Early Miocene parallel dike swarms in the Tsuruga Bay area, back-arc side of central Japan

Abstract

We analyzed Early Miocene dike swarms in the Tsuruga Bay area, Fukui Prefecture, Japan, with the aim of understanding the stress field that accompanied the formation of the Japan Sea back-arc basin. The dikes have coherent strikes of N50−60° E and are about 20 Ma in age, based on new K/Ar data and previous radiometric ages. The dominant NE−SW strike is consistent with the trends of other 20-Ma parallel dike swarms found on the back-arc side of central Honshu, indicating a regional tectonic stress with a NE−SW maximum horizontal compressive axis and a NW−SE minimum principal stress axis. The back-arc region of central Honshu was probably subject to a NW−SE extension at ∼20 Ma.