宮崎県北部の四万十帯の層序ならびに緑色岩の層序・構造的位置

The Shimanto Terrain in the northern part of Miyazaki Prefecture is divided into three subparallel major belts, which are subdivided into some to several subbelts. These belts are separated by northely gently inclined thrusts and form an imbricate structure. In each belt the strata face their top side to the north and show a normal stratigraphic sequence. The three major belts are occupied by turns from north to south by the Morotsuka, Kitagawa and Hyuga Groups, of which the former two are considered to be mainly Early Cretaceous and Late Cretaceous in age, respectively and the last is Paleogene. Thus these groups show a southward younging in age as a whole despite their inclination towards the north. The three groups are similar in lithostratigraphic sequence and geologic structure to one another and display a three-part division: the pre-flysch unit of strongly deformed argillite with some intercalations of greenstone, the lower flysch comprising clayslate in the lower and thin-bedded alternation of fine-grained graded sandstone and shale showing an upward coarsening sequence, and the upper flysch of thin-to thick-bedded medium to coarse-grained sandstone and shale with thick beds of sandstone, representing two or three cycles of an upward coarsening and thickening sequence. The pre-flysch unit shows a complex structure characterized by many, small, tight asymmetrical folding, pervasive scaly foliatior and transposition of bedding, and is referable to tectonite. The lower flysch is also characterized by small, asymmetrical folding and slaty cleavage, but the upper flysch is gently folded and not cleaved. Greenstones exclusively occur in the pre-flysch unit mostly as tectonic inclusions of various sizes, ranging from smaller than 50 cm x 1 m to 1 km x 10 km in thickness and extension. They are composed of pillow basalt, pillow breccia and aquagene tuff. Associated with these effusives or independently, intrusive sills of gabbro, dolerite and basalt were also commonly found. In the Morotsuka Group chert is also associated and always lies directly on aquagene tuff. The greenstones are usually faulted against the argillite, but a conformable relation of pillow lavas lying on argilite was observed at several outcrops. Moreover, xenoliths of mudstone and micritic limestone captured in pillows and sills were found not uncommonly. The pillow lavas mostly lack vesicularity, but the top part of large bodies as thick as 1 km contains a little amount of small cavities. They are chemically most similar to oceanic tholeiite. These facts indicate the pillow lavas were extruded on the deep ocean floor where muddy sediments were being accumulated, and are probably referable to off-ridge oceanic basalt. The disposition of strata showing an oceanward orogenic polarity and the stratigraphic and structural characteristics of the Shimantogawa Supergroup in the investigated area can be best explained in terms of subduction model. The pre-flysch and lower flysch units are referable to an accretionary prism of oceanic and trench-fill sediments and the upper flysch to forearc basin and slope sediments which covered the accretionary prism.