Evolution of the Miocene Waup Basin, SE Korea, in response to dextral shear along the southwestern margin of the East Sea (Sea of Japan)

The Waup Basin, SE Korea, is a nonmarine basin created coevally with the back-arc opening of the East Sea (Sea of Japan) during the Miocene. The basin is bounded by normal faults in the northwestern and southeastern margins and by dextral strike-slip faults in the northeastern and southwestern margins. A number of intrabasinal faults with dextral strike-slip and normal senses of slip are also present, delineating three structural domains of the basin, i.e. the NE, central, and SW blocks. The basin fills, composed of alluvial-to-lacustrine deposits and dacitic to basaltic volcanogenic rocks, have the geometry of a half graben in the NE and central blocks and a graben in the SW block. They are younging from the NE to the SW blocks, suggestive of propagating rifting of the basin and associated depocenter migration. Structural and stratigraphic data suggest that the NE and central blocks were produced by NW-SE extension during the Early Miocene (22-17 Ma), involving northwestward block tilting and subsequent clockwise horizontal block rotation. On the other hand, the SW block formed at about 17 Ma by NW-SE extension and experienced neither block tilting nor block rotation. The reconstructed configuration of the Waup Basin is that of a pull-apart basin produced by NNW-directed dextral strike-slip stress and associated NW-SE extensional stress. The ages of the basin fills (ca. 22 to <17 Ma), corresponding in timing with the active opening of the East Sea, suggest that the dextral stress is most likely due to spreading of the East Sea in a SSE direction. The maintenance of the NNW-directed dextral stress throughout the Early Miocene in the Waup area suggests that the southeastern margin of the Korean Peninsula acted as the western dextral shear zone associated with pull-apart opening of the East Sea.