At convergent tectonic plate boundaries rocks are brought down into the Earth’s mantle. Due to the deep burial, this material is either recycled and contributes to the formation of new and growing orogens or transported further into the mantle. Furthermore, subduction of both oceanic and continental crust assists the Earth’s water cycle, which is essential for life on Earth. H2O bound within the crystal lattice of minerals is brought down into great depth and released by dehydration processes. This released H2O plays a major role in fluid-rock interaction, since it triggers transformation processes, which subsequently initiate chemical-mechanical changes of the downgoing and surrounding material. Such interdependencies cause metamorphic transition as well as emerging rheological inhomogeneities and deformation of the affected rocks. Especially when dry and rigid rocks of the continental crust are subducted, infiltrating fluids, H2O in particular, mobilize, promote and increase the efficiency of the chemical-mechanical processes.
How fluid-rock interaction, deformation, rheology, and fluids are coupled and how they affect subducting rocks has long been part of the research. However, investigations of these processes are highly challenging because they occur at great depth where no in-situ analysis is possible. Therefore, studies often use either field-derived data, laboratory data, partially obtained by the analysis of synthetic materials, or complex numerical simulations. This thesis provides a comprehensive dataset arising from detailed field observations and selected samples being analyzed using various methods and equipment. Subsequently, the petrological results were employed for numerical simulations. With this interdisciplinary approach I give new insights into how an infiltrating fluid, mainly H2O, successively transforms a dry and metastable crustal rock. The fluid infiltration triggers a progressive eclogitization, a transient weakening, and a ductile deformation of the affected host rock. I highlight the role of water, stored in nominally anhydrous minerals (NAMs), which constitute large volumes of the continental crust. Additionally, I present new estimates about the physical-chemical properties, timing, and spatial scales of the addressed metamorphic and dynamic processes.
One of the best natural laboratories to study the transformation and deformation of crustal rocks based on an infiltrating external fluid are the rocks exposed on Holsnøy (western Norway). Various studies have shown that the eclogite-facies shear zone network developed on Holsnøy was formed due to an interplay of brittle and ductile deformation assisted by fluid infiltration. The shear zones widen during strain accumulation, deformation and progressive metamorphism and partially eclogitize the highly reactive granulite. However, it is still a matter of debate how long fluid was available and to what spatial extent. How does it affect the resulting geometries, microstructures and rheology of the system?
To better understand the addressed interconnections, the effect of fluid availability on the evolving shear zone geometry and widening was assed first. The results show that only a substantial amount of fluid enables the geometrical evolution as observed in the field. This fluid was either injected by numerous fluid pulses during individual events or by one large influx. Furthermore, it was possible to decipher that the hydration occurs in two contemporaneous types. By a diffusional hydrogen (H+/H2) influx, and simultaneous inflow of an aqueous fluid (H2O and H+/H2). The hydrogen influx caused a hydration of the NAMs, due to an incorporation of OH-groups within the crystal lattice, and progressed further into the wall rock. The supply of H2O and additional hydrogen through an inflow of aqueous fluid, caused further incorporation of OH-groups during recrystallization into a hydrated eclogite-facies mineral assemblage. Both influxes, where the diffusional hydrogen influx is one order of magnitude faster than the aqueous fluid inflow, initiate a transient weakening of the system. To fit the observed shear zone geometries with numerical simulations, the hydrated granulite must be two orders of magnitude weaker, and the equilibrating eclogite four orders of magnitude weaker compared to the dry and rigid granulite host rock. If inflow of aqueous fluid is modelled only, the eclogite is only three orders of magnitude weaker compared to the granulite. Hence, the hydrogen influx has an appreciable effect on the rheology of the granulite. Furthermore, the conducted numerical simulations provide new time constraints for the granulite hydration and shearing of less than ten years at low shear velocities of < 10-2 cm/a. Hence, the results presented here significantly contribute to a better understanding of the fluid-assisted transient weakening and eclogitization of subducted continental crust.
