Lead supervisor: David Wallis, Earth Sciences
Co-supervisor: Lars Hansen, School of Earth and Environmental Sciences, University of Minnesota
Brief summary:
This project will develop new models for transient creep of geological materials based on high-temperature deformation experiments and microstructural observations.
Importance of the area of research concerned:
Geodynamic processes operating over human timescales, such as postseismic deformation and anthropogenic glacial isostatic adjustment, apply rapid changes in stress to rocks at high temperatures in the lower crust and upper mantle. Unlike the long-term deformation associated with many tectonic processes, which can be assumed to involve steady-state flow, these rapid events involve non-steady-state deformation. Both laboratory experiments and geodetic observations indicate that rocks deforming in these contexts undergo transient creep, whereby viscosity evolves with strain/time. Modelling transient creep is therefore of critical importance to understanding the behaviour of the solid earth over human timescales. However, the microstructural processes that control transient creep of the key geological minerals remain poorly constrained. As such, it is difficult to parameterise models of transient creep in a manner that captures the essential microphysics, limiting our confidence in extrapolations of such models from the conditions under which they are calibrated in the laboratory to the conditions prevailing during deformation in the lower crust and upper mantle.
Project summary :
This project aims to develop new models of transient creep at high temperatures in major rock-forming minerals. Recent work has revealed new clues that transient creep of olivine arises from the evolution of stress heterogeneity generated by lattice dislocations, providing the basis for a new generation of transient-creep models rooted in the underlying microphysical processes. This project will develop this model and test its applicability to other minerals by providing new quantitative datasets on the evolution of microstructures in response to changes in stress. Key microstructural parameters will be the evolution of stress heterogeneity and the densities and distributions of dislocations and subgrain boundaries. These datasets will be used to formulate and calibrate new constitutive equations for transient creep that can be incorporated into geophysical and geodetic analyses.
What will the student do?:
This project will combine three main elements of deformation experiments, microstructural analyses, and modelling. Deformation experiments will be conducted at temperatures up to 1500°C on a new deformation apparatus designed to impose rapid changes in stress analogous to those imposed by earthquakes. The evolution of strain rate, and hence viscosity, will be monitored following stress increases and decreases under varied conditions. The microstructures of the deformed samples will be analysed in unprecedented detail using the new technique of high-angular resolution electron backscatter diffraction. Critically, this technique can map dislocation density and stress heterogeneity, which are hypothesised to control transient creep. The combined mechanical data and microstructures will be used to formulate and calibrate new constitutive equations describing transient creep. Microstructural analyses of natural rocks will test the validity of the new models. The student will be involved in the choice of minerals to analyse and all aspects of experiment design, execution, and analysis, and will be encouraged to develop and pursue their own research directions.
References - references should provide further reading about the project:
Wallis, D., Hansen, L.N., Wilkinson, A.J., and Lebensohn, R.A. (2021) Dislocation interactions in olivine control postseismic creep of the upper mantle. Nature Communications, 12, 3496, doi: 10.1038/s41467-021-23633-8.
Hansen, L.N., Wallis, D., Breithaupt, T., Thom, C.A., and Kempton, I. (2021) Dislocation creep of olivine: Backstress evolution controls transient creep at high temperatures. Journal of Geophysical Research: Solid Earth, 126, e2020JB021325, doi:10.1029/2020JB021325.
Breithaupt, T., Katz, R.F., Hansen, L.N., Kumamoto, K.M. (2023) Dislocation theory of steady and transient creep of crystalline solids: Predictions for olivine. Proceedings of the National Academy of Sciences, 120, e2203448120, doi:10.1073/pnas.2203448120.
Applying
You can find out about applying for this project on the Department of Earth Sciences page.