The presence of offsets, appearing at intervals ranging from 10s to 100s of kilometres, is a distinct characteristic of constructive tectonic plate margins. By comparison, boundaries associated with subduction exhibit uninterrupted continuity. Here, we present global mantle convection calculations that result in a mobile lithosphere featuring dynamically derived plate boundaries exhibiting a contrasting superficial structure which distinguishes convergence and divergence. Implementing a yield-stress that governs the viscosity in the lithosphere, spreading boundaries at the top of a vigorously convecting mantle form as divergent linear segments regularly offset by similar length zones that correlate with a large degree of shear but comparatively minimal divergence. Analogous offset segments do not emerge in the boundaries associated with surface convergence. Comparing the similarity in the morphologies of the model plate margins to the Earth’s plate boundaries demonstrates that transform-like offsets are a result of stress induced weakness in the lithosphere owing to passive rupturing.

SUMMARY Previous geodynamic studies have indicated that the presence of a compositionally anomalous and intrinsically dense (CAID) mantle component can impact both core heat flux and surface features, such as plate velocity, number and size. Implementing spherical annulus geometry mantle convection models, we investigate the influence of intrinsically dense material in the lower mantle on core heat flux and the surface velocity field. The dense component is introduced into a system that features an established plate-like surface velocity field, and subsequently we analyse the evolution of the surface velocity as well as the interior thermal structure of the mantle. The distribution and mobility of the CAID material is investigated by varying its buoyancy ratio relative to the ambient mantle (ranging from 0.7 to 1.5), its total volume (3.5–10 per cent of the mantle volume) and its intrinsic viscosity (0.01–100 times the ambient mantle viscosity). We find at least three distinct distributions of the dense material can occur adjacent to the core–mantle boundary (CMB), including multiple piles of varying topography, a core enveloping layer and two diametrically opposed provinces (which can on occasion break into three distinct piles). The latter distribution mimics the morphology of the seismically observed large low shear wave velocity provinces (LLSVPs) and can occur over the entire range of CAID material viscosities. However, diametrically opposed provinces occur primarily in cases with CAID material buoyancy numbers of 0.7–0.85 (corresponding to contrasts in density between ambient and CAID material of 130 and 160 kg m−3, respectively) in our model (with an effective Rayleigh number of order 106). Steep and high topography piles are also obtained for cases featuring buoyancy ratios of 0.85 and viscosities 10–100 times that of the ambient mantle. An increase in relative density, as well as larger volumes of CAID material, lead to the development of a core enveloping layer. Our findings show that when two provinces are present core heat flux can be reduced by up to 50 per cent relative to cases in which CAID material is absent. Surface deformation quantified by Plateness is minimally influenced by variation of the properties of the dense material. Surface velocity is found to be reduced in general but mostly substantially in cases featuring high CAID material viscosities and large volumes (i.e. 10 per cent) or buoyancy ratios.

The generation of a magnetic field and the presence of tectonic plates are fundamental aspects of Earth's evolution. Viable dynamic models of terrestrial mantle convection therefore require the existence of heat transport from the core and a surface characterized by piecewise uniform surface velocity domains (i.e., plates). To test the compatibility of these two requirements, we varied energy input, rheology, and compositional heterogeneity in more than 70 mantle convection simulations. Calculations are performed in a spherical annulus geometry. By systematic investigation, we demonstrate that Earth‐like core heat flux can be obtained with self‐consistent model plates. We find that, given an initial condition where model parameters are chosen to ensure a mobile surface, the mobility is not strongly influenced when H is varied by up to a factor of three. Consequently, in spherical models with steady core temperatures and internal heating rates, the latter quantity can be used to regulate core heat flow to fit within the bounds inferred for terrestrial values. In contrast, core heat flow is strongly sensitive to model yield stress. We systematically vary thermal viscosity contrast, an intrinsic depth‐dependent viscosity and the depth‐dependence of a stress‐dependent rheology and analyze how each factor affects both surface velocities and core heat flux. Finally, we show that the addition of an intrinsically dense component comprising 5% or less of the mantle volume does not affect surface mobility or plateness, although it can have a profound impact on heat loss from the core.

Summary More than two decades of systematic investigation has made steady progress towards generating plate-like surface behaviour in models of vigorous mantle convection. Accordingly, properties required to obtain dynamic plates from mantle convection have become widely recognized and employed in both 2D and 3D geometries. Improving our understanding of the properties required to obtain durable (or replenishable) deep mantle features with LLSVP-like characteristics has received interest for a period with similar longevity. Investigation ultimately focusses on discovering the properties able to produce the presence of a detached pair of three-dimensional features, distinct from the ambient mantle. Here, we assume the LLSVPs have a chemical origin by incorporating a Compositionally Anomalous and Intrinsically Dense (CAID) mantle component comprising 2-3.5 per cent of the total mantle volume. The feedback between plate formation and the presence of a CAID mantle component is investigated in both 2D and 3D spherical geometries. We explore the impact of both an intrinsic contrast in density and viscosity for the CAID component, with the objective of finding system parameter values that encourage the formation of a pair of LLSVP-like assemblages and a surface that exhibits the principle features of terrestrial plate tectonics; including recognizable and narrowly focussed divergent, convergent and (in 3D) transform plate boundaries that separate 8-16 distinct plate interiors. We present the results of nine two-dimensional and eleven three-dimensional calculations and show that for some of the cases examined, a pair of CAID material provinces can be freely obtained in two-dimensional cases while maintaining a surface characterized by plate-like behaviour. However, specifying the same system parameters in the three-dimensional model does not readily yield a pair of enduring provinces for any values of the parameters investigated. Moreover, the inclusion of the CAID component in the mantle can affect the global geotherm so that in comparison to the surface behaviour obtained for the initial condition isochemical model, the surface behaviour of the cases incorporating the dense component are less exemplary of plate tectonics. In general, CAID material components that are 3.75 per cent-5 per cent denser than the surrounding mantle (at surface temperatures), and up to a factor of 100 times greater in intrinsic viscosity, form layers populated by voids, or nodes connected by tendril-like ridges that reach across the core-mantle-boundary, rather than distinct piles resembling LLSVPs. Due to its inherently heavy and stiff character, in equilibrated systems, we find the CAID material becomes especially hot so that the temperature-dependence of its density and viscosity results in reduced distinction between the intrinsically dense assemblages and the ambient mantle. Accordingly, the CAID material forms masses on the CMB that are relatively less dense (0.625 per cent-1.5 per cent) and viscous than the adjacent mantle material, in comparison to the percentage differences obtained at common temperatures. We find that by adjusting our yield stress model to account for the influence of the CAID material on the geotherm, a highly satisfactory plate-like surface can be re-attained, however, the formation of a pair of LLSVP-shaped masses remains elusive.