U–Pb and fission‐track dating of a submarine pyroclastic rock from southwest Japan

The Okinoshima Formation crops out on Okinoshima Island and comprises a thick sequence (> 200 m) of pyroclastic rocks and alternating beds of sandstone and mudstone. Because Okinoshima Island is located between Honshu and Tsushima Island, the Okinoshima Formation potentially provides an important record of volcanism during the opening of the Japan Sea in northwest Kyushu, as well as a record of the formation of the present Genkai Sea region. In consideration of the lack of previous geochronological work, dating (fission‐track and U–Pb) of igneous zircons extracted from the Okinoshima Formation 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 Okinoshima Formation was deposited at 16.2 Ma. The present Genkai Sea 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 Japan Sea formation. After 16 Ma, the tectonic setting of the present Genkai Sea region changed from one of extension (related to the formation of the Japan Sea) to one of compression, with uplift occurring under the influence of the clockwise rotation of southwest Japan. Consequently, after 16 Ma the present Genkai Sea region became isolated from the forming processes of the Japan Sea. 本州と対馬の間に位置する沖ノ島は, 日本海拡大期の火成活動や現在の玄界灘地域の形成について重要な情報を記録していると考えられる. 著者らは九州北西部に分布する古第三系および新第三系の形成年代を再検討しており, これまで不明だった沖ノ島層と玄界灘地域の形成年代を, 火砕岩のフィッション・トラック(FT)年代, U–Pb年代, 熱履歴に基づいて考察した. その結果, 沖ノ島層は堆積後にFT年代を若返らせる熱イベントを受けていないこと, 沖ノ島層が16.2 Maに堆積したこと, 玄界灘地域は日本海拡大最盛期(16.2 Ma)に, 水中火成活動を伴う急速な沈降により形成された深海海盆であったことが明らかになった. 16 Ma以降, 日本海中部では, さらに日本海拡大が進み玄界灘地域の応力場が伸長場から圧縮場へ変化したことにより, 玄界灘地域は日本海拡大過程から分離され, 独自の地史を辿ったと考えられる. The present Genkai Sea 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 Japan Sea formation. However, after 16 Ma, the tectonic setting of the present Genkai Sea region changed from one of extension (related to the formation of the Japan Sea) to one of compression, with uplift occurring under the influence of the clockwise rotation of southwest Japan. Consequently, after 16 Ma the present Genkai Sea region