Age and associated stress field of middle Miocene back‐arc basalt magmatism in Northeast Japan

Abstract

AbstractA large volume of middle Miocene basaltic rocks is widely distributed across the back‐arc region of Northeast Japan, including around the Dewa Mountains. Petrological research has shown that basaltic rocks of the Aosawa Formation around the Dewa Mountains were generated as a result of the opening of the Sea of Japan. To determine the precise ages of the middle Miocene basaltic magmatism, we conducted U–Pb and fission‐track (FT) dating of a rhyolite lava that constitutes the uppermost part of the Aosawa Formation. In addition, we estimated the paleostress field of the volcanism using data from a basaltic dike swarm in the same formation. The rhyolite lava yields a U–Pb age of 10.73 ±0.22 Ma (2σ) and a FT age of 10.6 ±1.6 Ma (2σ), and the paleostress analysis suggests a normal‐faulting stress regime with a NW–SE‐trending σ3‐axis, a relatively high stress ratio, and a relatively high magma pressure. Our results show that the late Aosawa magmatism occurred under NW–SE extensional stress and ended at ~ 11 Ma.