Tectonics in the Forearc Region of Central and SW Japan during the Japan Sea Opening

Abstract

Studies on geologic structures including some deformation microstructures in both areas along the Median Tectonic Line （MTL） in western Shikoku and of the South Fossa Magna indicate that N-S trending horizontal compression tectonics prevailed in these areas during the earliest Middle Miocene （ca. 15 Ma）. Also, a review of structural development in Nankai province indicates that similar compression tectonics prevailed during the periods from the Early to earliest Middle Miocene （ca. 20-15 Ma）. These periods are nearly coeval with the timing of the Japan Sea opening. Although there is uncertainty about the reactivation of the MTL around this time, south-dipping normal faults were formed in the Sambagawa belt truncating the MTL to produce the Kuma basin （18-16 Ma） , which were later reactivated as thrusts （Tobe and Hanayama thrusts） , indicating inversion tectonics. In the South Fossa Magna region, not only a large amount of shortening-as much as 40-50％-was caused in the Momonoki Subgroup （earliest Middle Miocence, 16-15 Ma） by folding strata, but also the constituent quartz grains were moderately plastically deformed and microcracked followed by healing （i.e. healed microcracks） , indicating deformation under brittle-ductile transition conditions （ca. 300℃） at ca. 15 Ma. Furthermore, slaty cleavage caused by pressure solution developed in the strata constituting the Oligocene-Early Miocene （＞20 Ma） Southern Shimanto belt, and forearc sediments （18-15 Ma） as thick as 4000 m overlying these sediments with angular unconformity （e.g. Tanabe and Kumano Groups in the Kii peninsula） were also deformed by folding with a shortening ratio of 10-20％. All these onland geological facts in the forearc region indicate that the forearc region was in a regime of compression, while the back-arc region was in a regime of extension at the same time during the Japan Sea opening. Similar tectonics also occurred in Italy, where the Tyrrhenian Sea （back-arc basin） has opened since the Pliocene （5 Ma） , while shortening tectonics shown by the development of fold and thrust belts and deep （＞7 km） basins occurred at the same time in the forearc region. Although these concurrent back-arc extension and forearc shortening are difficult to interpret, they may be caused by ridge push force （i.e. gravity tectonics） after back-arc spreading commences so that extensional force is no longer sustained in the arc. Key words： the Japan Sea opening, back-arc extension, forearc shortening, Median Tectonic Line, South Fossa Magna ：日本海拡大背弧伸張前弧短縮中央構造線南部フォッサマグナ ＊ 北海道大学大学院理学院