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. Seismic faulting causes wall rock damage, which is driven by both mechanicaland thermal stress. In the lower crust, co-seismic damage increases wallrock permeability, permits fluid infiltration and triggers metamorphicreactions that transform rock rheology. Wall rock microstructures revealhigh-stress conditions near earthquake faults; however, there is limiteddocumentation on the effects of a thermal pulse coupled with fluidinfiltration. Here, we present a transmission electron microscopy study ofco-seismic microfractures in plagioclase feldspar from lower crustalgranulites from the Bergen Arcs, Western Norway. Focused ion beam foils arecollected 1.25 mm and 1.8 cm from a 1.3 mm thick eclogite faciespseudotachylyte vein. Dislocation-free plagioclase and K-feldspar aggregatesin the microfractures record a history of fluid introduction and recoveryfrom a short-lived high-stress state caused by slip along the nearby fault.The feldspar aggregates retain the crystallographic orientation of theirhost and are elongated subparallel to the pseudotachylyte. We propose thatplagioclase partially amorphized along the microfractures at peak stressconditions followed by repolymerization to form dislocation-free grainaggregates. Repolymerization and recrystallization were enhanced by theinfiltration of fluids that transported Ca and K into the microfractures.Subsequent cooling led to exsolution of intermediate plagioclasecompositions and the formation of the Bøggild–Huttenlocher intergrowth inthe grains from the fracture closest to the pseudotachylyte. Our findingsprovide unequivocal evidence that the introduction of fluids in themicrofractures occurred within the timescale of the thermal perturbation,prompting rapid annealing of damaged wall rock soon after earthquakerupture.

Nanoscale earthquake records preserved in plagioclase microfractures from the lower continental crust