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Cambridge NERC Doctoral Training Partnerships

Graduate Research Opportunities
 

Lead Supervisor: David Wallis, Earth Sciences

Co-Supervisor: Lars Hansen, Department of Earth and Environmental Sciences, University of Minnesota

Brief summary: 
This project will develop new models for transient creep in the seismic cycle based on high-temperature deformation experiments and microstructural observations.
Importance of the area of research concerned: 
Major earthquakes impose rapid stress changes on broad regions of the lower crust and upper mantle. The rocks in these regions respond by undergoing viscoelastic deformation during the postseismic period. This deformation plays a crucial role in controlling the evolution of stress state on the fault zone and provides a potential mechanism for delayed triggering of additional earthquakes. Postseismic deformation also generates geodetic signals used to monitor fault behaviour and infer the rheological properties of the lower crust and upper mantle. A major challenge in the effort to model postseismic deformation is that the viscosities of rocks are not constant but undergo a period of evolution in response to stress changes. This transient creep is exhibited by most crystalline materials but the microscale processes that generate such behaviour remain poorly understood. Consequently, there is great uncertainty regarding the most appropriate models with which to analyse postseismic deformation, which limits the reliability of model results.
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 underling 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.
Wallis, D., Hansen, L.N., Kumamoto, K.M., Thom, C.A., Plümper, O., Ohl, M., Durham, W.B., Goldsby, D.L., Armstrong, D.E.J., Meyers, C.D., Goddard, R., Warren, J.M., Breithaupt, T., Drury, M., and Wilkinson, A.J. (2020) Dislocation interactions during low-temperature plasticity of olivine and their impact on the evolution of lithospheric strength. Earth and Planetary Science Letters, 543, 116349, doi:10.1016/j.epsl.2020.116349.
Applying
You can find out about applying for this project on the Department of Earth Sciences page.
Dr David Wallis
Department of Earth Sciences Graduate Administrator