Seismogenesis in the brittle–plastic transition zone of the Yecheon shear zone, South Korea

This study investigates seismogenesis in the brittle–plastic transition zone of the Yecheon Shear Zone, South Korea. The NE-to NNE-striking right-lateral shear zone cuts a Mesozoic granitoid pluton in an outcrop that was investigated in this study. We focus on light green layers within granite mylonitic rocks that preserve evidence of seismic slip events and subsequent plastic deformation. Field observations and microstructural analysis reveal a complex history of brittle and plastic deformations. The light green layers, formed by frictional melting during seismic events, contain injection structures, flow textures, and euhedral to subhedral microlites with inclusions. Elevated pore-fluid pressure might have played a crucial role in initiating seismic ruptures in the brittle–plastic transition zone, forming shear band ruptures or R shear planes and implosion breccia in the releasing stepovers in shear band ruptures. We propose a seismogenic model involving fluid pressure buildup, formation of shear band ruptures, rupture propagation into the C foliation, and the formation and subsequent alteration of pseudotachylytes. The fine-grained nature of the altered pseudotachylytes would have promoted strain localization during interseismic periods, leading to their transformation into ultramylonite. These findings improve our understanding of earthquake nucleation processes in the brittle–plastic transition zone and highlight the importance of fluids in inducing seismic events. • Granitic mylonites of brittle-plastic transition exhibit cyclic aseismic and seismic slips. • Seismic slip nucleates along shear bands and propagates into C -foliation. • Pseudotachylytes formed by seismic slip transform to ultramylonites during aseismic slip.