Cenozoic extension structures in the continental framework of the Japan Sea

The results of on-land studies of Cenozoic extension zones in Korea, petrologic investigations, as well as seismic profiling and deep-sea drilling confirmed the genetic similarity of extension structures of the continental margin and the adjacent Japan Sea. All of these structures may be regarded as pull-apart basins. The correlation of the structures in the Japan Sea and the framing structures of the marginal continental rifting revealed their synchronous evolution and a similar tendency in their dynamics. The extension, gradually increasing since the late Eocene, reached a maximum in the early-middle Miocene. The extension structures were closed in the middle-late Miocene giving way to the postgraben (post-marginal-sea) stage. The compositional variation of volcanic rocks in the extension structures testifies to the variable extension intensity, so that the volcanics may be used as indicators of the continental rifting stages. Cenozoic continental rifting in eastern Asia (China, Korea, and Japan Sea) was controlled by numerous NE and N-S dextral strike-slip faults accompanied by a local transtension. The pull-apart basins are typical structures of this geodynamic environment. The rate of extension varied from marginal continental basins without the crust having been pulled apart to spreading in the marginal basin with the formation of a new oceanic crust. The data obtained indicate the passive character of Cenozoic rifting in eastern Asia; i.e., they support the crucial role of the structural factor as a consequence of global plate-tectonic events.