Semi-brittle deformation in plagioclase-rich rocks

对下大陆地壳（约20至60公里深）的变形行为仍然知之甚少。由于缺乏数据，特别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争论的问题。除了地震台站对下地壳地震的记录外，还有证据表明，下地壳岩石以所谓的假玄武玻璃脉的形式暴露在地球表面。这些矿脉的形成是由于断层表面的快速移动导致彼此熔化和随后的淬火。这些观测结果与先前的实验数据和模型形成了鲜明对比，这些数据和模型倾向于认为下大陆地壳主要通过晶体塑性流动变形，因此在地质时间尺度上不应承受重大应力。因此，下陆壳通常被认为是脆弱和不稳定的。斜长石的体积含量约为40%地壳中最常见的造岩矿物群。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，石英仅占地壳岩石的约10%。10体积%大陆地壳此外，斜长石变形的定量研究已经进行了样品和条件下，有利于晶体塑性变形。然而，除了假玄武玻璃岩脉，在富含斜长石的下地壳岩石中有大量证据表明，变形是由脆性和塑性变形的混合物发生的，因此在所谓的半脆性变形状态下，岩石预计是最坚固的。因此，至少在某些情况下，下大陆地壳可能比目前预期的要坚固得多。因此，计划中的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装置以及实验岩石变形中使用的标准技术，再加上对回收实验样品的微观和纳米结构的广泛分析。我将使用微压缩试验，一种材料科学界常用的技术，来研究斜长石中的变形孪晶。微压缩阶段安装在一个SEM使我能够量化必要的应力，形成变形孪晶以及在斜长石中使用电子背散射衍射原位变形过程中的孪晶形态。此外，为了测试压力对形变孪晶的影响，也为了量化形变孪晶和微裂纹的分配，我将使用高压、高温气体介质形变压力机。我的最终目标是将在受控实验室条件下获得的斜长石变形孪晶结果应用于从下大陆地壳中挖掘出的自然变形斜长石丰富的样品，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.