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 and integration with constraints of the deep Earth from other disciplines.
Strain-weakening rheology in Earth’s lower mantle
Anna Gülcher, Gregor Golabek, Marcel Thielmann, Maxim Ballmer, and Paul Tackley
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 the effect of strain-weakening rheology on on global-scale mantle convection and ultimately plate tectonics.
Manuscript in preparation
Coupled dynamics and evolution of primordial and recycled heterogeneity
Anna Gülcher, Maxim Ballmer, and Paul Tackley
Topics of particular attention are the nature of chemical lower-mantle heterogeneity, how it has evolved over time, and how it has affected our planet’s evolution. In this sub-project, we investigate the coupled dynamics and evolution of primordial domains and recycled material in Earth-like numerical models. Primordial and recycled materials are robustly predicted to co-exist in models of Earth-like planets. Our models provide a new integration of independent hypotheses of present-day lower-mantle heterogeneity, which we link to geochemical and geophysical observations.
Dynamic styles of primordial material preservation in Earth’s interior
Anna Gülcher, David Gebhardt, Maxim Ballmer, and Paul Tackley
Cosmo- 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. We explore the effects of composition-dependent rheology on heterogeneity preservation and the dynamics of mantle mixing. 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 are highly relevant for Earth as they can reconcile the preservation of primordial domains in a convecting mantle.
Corona structures driven by plume-lithosphere interactions and evidence for ongoing plume activity on Venus
Anna Gülcher, Taras Gerya, Laurent Montési, and Jessica Munch
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, remains 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. We show how coronae provide unique insights into the present-day geological activity of Venus.
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. Therefore, corona structures related to ongoing plume-lithosphere interaction may be distinguished from fossil corona structures. Guided by these outcomes, we systematically investigated the topographic patterns of large coronae on the Venusian surface. 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.
On the formation of detachment faults and their influence on intra-oceanic subduction initiation
Anna Gülcher, Stéphane Beaussier, and Taras Gerya
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. We conducted a series of 3D numerical 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).