Thesis: Fluid Control on High-Temperature Fracture Propagation in Crustal Rocks
The aim of my doctoral research is to gain a better understanding of material transport through lower and mid-crustal regions. The focus is on the coupling of deformation and chemical reactions; with an emphasis on the water related weakening known as “hydrolytic weakening”. This coupling mechanism increases its efficiency with decreasing grain size. With a smaller average grain size the volumetric percentage of grain boundaries and triple junctions becomes larger. Grain boundaries and triple junctions act as fast diffusion pathways for water related species. In nature grain size reduction is observed in mid-crustal shear zones, with a progressive decrease towards the core of the shear zone. Therefore, shear zones are not only places of enhanced material transport, they also have to be regarded as chemical reactors.
My research approach is numerical grain scale modelling. I am mainly using the Finite Element code ABAQUS to model the coupling of chemical diffusion and reactions with semi-brittle failure and plastic deformation. The results of these numerical experiments are tied into a thermodynamic frame work in order to check their compliance with basic thermodynamic laws. The idea behind this approach is not to obtain a numeric value for a specific parameter, but to understand the relationship between all the parameters involved.
Why my research is important
The migration of fluids through the ductile part of the earth’s crust is poorly understood as the conventional notion of permeability associated with rock fragmentation is not applicable. The presence of fluid pathways will be episodically and linked to high temperature creep fracturing in shear zones. Creep fracturing in turn is facilitated by hydrolytic weakening of the rock.
Fluids play an important role in many, if not most, geological processes. Fluids are reactants and products of chemical reactions, such as metamorphic reactions or ore mineralisation. At the same time they are agents of transport for other reactants and products. On top of that, fluids can change the rheology and transport properties of the material they are situated in. All this makes them a prime suspect for coupling mechanisms, that can led to instabilities and strain localization in lower and mid-crustal regions.