The lower mantle is the largest geochemical reservoir in the Earth’s interior. It controls the style of mantle convection and, through it, the evolution of our planet over billions of years. Constraining the composition and structure of Earth’s lower mantle, however, remains a scientific challenge that requires cross-disciplinary efforts. In my PhD project, I search for the chemical and rheological recipe of Earth’s lower mantle through numerical models of long-term mantle convection. In particular, I test recent hypotheses of primordial and/or rheological heterogeneity in the deep Earth.
Dynamic styles of primordial material preservation in Earth’s interior
Cosmochemical and geochemical constraints indicate that the lower mantle hosts an ancient primordial reservoir enriched in silica with respect to the upper mantle. Yet, geophysical observations and models point to efficient convective mixing across the entire mantle. Recent hypotheses of primordial-material preservation in a convecting mantle involve delayed mixing of intrinsically dense and/or intrinsically strong heterogeneity. In our 2020 EPSL paper, we explore the effects of composition-dependent rheology on heterogeneity preservation and the dynamics of mantle mixing. By conducting a systematic parameter study using 2D global mantle convection simulations, we establish multiple regimes of primordial material preservation that can occur in terrestrial planets. Some of these regimes are characterised for the first time and some regimes can reconcile the preservation of primordial domains in a convecting mantle.
The coexistence of primordial and recycled heterogeneity in a convecting mantle
In a subsequent study, we investigate the dynamics and interplay between primordial, viscous domains and recycled, dense material in Earth-like numerical models. Moreover, integration of the geodynamic models with observations from seismology, mineral physics and geochemistry is on its way.
Strain-dependent rheology in Earth’s lower mantle
The lower mantle’s main constituents are the strong mineral Bridgmanite (~80%) and the weaker Ferropericlase (~20%). The viscosity of such a multiphase rock depends on the modal abundance of weak versus strong minerals as well as the fabric of the rock. Using geodynamic models with a newly implemented strain- and history dependent rheology, we study its effect on global-scale mantle convection, plate tectonics and long-term planetary evolution.
Our neighbouring planet Venus holds key insights into terrestrial planet evolution. Despite the absence of plate tectonics, its surface is littered with volcanic structures, rifts and mountains. To what extend these surface features reflect the current state of the planet’s interior, remain in question. The large ring-shaped volcano-tectonic corona structures may bear testimony of turbulent interior processes of Venus as their formation is often linked to underlying mantle plumes. In our 2020 Nature Geoscience paper, we show how coronae provide unique insights into the present-day geological activity of Venus.
Plume-induced coronae formation
We systematically ran 3D computer models of plume-lithosphere interactions on Venus to assess the origin coronae, and the reason behind their morphological differences. We found that different corona morphologies not only represent different dynamic styles of plume-lithosphere interactions, but also different stages in evolution. We concluded that corona structures related to ongoing plume-lithosphere interaction may be distinguished from fossil corona structures.
Evidence for ongoing plume activity on Venus
Guided by the conclusions from our numerical modelling study, we systematically investigated the topographic patterns of large coronae on the Venusian surface. Subsequently, we identified which structures are currently active and which are currently inactive. This assessment revealed broad regions of ongoing plume activity on the planet, presenting new evidence for widespread recent magmatic activity on the surface of Venus. The global distribution of this proposed tectono-magmatic activity sparks intriguing questions on Venusian deep interior circulation and dynamics.
Extensional detachment faults, widely documented in slow-spreading and ultraslow-spreading ridges on Earth, can effectively localise deformation due to their weakness. After the onset of oceanic closure, these weak faults may directly control the nucleation of a subduction zone parallel to the former mid-ocean ridge. In our 2019 EPSL paper, we conducted a series of 3D numerical thermomechanical experiments in order to investigate the formation of detachment faults in slow oceanic spreading systems and their subsequent response upon inversion from spreading to convergence. We define the controlling parameters for detachment fault formation, and show how these faults affect the dynamics of intra-oceanic subduction initiation.
Schematic sketch of the typical model evolution found in Gülcher et al. (2019), EPSL. Onset of slow oceanic spreading, detachment fault formation near the ridge (A); mature oceanic spreading stage with “Christmas tree” faulting pattern (B); and convergence and underthrusting stage, with intra-oceanic subduction initiation (C).