Characteristics of the Cenozoic crustal deformations in SE Korea and its vicinity due to major tectonic events

The southeastern Korean Peninsula has experienced multiple crustal deformations according to changes of global tectonic setting during the Cenozoic. Characteristics of the crustal deformations in relation to major Cenozoic tectonic events are summarized as follows. (1) Collision of Indian and Eurasian continents and abrupt change of movement direction of the Pacific plate (50 ~ 43 Ma): The collision of Indian and Eurasian continents caused the eastward extrusion of East Asia block as a trench-rollback, and then the movement direction of the Pacific plate was abruptly changed from NNW to WNW. As a result, the strong suction-force along the plate boundary produced a tensional stress field trending EW or WNW-ESE in southeastern Korea, which resultantly induced the passive intrusion of NS or NNE trending mafic dike swarm pervasively. (2) Opening of the East Sea (25 ~ 16 Ma): The NS or NNW-SSE trending opening of the East Sea generated a dextral shear stress regime trending NNW-SSE along the eastern coast line of the Korean Peninsula. As a result, pull-apart basins were developed in right bending and overstepping parts along major dextral strike slip faults trending NNW-SSE in southeastern Korea. The basins can be divided into two types on the basis of geometry and kinematics: Parallelogram-shaped basin (rhombochasm) and wedged-shaped basin (sphenochasm), respectively. At that time, the basins and adjacent basement blocks experienced clockwise rotation and northwestward tilting, and the eastward propagating rifting also occurred. At about 17 Ma, the Yeonil Tectonic Line, which is the westernmost border fault of the Miocene crustal deformation in southeastern Korea, began to move as a major dextral strike slip fault. (3) Clockwise rotation of southeastern Japan Island (16~15 Ma): The collision of the Izu-Bonin Arc and southeastern Japan Island, as a result of northward movement of the Philippine sea-plate, induced the clockwise rotation of southeastern Japan Islands. The event caused a NW-SE compression in the Korea Strait as a tectonic inversion, which resultantly terminated the basin extension and caused locally counterclockwise rotation of blocks in southeastern Korea. At that time, the folding of the San'in folded zone commenced. The folding in the Tsushima Island was almost completely accomplished at about 15 Ma and then an extensive intrusion of felsic magma occurred in the southern part of the island which led the island to be tilted about 20° into northeast. Simultaneously, the Tsushima-Goto fault was reactivated as major sinistral strike-slip faults owing to the accumulated NNW-trending compressional stress. The adjacent blocks to the fault were rotated horizontally about 28° counterclockwise due to the sinistral movement, and resultantly the western part of the San'in folded zone was dragged counterclockwise. (4) E-W compression in the East Asia (after about 5 Ma): Decreasing subduction angle of the Pacific plate and eastward movement of the Amurian plate have constructed the-top-to-west thrusts and become a major cause for earthquakes in southeastern Korea.