Geodynamica

Anisotropic viscosity is likely prevalent within the upper mantle, but is usually disregarded in geodynamics models. On a crystal scale, olivine’s intrinsic properties are such that dislocation creep occurs over an order of magnitude more easily along olivine’s [100] symmetry axis than along its [001] axis. However, deforming olivine aggregates generate crystallographic preferred orientations (CPO) with their own macroscopically effective anisotropic viscosities that have proven difficult to estimate from the microscopic anisotropies of individual olivine crystals. Here we present a simple method to derive anisotropic viscosity parameters directly from the CPO mean orientation tensors. To calibrate the method, we created a large database of textures likely to occur in geodynamic simulations. We tested our method within numerical simulations of simple shear with both constant and varying shear directions. Finally, we integrated our method into the geodynamic code ASPECT, where it can be used to explore the geodynamic importance of anisotropic viscosity.

Chemical buoyancy within the lithosphere may be prevalent in certain depth regions. To constrain those depths, we follow an approach similar to Wang et al. (2023) and plot the difference between residual topography—where only crustal isostatic topography has been removed—and dynamic topography—where only sub-lithospheric density anomalies are considered—against lithosphere thickness. In theory, this difference should be caused by density anomalies in the lithosphere, and the slope of the fitting line versus lithosphere thickness should hence indicate depth-dependent density anomalies in the lithosphere. When computing dynamic topography, lithosphere thickness and density anomalies outside the continental lithosphere are derived from tomography. Within the continental lithosphere, density anomalies are set to a reference value, for which we use either the global mean, or the depth-dependent mean values below mid-ocean ridges. With densities in the continental lithosphere set to global mean, we typically find a break in slope around 150 km thickness, from nearly zero above to a negative slope below. With densities set to mid-ocean ridge values, there is still a break in slope but the slope above 150 km is also negative. This indicates that chemical density anomalies that cause lithospheric buoyancy are concentrated in the upper ≈150 km. Whether or not there are substantial density anomalies also above 150 km mainly depends on whether the global mean, or mid-ocean ridges are used for reference.

Long-lived lateral variations in radiogenic heat production create persistent thermal heterogeneities that shape continental lithosphere over geological timescales. We introduce a steady-state concept of thermal inheritance, linking these variations to crustal-scale strain localization and tectonic architecture.
Using numerical models, we explore both crustal- and lithospheric-scale consequences of heterogeneous heat production. A key finding is that lateral variations in heat production leave a distinct crustal-scale tectonic signature, controlling patterns of strain localization. The South China Sea serves as a proof-of-concept: the segmented, oblique extension observed there aligns with zones of mechanically weaker crust, reflecting the underlying inherited thermal heterogeneity.
These results highlight that crustal-scale tectonic features can emerge from steady-state thermal conditions, independently of transient anomalies. They provide a quantitative framework linking inherited thermal structure to observable deformation patterns. More broadly, our study suggests that laterally heterogeneous heat production offers a physically motivated alternative to traditional exponential-decay models, better capturing the spatial complexity and persistence of lithospheric thermal structure.
By emphasizing the crustal imprint of thermal inheritance, we demonstrate that radiogenic heat variations are a fundamental control on strain localization and tectonic segmentation. This approach opens a new perspective on how long-lived thermal heterogeneities shape continental deformation and the architecture of lithospheric structures over hundreds of millions of years.

Most rocky planets operate in a stagnant lid convective regime. They are characterized by the strongly temperature-dependent viscosity that locks the cold mantle silicates near the surface into a stiff layer, or lid, which does not participate in mantle convection. Stagnant lid planets have lower surface heat flows than mobile lid planets like Earth; however, Earth-like heat flows have been inferred from elastic lithosphere thickness estimates for Venus leading researchers to wonder how these higher heat fluxes occur. Mineral phase transitions in the upper mantle can inhibit or enhance mass exchange through the mantle transition region at pressures and temperatures relevant for larger terrestrial mantles such as Earth’s or Venus’s. For a multiphase composition of anhydrous pyrolite, small cold plume instabilities originating at the base of the stagnant lid sink to a depth of about 600 km.  When a critical amount of material accumulates, the drips avalanche into the lower mantle as a large coherent downwelling, generating warm return flow into the upper mantle.  Previous work using 2D numerical models of mantle convection with an assumed anhydrous pyrolite composition showed wadsleyite transforms into majorite plus ferropericlase for temperatures warmer than 1950 K. This study explores how the endothermic WMF transition demonstrates weaker upper-lower mantle layering for increasingly higher Rayleigh numbers and higher mantle temperatures. This counterintuitive weakening induces more numerous and continuous flushing events (mantle avalanches) between the upper and lower mantle, inducing a state of sustained, higher-efficiency heat flow throughout the mantle, manifesting as a “jump” or upwards shift in the Rayleigh-Nusselt scaling compared to colder models that also exhibit the effects of mantle layering. This higher efficiency state of continuously- layered-and-flushed stagnant lid convection may be relevant for Venus which lacks apparent plate tectonics yet has regions which show evidence of Earth-like (high) surface heat fluxes.

Hydrocarbons are observed on the surfaces of many icy moons and planets. At high-pressure high-temperature conditions, such as occur in larger planets, they are known to dissociate to form diamond and hydrogen. Within our solar system Uranus and Neptune easily reach the requisite 10 GPa and 2000 K for this process, while icy moons do not.  Densities indicative of icy compositions are commonly observed for exoplanets, many of which have radii intermediate between the local icy moons and icy planets. These so-called 'mini-neptunes' are a common class of exoplanet and, where hydrocarbons are incorporated within their ices, are candidates for diamond formation. Here we simulate model icy exoplanets to investigate the size required to induce diamond formation.  Where the conditions are met, the denser diamond will sink through the ices deeper into the planet under gravity. This provides a source of internal heating, and will sequester the carbon deep within the planet. As a consequence, exoplanets with deep ice layers likely have shallow regions depleted in carbon.