Crustal deformation of SE Korea caused by back-arc opening of the East Sea (Sea of Japan) commenced locally in the Late Oligocene. Intense deformation occurred during the Early Miocene, accompanying extension of parallelogram-shaped pull-apart basins between NNW-striking principal displacement zones, clockwise rotation and northwestward tilting of blocks, and southwestward propagating rifting. At about 17 Ma, the crustal deformation suddenly expanded westward and northward owing to activation of the Yeonil Tectonic Line, resulting in a dramatic change of depositional environment and the subsidence of wedge-shaped pull-apart basins. The Yeonil Tectonic Line and western border faults of the Pohang Basin are the westernmost limit of the Miocene crustal deformation. All the features of crustal deformation in SE Korea support the pull-apart model for the East Sea opening. However, they indicate that the NNW-striking faults such as the Yeonil Tectonic Line acted as the principal displacement zones rather than the NNE-striking Yangsan Fault under a consistent dextral simple shear. At about 16 Ma, the collision of the Philippine Sea Plate with the Japanese Islands caused a tectonic inversion. This inversion resulted in a compositional change of basaltic magma at about 15 Ma and crustal uplift in SE Korea, which caused the cessation of sedimentation at about 10 Ma.

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