Interplay of metamorphism and deformation mechanisms in eclogite. Implications on transient mechanical behaviour at convergent plate boundaries.

The geodynamic evolution of the earth is highly governed by the deformation behaviour of rocks at plate boundaries. In convergent settings, dry mafic rocks are subducted to great depths where they experience high pressures and temperatures. These increased pressure and temperature conditions cause a mineral transformation which subsequently results in the change of the mechanical behaviour of the rock. A frequent, and therefore important rock in such continental and oceanic subduction zones is eclogite. In the last decades, the deformation behaviour of eclogite has been subject of a large number of field based, experimental and numerical studies. One of the main interests is a better understanding of the deformation behaviour and strain weakening of eclogites, being highly relevant for the tectonic evolution at convergent margins. However, there is no consensus on the deformation behaviour of eclogite. Authors identified diffusion dominated processes, dislocation creep and/or brittle failure as main strain accommodating mechanism in eclogite. This initially conflicting observation is raising the question of the factors controlling the deformation behavior of eclogite.

Herein I am going to present two case studies from the eclogite type locality (Saualpe-Koralpe Complex. Eastern Alps, Austria). Intergraded field, microstructural and geochemical investigations document strong interactions between metamorphism and deformation processes operating at eclogite-facies conditions. The investigations illustrate how metamorphic reactions dictate the deformation style of eclogite. Fluid-supported syntectonic prograde eclogitization was initially accommodated by diffusion and dissolution-precipitation creep and accompanied by the development of fluid pathways and formation of hydrous minerals. Subsequent prograde metamorphism resulted in progressive dehydration and melting of the eclogite. Concomitant increase in the pore-fluid pressure induced brittle failure and allowed precipitation of an eclogite facies assemblage in synmetamorphic veins. Finally, the quartz-enriched veins localized ductile strain and deformed by crystal plasticity. The investigated samples testify that eclogite deformation is highly sensitive to small metamorphic changes.

Finally, in an attempt to improve our understanding of strain-localization processes operating in equilibrated (non-reacting) eclogite we performed a series of high temperature-high pressure deformation experiments. Deformation experiments were complemented by numerical simulations. Special focus lied on the role of varying garnet content on strain weakening of eclogite. The triaxial deformation experiments and numerical simulations were performed at a temperature of 1000°C, a confining pressure of 2.5 GPa, and a strain rate of 3 × 10-6 s-1. The combination of the results from the numerical simulations with experimentally derived microstructures enables us to determine the spatial and temporal evolution of strain and strain rate and identify microstructures characteristic for strain localization. Results emphasize once more the importance of structural and/or compositional heterogeneities for the localization of strain.

Above case studies allow to illustrate the rheological evolution of a dry mafic gabbro into an equilibrated eclogite. Deformation mechanisms operating on a prograde metamorphic path in a convergent setting have been identified. The results have potential to improve our understanding of the geodynamic evolution of convergent settings.

Zoom link: https://ubc.zoom.us/j/61227908464?pwd=eE9IaGhPZFF6OW5OSWROL3U4cmJYQT09(link is external)

Meeting ID: 612 2790 8464

Passcode: 009405

Please note that after the seminar, the candidate will give a chalk-talk regarding their research plan over the next five years at ESB 5104 @2:30 pm, you are welcome to join as well!