Air-gun profiles delineate a large-scale strike-slip fault system along the eastern continental margin of Korea. This system comprises two right-stepping curvilinear master faults (Hupo and Yangsan faults) that are closely associated with en echelon nominal faults, and reverse faults and folds. These structural features indicate two phases of regional deformation. In the earlier phase (late Oligocene to early Miocene), the margin experienced extensional shear strain with dextral strike-slip movements along the Hupo and Yangsan faults that guided a pull-apart opening of the PohangYoungduk Basin. The later phase deformation (late Miocene to early Pliocene) was governed by shortening strain that induced a convergent strike-slip reactivation of the Hupo fault as well as widespread contractile faulting and folding. The regional deformation in the eastern Korean continental margin reflects three stages of tectonic evolution of the Ulleung Basin: (1) pull-apart opening stage from late Oligocene to early Miocene, (2) rotational opening stage with a differential rotation of the Japanese Arc in middle Miocene, and (3) back-arc closing stage from the end of middle Miocene to present.

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