日本海拡大に伴う西南日本の変形特にメガキンク帯の形成とその応力場

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The deformation of SW Japan arc, due to the Neogene clockwise rotation of the arc and resultant back arc spreading of Japan Sea, is interpteted dynamically using a horizontal buckle model. The Neogene rotational drift of SW Japan arc induced various strain features within the arc. The megakink bands are one of the actual manifestation of such crustal deformations (YANAI 1986). The megakink bands are discrete narrow zones of kilometric width, which are similar in geometry with meso-and microscopic kink bands. The megakink bands are tectonic, because the Butsuzo Tectonic Line in Kyushu and Shikoku and slaty cleavage in Shikoku are bent by them. The zigzag distribution of strata in the Shimanto belt is well explained by the occurrence of megakink bands. The typical megakink bands are Oritate megakink bands (MKB) and Komatagawa MKB in Kii Peninsula, and Sakihama MKB and Muroto MKB in eastern Shikoku, and Kubokawa MKB and Oyu MKB in western Shikoku, and Hokusatsu-Nojiri MKB in southern Kyushu. Conjugated megakink bands are outstanding. The hinge zone (megakink band boundary) is angular or rounded. The orientation of megakink band changes systematically from Kii Peninsula to southern Kyushu ; the dextral megakink bands trend NNE in Kii Peninsula and N-S to NNW in southern Kyushu, and sinistral ones trend NW in Kii and WNW to E-W in Kyushu. The famous Hokusatsu Bend is actually just the northern megakink band boundary of the biggest Hokusatsu-Nojiri Nojiri MKB. The kinematic rotation axis of megakink bands is almost vertical everywhere. The timing of megakink band development is constrained to 20 to 14 Ma by geological data; the megakink deformation involved early Miocene (20 Ma) strata, while the Omine acid dikes (14 Ma) are not rotated by megakink bands. The megakink bands are indicators of regional stress field (axial orientation) and shortening magnitude. The δ1 axial trajectory map using conjugated pairs of megakinkbands is delineated ; the al trajectory is horizontal and parallel to the arc trend, i.e., N 80° in central Kii, N55° in western Shikoku and N25° to 50° in Kyushu, and δ2 axis is vertical. The shortening magnitude due to the megakink band development, along the reference lines from Kii Peninsula to Kyushu, increases southwards up to 22% from the line just to north of the Butsuzo Tectonic Line. The stress and strain pictures of the megakink bands implicate that the rotation mode of SW Japan arc is approximated by buckling with subvertical kinematic rotation axis. In this gigantic buckle fold model, the SW Japan wing was rotated with respect to the almost-fixed Ryukyu wing, under the operation of N30°-oriented horizontal push. The fold would have two hinge zones, i.e., the Shibisan-Nojiri hinge and Bungo hinge. The megakink shortening is interpreted to be resulted from the inner overcompression of Ryuky-SW Japan buckling.
社団法人東京地学協会の論文

日本海拡大に伴う西南日本の変形特にメガキンク帯の形成とその応力場

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日本海拡大に伴う西南日本の変形 特にメガキンク帯の形成とその応力場

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日本海拡大に伴う西南日本の変形特にメガキンク帯の形成とその応力場