Transform-to-thrust reactivation of a fossil back-arc spreading system: Evidence from marine magnetic anomalies in the East Sea (Japan Sea)

Transform faults contain rheological weaknesses that can be reactivated into either divergent or convergent boundaries, depending on tectonic stress regime. While rifting triggered from transform fault is well documented, geophysical evidence demonstrating compressional reactivation of fossil transform boundaries, and the lithospheric controls that govern it remains poorly understood. The East Sea (Japan Sea), a back-arc system at the junction of the Eurasian, Pacific, North American, and Philippine Sea plates, offers an ideal setting to understand this process. However, the tectonic origin and opening history of its southeastern part, the Ulleung Basin, have remained controversial due to thick sedimentary cover and previously ambiguous magnetic signatures. Here we show that the Ulleung Basin underwent seafloor spreading between 20–17 Ma, and that its paleo-transform boundary is currently reactivating into a thrust system. Using a newly compiled high-resolution marine magnetic anomaly map and two-dimensional forward modeling, we identify coherent ENE-WSW trending magnetic lineations (Chron C6An.1n–C5Dn), indicating a past half-spreading rate of 20 km/Myr. Furthermore, we identify the western margin of the basin as the East Korean Fracture Zone, a fossil transform boundary that spatially coincides with the modern Ulleung Major Thrust indicating that the rheological weakness of this transform fault has facilitated its compressional inversion under E-W tectonic stress regime. These results provide a kinematic framework linking the back-arc opening of the East Sea to its Late Cenozoic inversion, and document a rare example of transform-to-thrust reactivation in a continental back-arc setting.