日本海拡大時の中央および西南日本前弧域テクトニクス

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°C) 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.