The Microphysics of Plate Boundary Formation: Dynamic Recrystallization and Phase Mixing

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The modern theory of plate tectonics explains much about the structure and workings of our planet from sea-floor spreading to subduction and mountain building. Although this powerful theory tells us so much about activity on the surface of our planet, relatively little is known about how plate tectonics starts and the underlying physics that allows it to continue over time. There is little doubt that convection of the mantle drives the motion of these plates. However, the details of how this process occurs and how it originated are far from complete. Plate tectonics is defined by special boundaries, namely subduction zones, ridges, and strike-slip fault margins at the edges of continents. These boundaries are formed at the top of the mantle, which requires some special physical properties that localize deformation through certain mechanisms. Thankfully, the Earth has provided an important clue through the abundance of rocks called mylonites that are found at plate boundaries. These fine-grain rocks provide evidence of the physics that might tell us how plate tectonics started and how it persists today. The investigator's research captures the essential physics of the mechanisms that lead to development of mylonites that persist as weak zones and ultimately develop as plate boundaries. This work is the primary focus of the doctoral dissertation project of a graduate student in the PI's lab and will help develop the student's skills as a scientist in addition to providing fundamental research necessary for the future progress of our understanding of the generation of tectonic plates. The experimental techniques developed in this research will be applied to new undergraduate lab exercises that demonstrate complex processes involving feedback between microstructural development and mechanical properties of the material. The Earth is unique in the solar system in that it displays plate tectonics. The physical explanation for this behavior remains a major challenge. This research focuses on the influence of (i) a secondary solid phase and (ii) localized deformation on lithospheric strength with implications for plate tectonics. The project emphasizes a transformative approach to laboratory experiments designed to investigate the role of a second phase in the deformation of olivine, the primary phase in Earth's upper mantle. To achieve steady-state microstructures and thus steady state creep rates, the team will fabricate two-phase samples with grain sizes significantly larger than the steady-state grain size. They will then deform these samples in torsion to large shear strains, such that dynamic recrystallization produces a steady-state grain size. The dependence of strain rate on stress is determined through rate or load steps. Some samples will be annealed after large strain to measure grain growth in thoroughly mixed, dynamically recrystallized samples. Grain-size reduction due to dynamic recrystallization of coarse-grained rocks is well established both experimentally and in observations of natural shear zones. This process leads to a grain-size sensitive process and associated strain weakening. However, without continued deformation and dynamic recrystallization, grains will grow and ultimately lead to hardening that makes reactivation of a shear zone difficult. In polyphase materials, pinning of grain boundaries by a secondary phase inhibits grain growth, allowing localization to persist or to be more easily reactivated. For this process to be effective, the phases must be thoroughly mixed. Understanding both the chemical and mechanical processes that lead to phase mixing and localization is the primary goal of this proposal.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.