Gi‐Bom Kim

This study reports a rare case of failing incipient subduction found in the East Sea (Japan Sea), a back‐arc basin in the western circum‐Pacific, where an opposing pair of incipient‐diffuse subduction margins competes to attain self‐sustainability. Seismic reflection analysis of the post‐4 Ma structural components around the western East Sea margin revealed three juxtaposed compressional neotectonic components: (a) the Ulleung Major Thrust, (b) the Ulleung Syncline, and (c) the East Korean Thrust Belt. The Ulleung Major Thrust, marked by the NS‐elongated west‐dipping reverse tri‐shear zone along the continent–back‐arc transition boundary, is interpreted as an embryonic subduction interface that began to guide the underthrust of the Ulleung back‐arc crust below the Korean continental block. With the growth of the Ulleung Major Thrust, the crustal downbuckle on the western end of the Ulleung Basin became progressively asymmetric and formed the Ulleung Syncline, a proto‐trench. Underthrusting of the Ulleung Basin crust induced regional uplift over the eastern Korean margin, accommodated by arrays of east‐dipping thrust faults referred to as the East Korean Thrust Belt. The east‐upward thrust movements formed NS‐elongated thrust‐bounded sedimentary basins, classified as piggyback basins. The subduction initiation on the western East Sea margin appears destined to fail, because a convergent proto‐plate boundary is now nucleating on its opposing side. However, lines of evidence, such as marine terraces and frequent compressional earthquakes on Quaternary faults along the eastern Korean margin imply that the subduction initiation on the western East Sea is failing but has not yet failed.