K‐Ar dating studies of Ashigawa and Tokuwa granodiorite bodies and plutonic geochronology in the South Fossa Magna, central Japan

Abstract

K-Ar age studies in the Ashigawa and the southern part of the Tokuwa granodioritic bodies, which consist of the southern part of the Kofu plutonic complex, revealed that they formed between 12 and 9 Ma. Quite a narrow range of ages obtained from the Ashigawa southernmost part of the Tokuwa pluton implies that they cooled rapidly. The southern part of the Tokuwa pluton, as a whole, shows a systematic age distribution with a decrease in age to the north. Compilation of currently available plutonic ages in the South Fossa Magna suggests that the plutonic activities occurred three times in this region. Episodic activity like this could be argued in relation to the tectonic development of this region.

Volcano-tectonic interactions during rapid plate-boundary evolution in the Kyushu region, SW Japan

“…15 Ma, and assumes that the confi guration of tectonics in southwest and central Japan has changed little since that time. The primary evidence supporting a mid-Miocene timing for the Izu collision is the intrusion of the 15.7-7.4 Ma Kofu Granitic Complex (which is thought to be derived from the Izu arc) into the Cretaceous-Paleogene Shimanto belt (Kawano and Ueda, 1966;Shibata et al, 1984;Saito and Kato, 1996;Saito et al, 1997). The Shimanto Belt is an ancient accretionary complex and one of the primary bedrock terranes in central Japan in the region of the Izu collision.…”

Late Cenozoic evolution of the Nankai trench-slope system: evidence from sand petrography and clay mineralogy

“…Japan was connected to the Asian mainland prior to opening of the Japan Sea, which started at 15-18 Ma (Tamaki et al 1999). At approximately the same time, the Outer Zone was affected by rotation and widespread neartrench magmatic activity (Oba 1977;Hisatomi 1988;Terakado et al 1988;Saito et al 1997). The Izu-Bonin arc ( Fig.…”

“…The oldest Miocene turbidites on the abyssal floor of the Shikoku Basin (c. 10 to 15Ma) coincide in age with opening of the Japan Sea (Otofuji 1996;Lee et al 1999) and near-trench magmatism within the Outer Zone (Oba 1977;Terakado et al 1988;Saito et al 1997). Subduction rates at that time may have been inconsistent, as indicated by: (1) discrete phases of collision between the Honshu arc and the Izu-Bonin island arc (Amano 1991); (2) crustal shortening along the north coast of SW Japan in the late Miocene (Itoh & Nagasaki 1996); and (3) resumption of widespread calc-alkaline volcanic activity in Kyushu at around 6Ma (Kamata & Kodama 1994).…”

Mid-Quaternary decoupling of sediment routing in the Nankai Forearc revealed by provenance analysis of turbiditic sands

“…1) contains low-pressure, hightemperature metamorphic rocks of the Ryoke Belt and associated granitic rocks (nakajima 1997). The outer zone geology, which is closely related to the provenance of the nankai Trough clastics, consists of slivers of high-pressure metamorphic rocks and low-grade metasedimentary strata assigned to the Sanbagawa, Chichibu and Shimanto belts (Taira et al 1988;Higashino 1990 at about 15-18 Ma, the outer trench was affected by rotation and near-trench magmatic activity (Saito et al 1997). The Izu-Bonin arc started colliding with the Honshu arc at approximately 12 Ma with four major episodes of accretion occurring at 12, 7-9, 3-5 and 1 Ma (amano 1991).…”