Seismic faulting causes wall rock damage, which is driven by both mechanical and thermal stress. In the lower crust, co-seismic damage increases wall rock permeability, permits fluid infiltration and triggers metamorphic reactions that transform rock rheology. Wall rock microstructures reveal high-stress conditions near earthquake faults; however, there is limited documentation on the effects of a thermal pulse coupled with fluid infiltration. Here, we present a transmission electron microscopy study of co-seismic 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 1.3 mm thick eclogite facies pseudotachylyte vein. Dislocation-free plagioclase and K-feldspar aggregates in the microfractures record a history of fluid introduction and recovery from a short-lived high-stress state caused by slip along the nearby fault. The feldspar aggregates retain the crystallographic orientation of their host and are elongated subparallel to the pseudotachylyte. We propose that plagioclase partially amorphized along the microfractures at peak stress conditions followed by repolymerization to form dislocation-free grain aggregates. Repolymerization and recrystallization were enhanced by the infiltration of fluids that transported Ca and K into the microfractures. Subsequent cooling led to exsolution of intermediate plagioclase compositions and the formation of the Bøggild–Huttenlocher intergrowth in the grains from the fracture closest to the pseudotachylyte. Our findings provide unequivocal evidence that the introduction of fluids in the microfractures occurred within the timescale of the thermal perturbation, prompting rapid annealing of damaged wall rock soon after earthquake rupture.

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

地震断层会引起围岩损坏，这是由机械应力和热应力共同驱动的。在下地壳中，同震破坏会增加围岩的渗透性，使流体渗透并引发变质反应，从而改变岩石的流变性。围岩微结构揭示了地震断层附近的高应力条件。但是，有关热脉冲与流体渗透的影响的文献很少。在这里，我们介绍了来自挪威西部卑尔根弧的下部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1.3 mm厚的榴辉岩相假速溶岩脉收集1.25 mm和1.8 cm的聚焦离子束箔。微裂缝中无位错的斜长石和钾长石聚集体记录了流体的引入和从沿附近断层滑移引起的短寿命高应力状态恢复的历史。长石聚集体保留了其主体的晶体学取向，并被拉长至与假速溶膜平行。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，然后再聚合形成无位错的晶粒聚集体。通过将Ca和K转运到微裂缝中的流体的渗透增强了再聚合和重结晶。随后的冷却导致中间斜长石组合物的析出，并从最接近假速溶裂缝的裂缝中的晶粒中形成了Bøggild-Huttenlocher共生体。