LAUSR

Abstract Basalt is one of the main components of megathrust faults in subduction zones where the world's largest earthquakes are generated. Paradoxically, pseudotachylytes have never been reported from basaltic rocks deformed at conditions compatible with the thermally-defined seismogenic zone. We report the first discovery of pseudotachylyte from oceanic crustal basalt, identified by microstructural criteria. The fault is located within a Late Cretaceous subduction complex, the Mugi Melange, and either formed along the plate interface during subduction or within the upper plate during underplating. After solidifying, the pseudotachylytes were fragmented by cataclasis and subsequently partially hydrated to produce phyllosilicates. Previous experiments using dry, felsic lithologies as starting material have shown that pseudotachylytes can be as strong as their host rock, deterring re-activation and favoring their preservation over geologic time. However, we show that where fluid is present, pseudotachylyte can be replaced by phyllosilicate-rich layers with a low frictional strength. The rate of dissolution is dependent on the bulk composition of the pseudotachylyte, with dissolution of mafic glasses occurring at least three orders of magnitude faster than felsic glasses. Under hydrothermal conditions, the replacement of pseudotachylytes by layers of frictionally weak phyllosilicates is predicted to occur over time intervals shorter than the megathrust earthquake cycle. This process likely reduces fault strength and promotes reactivation.