A new method for mapping stresses in mantle rocks: Dislocation density from electron-backscatter diffraction

The viscous flow of solid rock in Earth's deep interior fundamentally controls a variety of large-scale processes ranging from the formation of tectonic plate boundaries to the timescale of sea-level rise associated with receding ice sheets to the dynamics of the deepest portions of earthquake-generating faults. The most common approach to learning about the flow of rocks at extreme conditions is to conduct experiments in a laboratory on small samples at relatively short timescales. The relevant processes in the Earth, however, occur over many kilometers and millions of years, scales much larger than those accessible in the laboratory. For scientists to know how best to extrapolate from small scales to large scales when predicting the behavior of real Earth processes, they need a robust understanding of the microscopic physical mechanisms controlling the flow of rocks at extreme conditions.

Several sophisticated models exist describing the viscous behavior of rocks. However, considerable uncertainty persists regarding their details, and there is a corresponding lack of suitable analytical techniques to test those details. Therefore, we propose to adapt a new technique for quantifying the intricacies of the viscous flow of rocks. This technique, recently developed for analyzing small-scale structures in metals, allows defects in the constituent crystals-known as dislocations-to be mapped quickly and precisely at high resolution in an electron microscope. We will conduct laboratory deformation experiments on single crystals of olivine, the dominant mineral in the upper mantle, to establish a relationship between the density of dislocations and the deformation conditions. Determining this relationship will enable us to map deformation conditions at very fine scales in synthetic and natural olivine-rich rocks deformed in variety of settings. We will then compare the results of this mapping to predictions from current models for olivine deformation, using the results to rule out models that do not capture the essential physical processes and to refine and update the ones that do. The project will thus yield a new method in the Earth sciences applicable to a wide range of minerals, and it will address several outstanding problems in the deformation of upper-mantle rocks.

The proposed research forms an interdisciplinary blue skies project, focussed on providing fundamental understanding that can underpin the application of laboratory-derived models of rock deformation to large-scale geological processes. The progress of such processes - over timescales ranging from very short to geologic in length - is hidden from direct observation and measurement and yet is crucial to addressing key outstanding problems in geodynamics with major societal and economic impact on communities worldwide. Such issues include the identification, assessment and mitigation of (intra-plate) earthquake and volcanic hazards and even the extraction of oil, gas and other mineral wealth using modern techniques such as induced hydraulic fracturing or 'fracking'.

The immediate end users engaged in modelling rock deformation, who would directly benefit from the project results, are primarily engaged within the academic community. However, the results of the project are likely to have down the line benefits for non-academic users - both industrial and governmental - involved in the application of rock deformation modelling for fossil fuel and mineral extraction, large scale civil engineering, construction, earthquake hazard mitigation and volcanic threat monitoring.

There is clearly, therefore, potential for the proposed work to have very significant impact and I can outline a number of clear mechanisms in order to deliver impact to a range of beneficiaries. Academic peers and end-users will be addressed via standard direct and indirect routes. The indirect path features dissemination of results and engagement with the academic modelling community via international conference presentations, collaborative travel, and publishing of data on University websites and NERC data repositories in order to drive the model development from which end users will benefit. Potential end users will also be engaged directly and informed of the progress of the project via delivery of presentations at a variety of general Earth Sciences and focused rock deformation academic conferences, which have historically featured presentation and discussion amongst the academic community and wider stakeholders in industries ranging from mineral/fossil fuel extraction to civil engineering.

Locally, presence of the Oxford-Shell Geoscience Lbaoratory will facilitate immediate and on-goining interaction with the fossil fuel extraction industry, which has a current research focus on teh deformation of mudrocks (rather than crystalline rocks) in sedimentary basins. Oxford Earth Sciences also has a strong mineral studies group, led by Professor Laurence Robb, with numerous direct links with mineral and mining companies whose interests will rock deformation are closely aligned with the research focus of this project.

More broadly, Oxford has a highly effective press office, with one officer dedicated to the work of the division that includes Earth Sciences. The Oxford science press officer also runs the Oxford Science Blog that has a wide reach (including social media such as Twitter and Facebook) and aims to communicate to the public, via the Internet, current research outputs by Oxford researchers. Oxford is also part of i-Tunes U, with considerable potential for the dissemination of material to the global public in the form of downloads and podcasts. Through the University and NERC, there are also substantial opportunities to deliver impact via outreach science, and we expect to allow the project PDRA to take a lead in these activities.