The ages for the Pohang basin fill, ranging from about 17 to 10 Ma, were obtained mostly from biostratigraphical studies. On the other hand, the radiometric ages of the volcanogenic rocks in other basins south of the Ocheon Fault System indicate an Early Miocene (24?18 Ma). These data suggest that the depocentre abruptly migrated northward owing to dramatic subsidence of the Pohang Basin in the late Early Miocene Epoch (about 17 Ma). A number of previous studies have suggested that the Pohang Basin is a pull-apart basin extended by the strike-slip faulting of the Yangsan Fault, which was erroneously assumed to bound the western margin of the basin as a principal displacement zone. Detailed geological mapping of the western basin margin in this study shows, however, that the actual border faults of the basin occur 2?7 km east of the Yangsan Fault. The border faults of the Pohang Basin are composed of Yeonil Tectonic Line in south, which is connected with the western border faults in west, and Ocheon Fault System in east. Yeonil tectonic line (YTL) was a NNW-striking dextral strike-slip fault zone and is a westernmost limit of the Miocene crustal deformation in southeastern Korea, characterized by basin formation, clockwise rotation and tilting of faulted blocks, and WNW?ESE crustal extension. Toward the north, the Tectonic Line connects with the western border fault of the Pohang Basin at an angle of 50°. Activation of YTL was enlarged at about 17 Ma, resulting in WNW-ESE-directed tensional stress led to the NNE-striking en echelon normal fault array along the western boundary of the basin. Development of the normal fault array and NW-striking transfer fault connecting the normal faults produced the western border fault of the basin. The western border fault constitutes a right-stepping zigzag-shaped fault zone composed of a series of smaller NNE- and NW-striking fault segments that acted as normal faults and extensional transfer faults, respectively, during subsidence of the basin. NNE-striking fault segments have a left stepping geometry and between of them is connected by NW-striking segments. A series of coarse grained fan-delta complexes are developed near these extensional transfer zones. The NNE-striking fault segments dip about 60°SE and exhibit an apparent fault scarp at outcrop. The basement rocks close to the fault are pervasively fractured and the nearby basin fill consists of disorganized, poorly sorted, angular to subangular, cobble?boulder conglomerates derived from the fractured basements. On the other hand, the NW-striking fault segments generally dip toward the basin at angles of 70?80° and the fault surfaces show a dextral strike-slip sense, indicating that the fault segments acted as a transfer fault connecting the normal faults. The reconstruction of palaeostress from the slip data of small faults adjacent to the border faults suggests that the basin was extended under a stress field of WNW?ESE tension. These results indicate that the subsidence of the Pohang Basin was induced by normal faulting of the NNE-striking western border faults segmented by a number of NW-trending transfer faults. Continuous stress field of WNW?ESE tension resulted in the NNE-striking normal faults in the interior of the basin, and migration of the normal faults basinward led to the eastward migration of depocenter, development of domino-type normal fault system, and expansion of the Pohang Basin. The eastern margin of the Pohang Basin is bounded by the Ocheon Fault System, which is a NE-striking fault system composed of a number of relayed NE- or NNE-striking normal-slip and sinistral-normal oblique-slip faults. The largest one within the system is developed along the boundary between the Middle Miocene Pohang and Early Miocene Janggi basins; this is a normal fault with a high-angle dip toward the NW and it has a scissor fault geometry along which its vertical offset decreases southwestward. This fault was initially the northwestern border fault of the Janggi Basin, but was reactivated as the eastern border faults of the Pohang Basin owing to the sudden northward migration of the depocentre at about 17 Ma. The Ocheon Fault System is interpreted to have experienced clockwise rotation with change of slip sense from normal-slip to sinistral-normal oblique-slip in response to the progressive dextral simple shear. ENE-striking normal faults, such as the Hyeongsan and Heunghae Fault, are developed in the interior of the basin at about 15 Ma. These faults have a conjugate geometry dipping toward each other and cut the NNE-striking normal faults. The southeastern part of the Doumsan area, which is located between the ENE-striking faults, is the deepest part of the basin, because the northern dipping Hyeongsan Fault has larger vertical displacement than the southern dipping Heunghae Fault. Activation of the Hyeongsan Fault led to the E-W trending clastic dikes and monoclinal folds with E-W trending axis, which indicate a N-S trending minimum horizontal stress. Collision of the Izu-Bonin Arc with the southwesternmost part of the Japanese Islands resulted in a tectonic inversion around the East Asia at about 16 Ma. This tectonic inversion had gradually upheaved the southeastern part of the Korean Peninsula and resulted in the cessation of subsidence and the compositional change of basaltic magma from subalkaline to alkaline at about 14 Ma. Afterwards, continuous E-W compressional stress field led to the reactivation of the preexisting faults, especially NNE-striking faults, as reverse fault, later folding of the basin fill with NNE trending axis, and finally cessation of sedimentation in the Poahng basin fill at about 10 Ma.