「九州西縁構造帯」とその意義
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概要
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The marginal area of western Kyushu represents a peculiar situation from the geological as well as structural points of view. The Nagasaki metamorphic rocks of high pressure-low temperature type, have long been controversial on their tectonic situation since the "Nagasaki-Dreiecke" was proposed by RICHTHOFEN in 1903. The structure of the metamorphic rocks is revealed to be a superimposed fold system and characterized by thrust fault of the NE-SW trend in which the basement rocks of 400 m. y. were brought up accompanied by the thrust movement. This movement rotated anticlockwise the axis of the sedimentary basins of the Late Cretaceous age (Himenoura Group) in association with the uplift of central Kyushu. Thus, started a new tectonic situation for the Ryukyu arc formation in the Late Cretaceous time. The Paleogene formations of western Kyushu are deformed more or less to show dome-basin structure and faults. The principal directions of the faults are the NE-SW and NW-SE which occurred successively so that the structure is characterized by "Twilled Structure" signifying the superimposed structure. The older NE-SW direction has been modified by the NW-SE direction to form the twilled structure in Oligocene to the Middle Miocene time. The "Hokusatsu Bend" which represents an oroclinal deformation of the Shimanto belt at southwestern Kyushu, has been accomplished by this time. The Kyushu Western Marginal Shear started to activate in the Late Cretaceous and culminated in the Middle Miocene at the contact zone between the Honshu and Ryukyu arcs along which the Ryukyu arc shifted southwards.
- 地学団体研究会の論文
- 1979-05-25
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