while the overriding plate moves horizontally (one-sided subduction). In contrast, global mantle convection models generally predict downwelling of both plates at convergent margins (two-sided subduction). We carried out two-dimensional (2-D) numerical experiments with a mineralogical-thermomechanical viscoelastic-plastic model to elucidate the cause of one-sided subduction. Our experiments show that the stability, intensity, and mode of subduction depend mainly on slab strength and the amount of weak hydrated rocks present above the slab. Two-sided subduction occurs at low slab strength (sin[ϕ] < 0.15, where ϕ is effective internal friction angle), regardless of the extent of hydration. In contrast, steadystate one-sided subduction requires a weak hydrated slab interface and high slab strength (sin[ϕ] > 0.15). The weak interface is maintained by the release of fl uids from the subducted oceanic crust as a consequence of metamorphism. The resulting weak interplate zone localizes deformation at the interface and decouples the strong plates, facilitating asymmetric plate movement. Our work suggests that high plate strength and the presence of water are major factors controlling the style of plate tectonics driven by self-sustaining one-sided subduction processes.

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