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.
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