The Miocene bending of Southwest Japan: new 39Ar/40Ar and microtectonic constraints from the Nagasaki schists (western Kyushu), an extension of the Sanbagawa high-pressure belt

Details The Miocene bending of Southwest Japan: new 39Ar/40Ar and microtectonic constraints from the Nagasaki schists (western Kyushu), an extension of the Sanbagawa high-pressure belt The Miocene bending of Southwest Japan: new 39Ar/40Ar and microtectonic constraints from the Nagasaki schists (western Kyushu), an extension of the Sanbagawa high-pressure belt

Dec 1988

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988e%26psl..91..105f&link_type=abstract

Earth and Planetary Science Letters, Volume 91, Issue 1-2, p. 105-116.

Physics

8

Scientific paper

In the westernmost part of Kyushu, the Nagasaki metamorphics (also observed in Amakusa) consist of oceanic sediments and fragments of ophiolite. These rocks underwent a high-pressure metamorphism followed by an increase in temperature up to the upper greenschist facies prior to the Late Cretaceous. Microtectonic and kinematic analyses indicate a synmetamorphic ductile shear from south to north. Dating of glaucophane, phengite and biotite by 39Ar/40Ar method suggests that the peak of metamorphism and the related deformation occurred approximately 90-95 Ma ago (i.e. Cenomanian), but a resetting by an Oligo-Miocene tectonic event is also evidenced. Due to discontinuous exposures, the metamorphic rocks of Kyushu cannot be continuously traced to the metamorphic belts of Honshu and Shikoku. Nonetheless the lithostratigraphy, metamorphic environment, radiometric data and deformation style strongly support a correlation of the Nagasaki metamorphics with the Green Schist nappe of Southwest Japan which underwent the Late Jurassic/early Cretaceous high-pressure Sanbagawa metamorphism. Such a correlation implies that (1) in the Amakusa area, the Nagasaki metamorphics form a window north of a meta-olistostrome that belongs to the Tanba-Ryoke zone; (2) the trend discrepancy of the stretching lineations between West Kyushu and Southwest Japan is in agreement with a clockwise rotation of the latter with respect to the former assumed to be fixed to Eurasia. Such a bending occurred after Paleogene times, probably during the opening of the Sea of Japan.

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