Periodic Reporting for period 3 - RIFT-O-MAT (Magma-Assisted Tectonics: two-phase dynamics of oceanic and continental rifts)
Reporting period: 2022-03-01 to 2023-08-31
Geological evidence shows that over hundreds of millions of years, Earth's continents drift across the denser mantle, floating like icebergs. This drift sometimes brings the continents together into larger aggregates, and it sometimes rifts them apart, creating ocean basins between them. This process of rifting creates sedimentary basins, surface topography, volcanic activity, and continental shelves, all of which are important to human evolution, resources, hazards, and economic development. Rifting is also a fundamental part of plate-tectonic theory, and hence essential to our understanding of the mechanics of Earth. Rifting is almost always associated with magmatism: melting of rocks at depth and injection of the liquid through cracks toward (and sometimes onto) the surface. This liquid and the heat it carries are understood to modify the mechanical properties of the system, weakening it significantly. Hence magmatism plays an essential role in the mechanics and evolution of rifts, and yet models of rifts have not been able to quantify this except in the simplest of terms. The purpose of this project is to apply a careful theoretical analysis to this problem, to assess the magmatic controls on rifting, and hence on plate tectonics. The objectives of the project are as follows: (1) to derive mathematical theory based on the physics of partially molten poro-viscoelastic-brittle rocks. To model brittle failure, we will use plasticity theory. (2) To incorporate this theory into numerical models that can capture the forces and motions associated with rifts. (3) To model continental rifting process for various configurations of thermal and mechanics forcing. (3) To model oceanic rifting (mid-ocean ridges) for over a range of spreading rates, magma supply rates, and variance in supply. (4) To interpret the model outputs in terms of observations of rifts, including topography, seismic structure, gas emissions, and heat flow.
A particular focus of the research will be on the relationship between dikes and fault, and how this is expressed in the topography of rifts. This has a bearing on the question of whether variations in magma supply are recorded in the topography of abyssal hills, which flank mid-ocean ridges.
A more societally relevant consideration of the project is how magmatism interacts with continental rifting to shape the formation of sedimentary basins, and potentially to localise hydrothermal circulation and the deposition of mineral resources.
A particular focus of the research will be on the relationship between dikes and fault, and how this is expressed in the topography of rifts. This has a bearing on the question of whether variations in magma supply are recorded in the topography of abyssal hills, which flank mid-ocean ridges.
A more societally relevant consideration of the project is how magmatism interacts with continental rifting to shape the formation of sedimentary basins, and potentially to localise hydrothermal circulation and the deposition of mineral resources.
The first stage of the research has been to develop a code framework for numerical solution of the partial differential equations that arise from the two-phase physics. This frameworks builds on a powerful, open-source library (PETSc), but encodes the specific equations that are relevant for the project (and some others). The equations are discretised using a staggered-mesh, finite-volume scheme, which has properties that are ideally suited to this problem. This architecture allows for regression testing of the codebase, and flexible use in problems at various levels of material complexity and boundary conditions. Having this tool has accelerated subsequent research.
The second stage has involved two projects that build toward the main goals. One is the development of a poro-viscoelastic-viscoplastic theory that can approximate fault-like and dike-like solutions. Our research has exposed shortcomings of previous attempts at this, and is progressing toward a physically grounded theory that allows for robust and stable numerical solutions. The second is the development of poro-viscous models of partially molten rock beneath mid-ocean ridges. These models incorporate variations in bulk density with melt fraction and composition. This allows us to explore the consequences of body-force variations on mantle flow. We have discovered that chemical depletion associated with melting tends to reduce the level of buoyancy-driven upwelling but, paradoxically, increases the propensity for upwelling to shift off the ridge axis, creating asymmetry in melting, as observed at most ridges. These models are also paving the way for mid-ocean ridge models that will include the poro-viscoelastic-viscoplastic theory.
A third project is exploring the use of linear-elastic fracture mechanics for modelling of dike formation. But as a test case, it considers the formation of basal crevasses beneath ice sheets and glaciers. This is a problem with important implications for understanding how these systems behave in a warming climate. We have shown that stress variations on the bed of the ice sheet, that may arise from friction or topography, can promote the formation of basal hydrofracturing of ice.
These projects are building our capacity to address the main questions of the project.
The second stage has involved two projects that build toward the main goals. One is the development of a poro-viscoelastic-viscoplastic theory that can approximate fault-like and dike-like solutions. Our research has exposed shortcomings of previous attempts at this, and is progressing toward a physically grounded theory that allows for robust and stable numerical solutions. The second is the development of poro-viscous models of partially molten rock beneath mid-ocean ridges. These models incorporate variations in bulk density with melt fraction and composition. This allows us to explore the consequences of body-force variations on mantle flow. We have discovered that chemical depletion associated with melting tends to reduce the level of buoyancy-driven upwelling but, paradoxically, increases the propensity for upwelling to shift off the ridge axis, creating asymmetry in melting, as observed at most ridges. These models are also paving the way for mid-ocean ridge models that will include the poro-viscoelastic-viscoplastic theory.
A third project is exploring the use of linear-elastic fracture mechanics for modelling of dike formation. But as a test case, it considers the formation of basal crevasses beneath ice sheets and glaciers. This is a problem with important implications for understanding how these systems behave in a warming climate. We have shown that stress variations on the bed of the ice sheet, that may arise from friction or topography, can promote the formation of basal hydrofracturing of ice.
These projects are building our capacity to address the main questions of the project.
We anticipate that the above strands of research will come together to create the capacity for modelling melt propagation across the cold, exterior boundary of the Earth, the lithosphere. With this theoretical/numerical framework, we will be able to test the hypotheses of the proposal. We will also have a powerful tool for research on other problems related to magmatism on Earth and other planetary bodies. This capacity is unprecedented in that it correctly accounts for both liquid and solid phases and their mechanical and thermal interactions, from the hot and viscous asthenosphere, to the cold and brittle near-surface.