U– P b and fission‐track dating of a submarine pyroclastic rock from southwest J apan

The O kinoshima F ormation crops out on O kinoshima I sland and comprises a thick sequence (> 200 m) of pyroclastic rocks and alternating beds of sandstone and mudstone. Because O kinoshima I sland is located between H onshu and T sushima I sland, the O kinoshima F ormation potentially provides an important record of volcanism during the opening of the J apan S ea in northwest K yushu, as well as a record of the formation of the present G enkai S ea region. In consideration of the lack of previous geochronological work, dating (fission‐track and U – P b) of igneous zircons extracted from the O kinoshima F ormation were undertaken and studied the clay mineral alteration in the pyroclastic material in order to reveal its thermal history. These data are used to constrain the age of the Okinoshima Formation and the present Genkai Sea region. Our results show that no thermal event has reset the fission‐track age after deposition of the pyroclastic rocks, and that the O kinoshima F ormation was deposited at 16.2 Ma. The present G enkai S ea region is a deep‐sea basin, and its formation at 16.2 Ma was accompanied by submarine volcanism and rapid subsidence that marked the climactic stage of J apan S ea formation. After 16 Ma, the tectonic setting of the present G enkai S ea region changed from one of extension (related to the formation of the J apan S ea) to one of compression, with uplift occurring under the influence of the clockwise rotation of southwest J apan. Consequently, after 16 Ma the present G enkai S ea region became isolated from the forming processes of the J apan S ea.