Abstract. Seismic faulting causes wall rock damage driven by both mechanical stress and thermal energy. In the lower crust, coseismic damage has important implications for wall rock permeability, the progress of subsequent fluid-driven metamorphic reactions, and rock rheology. Wall rock microstructures reveal high-stress conditions near the slip surface during lower crustal earthquakes, however, there is limited documentation on the thermal effect. Here, we present a transmission electron microscopy study of coseismic microfractures in plagioclase feldspar from lower crustal granulites from the Bergen Arcs, Western Norway. Focused ion beam foils are collected 1.25 mm and 1.8 cm from a 2 mm thick eclogite facies pseudotachylyte vein. Dislocation-free plagioclase aggregates fill the microfractures and record a history of recovery from a short-lived high stress-temperature (σ-T) state caused by seismic slip and frictional melting along the nearby fault surface. The plagioclase aggregates retain the crystallographic orientation of the host rock and shape preferred orientation relative to the fault slip surface. We propose that plagioclase partially amorphized along the microfractures at peak stress conditions followed by repolymerization to form dislocation-free grain aggregates within the timeframe of pseudotachylyte formation. The heat from the slip surface dissipated into the wall rock causing a short-lived temperature peak. Subsequent cooling led to exsolution of intermediate plagioclase compositions by spinodal decomposition within a few millimeters distance to the fault surface. Our findings provide microstructural evidence for the high σ-T conditions that are expected in the proximity of seismic faults, highlighting the importance of micro- and nanostructures for the understanding of earthquakes ruptures.

中文翻译：

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下大陆壳斜长石微裂缝中保存的纳米级地震记录

摘要。地震断层由机械应力和热能共同驱动，导致围岩损坏。在下地壳中，同震破坏对围岩渗透性，随后的流体驱动的变质反应的进展以及岩石流变学具有重要意义。围岩微结构揭示了在低地壳地震期间滑面附近的高应力条件，但是，关于热效应的文献有限。在这里，我们介绍了来自挪威西部卑尔根弧的下地壳粒状斜长石长石中同震微裂缝的透射电子显微镜研究。从2 mm厚的榴辉岩相假速溶岩脉中收集1.25 mm和1.8 cm的聚焦离子束箔。无位错的斜长石聚集体填充了微裂缝，并记录了由于沿附近断层表面的地震滑动和摩擦融化而导致的短期高应力-温度（σ-T）状态的恢复历史。斜长石聚集体保留了基质岩石的晶体学取向，并且相对于断层滑动面具有优选的取向。我们提出斜长石在峰值应力条件下沿着微裂缝部分非晶化，然后在假速凝形成的时间范围内再聚合形成无位错的晶粒聚集体。来自滑动表面的热量散发到围岩中，从而导致短暂的温度峰值。随后的冷却导致在距断层表面几毫米的距离内通过旋节线分解而溶解了中间斜长石组合物。我们的发现为地震断层附近可能发生的高σ-T条件提供了微观结构证据，突出了微观和纳米结构对于理解地震破裂的重要性。