Abstract
Rhyolitic to basaltic dikes crop out along the west coast of the Oga Peninsula, NE Japan, and have a coherent N39° E trend in the volcanic piles of the Late Eocene Monzen Formation and the Early Miocene Nomuragawa Formation. The dikes are similar in petrographic features and isotopic ages to the volcanic successions, and some dikes are connected to lava flows in the host successions. Map-scale normal faults within the Late Eocene and Early Miocene successions have similar trends to the dikes, suggesting NW-SE extensional stress at the time of dike emplacement. The crust in this area was extended by~9% due solely to dike intrusion, as deduced from the total thickness of the dikes. The total thickness of Late Eocene dikes is ca. 1.5 times greater than that of Early Miocene dikes. Crustal extension was initially achieved mainly by igneous intrusions, but later by waxing normal faulting that resulted in the opening of the Japan Sea.
