Semi-brittle deformation in plagioclase-rich rocks

下大陆地壳（约20至60公里深）的变形行为仍然知之甚少。由于缺乏数据，尤其是由于下地壳地震的发生，下大陆地壳的变形行为在过去四十年中一直是一个有争议的问题。除了地震台站对下地壳地震的记录外，在地表暴露的下地壳岩石中，也可以找到所谓的伪岩脉的证据。这些矿脉是由于断层表面相互快速移动而形成的，导致熔化和随后的淬火。这些观测结果与先前的实验数据和模型形成鲜明对比，这些数据和模型倾向于主要由晶体塑性流动变形的下大陆地壳，因此不应该能够支持地质时间尺度上的重大应力。因此，下大陆地壳通常被认为是脆弱和抗震的。斜长石的体积约为40%，是地壳中最常见的造岩矿物。然而，大多数旨在研究地壳岩石变形行为的实验都是在石英上进行的，石英只占地壳岩石的大约一半。占大陆地壳的10%。此外，在有利于晶体塑性变形的条件下，对斜长石变形进行了少量的定量研究。然而，除了伪长石脉外，在富含斜长石的下地壳岩石中也有大量证据表明，变形是由脆性和塑性变形混合发生的，因此在所谓的半脆性变形区，岩石被认为是最强的。因此，至少在某些情况下，较低的大陆地壳可能比目前预期的要强得多。因此，计划研究项目的主要目标是填补关于斜长石半脆性变形的知识空白。为此，我计划使用最先进的变形设备以及用于实验岩石变形的标准技术，并对回收的实验样品的微观和纳米结构进行广泛分析。我将使用材料科学界常用的微压缩试验技术来研究斜长石中的变形孪晶。微压缩阶段安装在SEM中，使我能够量化变形孪晶所需的应力，以及变形过程中使用原位电子后向散射衍射的斜长石孪晶形貌。进一步，为了测试压力对变形孪晶的影响，也为了量化变形孪晶和微裂纹的分配，我将使用高压、高温气介质变形压力机。我的最终目标是将在受控的实验室条件下获得的斜长石变形孪生的结果应用于从下大陆地壳中挖掘出来的自然变形的富含斜长石的样品，以估计这些岩石所经历的古应力。

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.