Semi-brittle deformation in plagioclase-rich rocks

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The deformation behaviour of the lower continental crust (around 20 to 60 km depth), is still poorly understood. Due to the lack of data and most notably due to the occurrence of lower crustal earthquakes, the deformation behaviour of the lower continental crust has been a matter of debate for the past four decades. Besides the record of lower crustal earthquakes by seismic stations, there is evidence that can be found in lower crustal rocks exposed on Earth's surface in the form of so-called pseudotachylyte vein. These veins formed due to the rapid movement of fault surfaces past each other leading to melting and subsequent quenching. These observations stand in contrast to previous experimental data and models favouring a lower continental crust that is mainly deforming by crystal plastic flow and should therefore not be able to support significant stresses over geological timescales. As a result, the lower continental crust is often considered to be weak and aseismic. Plagioclase is with around 40 vol.% the most common rock-forming mineral group of Earth's crust. Yet, most of the experiments that aimed to investigate the deformation behaviour of crustal rocks have been conducted on quartz, which makes up only approx. 10 vol.% of the continental crust. Further, the few quantitative studies on plagioclase deformation have been conducted on samples and at conditions that favour crystal plastic deformation. Yet, besides pseudotachylyte veins, there is abundant evidence in plagioclase-rich lower crustal rocks showing that deformation took place by a mixture of brittle and plastic deformation and thus in the so-called semi-brittle deformation regime, where rocks are expected to be strongest. Therefore, the lower continental crust might, at least in some situations, be much stronger than currently expected. It is therefore the main objective of the planned research project to fill this knowledge gap regarding the semi-brittle deformation of plagioclase. To do so, I plan to use state-of-the-art deformation apparatus as well as standard techniques used in experimental rock deformation coupled with extensive analyses of the micro- and nanostructures of the recovered experimental samples. I will use microcompression tests, a technique commonly used in the material science community, to study deformation twinning in plagioclase. The microcompression stage is mounted in an SEM enabling me to quantify the stress necessary to form deformation twins as well as the twin morphology in plagioclase using electron-backscatter diffraction in-situ during deformation. Further, to test the influence of pressure on deformation twinning, but also to quantify the partitioning of deformation twinning and micro-cracking, I will use a high-pressure, high-temperature gas-medium deformation press. My ultimate goal is to apply the results for deformation twinning in plagioclase obtained under controlled laboratory conditions to naturally deformed plagioclase-rich samples exhumed from the lower continental crust to estimate the paleo stresses these rocks experienced.