Abstract
This paper reports new K-Ar ages of 23 volcanic and two plutonic rocks (23 whole rock ages, and three plagioclase, four hornblende, and two biotite mineral ages) from the southern part of the Izumo Basin. Based on these new ages and on previous geological, geochronological, and biostratigraphic data, we find that: (1) Strata dated at 20-19 Ma (21-18 Ma, including error) in the Izumo Basin and previously described as “Kawai Formation” are newly defined as the lower Miocene “Sada Formation”. Equivalent strata are sporadically distributed in areas along the Japan Sea coast of eastern Shimane Prefecture. (2) K-Ar ages of the Hata and Kawai-Kuri Formations range from 17 to 15 Ma. The Hata Formation has previously been defined as lower Miocene standard strata in the eastern part of Shimane Prefecture, but it is latest early to middle Miocene in age, and the time gap between the Hata and Kawai Formations is very small. Paleomagnetic directions were measured at 17 sites in Miocene strata in the southern parts of Izumo and Matsue Basins. The paleomagnetic features show that: (1) The declination of the “Sada Formation” is about 50°E. (2) The paleomagnetic directions of most Hata volcanic rocks and the Yoshida plutonic complex are similar to the present magnetic direction, or are reversed. (3) The declinations of some Hata and Daito Formation strata dated at 16.6-16.0 Ma are 26-19°E. Based on geology, ages and paleomagnetism, we conclude that clockwise rotation of the Chugoku district took place after 18 Ma, and ceased before 16 Ma. The Hata Formation was thus produced during the opening of the Japan Sea.
