4D quantification of micro-scale feedbacks in dehydrating, deforming rocks

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This research project uses a novel methodological approach to determine where mineral dehydration reactions can trigger failure in deforming rocks. This link between dehydration and failure is important at convergent plate boundaries. Where plates collide, the shallow portions of the Earth's crust are affected by so-called thin-skinned tectonics. There, dehydration reactions enable the emplacement of tectonic nappes, which shape mountain belts such as the Swiss Jura, or the Appalachians in the US. Plate collision also leads to the subduction of tectonic plates, where dehydration reactions are suspected to trigger seismic events at depths of several tens of kilometers. In both tectonic settings hydrous minerals in rocks become unstable as temperature increases. They start to transform into denser minerals by releasing water in dehydration reactions. The density increase produces pores, which are filled by the water. The pores, the fluid pressure in them, and the newly grown minerals weaken the reacting rock mechanically. It may become unable to support tectonic stresses and fail. The processes that control large-scale tectonics start at the grain scale. These grain scale processes entail a series of complicated, intertwined developments that involve the chemistry, hydraulics and mechanics of a dehydrating rock. Coupled chemical, hydraulic and mechanical processes may facilitate the self-organization of the dehydrating rock into a state where it ultimately fails. Unfortunately, neither classical laboratory experiments nor field-based studies allow a spatial and temporal (4D) characterization of these coupled processes on the micro-scale. Models to explain failure in dehydrating rocks therefore lack a robust observational basis.We will use a unique combination of new methods to overcome this severe limitation. Our interdisciplinary team of experienced researchers will establish a technique to directly observe dehydration reactions in deforming rocks. We will employ the most powerful x-ray sources in the UK and Switzerland to observe dehydration reactions in a new generation of experimental pressure vessels. These vessels are transparent to x-rays and allow us to reproduce conditions at the base of tectonic nappes and at intermediate depths in subduction zones. They are designed and built in Edinburgh. Combining these vessels with time-resolved (4D) x-ray microtomography will enable us to document mineral dehydration at a wide range of conditions. The resulting 4D microtomography data sets will have a volume of several tens of TB. New analysis techniques based on machine learning will allow us to extract the relevant information from these vast quantities of data. Our analyses will determine conditions where dehydration causes rocks to become unable to support tectonic stresses. Using these analyses, we will test and advance theoretical concepts used to link dehydration and deformation in numerical simulations.The first direct observation of the complex grain-scale developments during dehydration reactions will significantly advance our understanding of some key processes in tectonics. Because our data are time-resolved and dynamic, they will support the interpretation of field data that otherwise capture a static, fossilized picture of dehydration reactions. Our data will allow testing and refining existing mathematical models that provide a foundation for robust simulations of large-scale tectonic processes. Ultimately, our findings will support the assessment of risks associated with plate collision. Our project will also make a new experimental imaging method available for research on geothermal energy, CO2 sequestration and nuclear waste storage. The method combines time-resolved x-ray microtomography in our new experimental vessels with advanced data mining and image analysis and computational simulation.