Geodynamica

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.

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.