Opening of the Sea of Japan and Kuroko deposit formation

The opening of the Sea of Japan began with the bulge at the margin of the Asian continent at 21 Ma. Crustal subsidence and basaltic eruption prior to Kuroko deposit formation appear to represent the opening event. Between 16.5 to 15.5 Ma was the most probable time of the counterclockwise rotation of NE Japan. Kuroko ore formation is believed to have taken place between 15.4 and 15.2 Ma based on geological evidence and fossil ages. It is proposed that Kuroko deposits formed in a weakly extensional environment at the waning stage of opening of the Sea of Japan. This may be the only model which can account for the occurrence and simultaneous formation of Kuroko deposits in Japan. The process can be ascribed to decreasing porosity of the crust due to the stress change, most commonly from extensional to compressional.