南部北上山地における白亜紀前期の造構作用の研究

Study on the Early Cretaceous Tectonic Movement in the Southern Kitakami Massif, Northeast Japan

IKEDA Yukio

廣島大學地學研究報告

Geological report of the Hiroshima University

This paper describes and discusses the deformation structures of the Southern Kitakami Massif, Northeast Japan, in an attempt to clarify the genetic history and the origin of the structures with special reference to the early Cretaceous plutonism.
The area consists of a sequence of thick sedimentary strata from Permian to Jurassic in age. The major geological structure of this area is large synclinorium, called Tsunakizaka syncline, with its axis plunging to S by about 30°. The Kesengawa granodiorite intruded into the eastern limb of the synclinorium during the early Cretaceous time, and the sedimentary rocks and porphyritic dykes adjacent to the granodiorite are converted into schistose hornfelses owing to the contact metamorphism.
Slaty cleavage and schistosity (both characterized by parallel arrangement of platy mineral flakes) are developed remarkably well in the pelitic rocks of the Permian and Triassic strata. They are also recognized in the porphyritic dykes that were subjected to the contact metamorphism. The development of schistosity is restricted to the granodiorite and the zone of contact metamorphism in which constituent grains are coarser than those in the original rocks.
The slaty cleavage exhibits fairly uniform orientation throughout the Tsunakizaka synclinorium and are parallel or subparallel to its axial surface. In view of maximum flattening, this arrangement of slaty cleavage suggests a regional horizontal shortening in the EW direction. But the exact mechanism of the formation of the synclinorium is unknown as yet.
Penetrative schistosity is developed in the Kesengawa granodiorite, and it is parallel to the regional trend of schistosity and slaty cleavage developed in the ajacent sedimentary strata. At outcrops near Osabe no refraction of schistosity is observed between the granodiorite and the host rocks irrespective of the orientation of their interface, except at small portions in and around minor folds of granitic and aplitic dykes where the pattern of schistosity is more complex owing to the inohomogeneous deformation associated with the folding. It is argued that the development of schistosity in granodiorite is closely associated with the regional tectonic movements during the early Cretaceous time, rather than with the magmatic flow within the body, contrary to the previously advocated notion.
Under the assumption that the rocks behaved as viscous fluids, (apparent) viscosity ratios between various rocks at the time of tectonic movements are estimated from deformed pebbles employing GAY'S technique and from dominant wavelengths of folds by use of BIOT'S equation. The results show : (1) the granite pebbles were 5 to 10 times more viscous than limestone, rhyorite and andesitic rocks, (2) the viscosity of quartz vein was 4 to 7 times as large as that of slate, and (3) slate behaved as more viscous fluid than did the limestone. The examination of cleavage patern indicates that the folds used in the estimation of viscosity ratio are buckling or buckle folds due to compression parallel or subparallel to the layering.

jpn

廣島大學理學部地學教室

[ISSN] 0073-2303

[NCID] AN00213676