Architecture and fluid-assisted evolution of a basin-forming normal fault zone: Insight from a drillcore in Miocene Waup Basin, SE Korea

The evolution of basin-forming faults involves long-term brittle deformation coupled with fluid–rock interactions. We investigated a basin-bounding normal fault zone along the margin of the Miocene Waup Basin, southeastern Korea, using a vertical drillcore (812 m deep) that penetrates the fault zone between the Miocene basin fill and Late Cretaceous basement rocks. The ∼150 m-thick fault zone, developed within Late Cretaceous felsic volcanic basement, consists of an anastomosing network of fault-related hydrothermal breccias with locally developed gouge strands. Deformation is spatially heterogeneous: localized slip zones and injected fine-grained materials are concentrated at the basin fill–basement contact, whereas deeper, widespread hydrothermal breccia domains exhibit variable deformation intensity and pervasive calcite cement. Diverse vein assemblages, authigenic metallic minerals, zircon U–Pb discordia, and fractured and altered zircon textures indicate that the fault zone acted as a conduit for episodic fluid flow. Zircon U–Pb ages record multiple episodes of fluid infiltration, including an event at ca. 21 Ma that was broadly coeval with the eruption of Miocene pyroclastic rocks, suggesting a temporal association between fault activity, fluid migration, and volcanism. Our observations indicate that the fault zone evolved at shallow crustal levels (