棚倉破砕帯の地質構造

The Tanakura Shear Zone constitutes the major tectonic zone which bounds the Abukuma Belt on the east and the Yamizo Belt on the west. This tectonic zone extends from Tanakura to Hitachiota along a NNW-SSE trend in the area studied. The Abukuma Belt is mainly composed of metamorphic and granitic rocks, while the Yamizo Belt is mainly composed of geosynclinal sedimentary rocks of Paleozoic and Mesozoic age. The shear zone is 3-4km in width and is composed of lens-shaped masses of cataclasite and mylonite which originated from the Abukuma and Yamizo belts. These masses are bounded by faults which are arranged in en echelon pattern as a whole. The Tanakura Shear Zone was originated and had primary development during the Cretaceous being closely related to the plutonic activities both in the Abukuma and Yamizo belts. During that period, left lateral displacements may have been occurred along the shear zone. In the areas adjacent to the Tanakura Shear Zone, on the west and east, the Neogene sediments are distributed with clino-unconformity on the basement rocks of the Yamizo and Abukuma belts. The Neogene sediments in area west of the Tanakura Shear Zone are separated by several basins about 10km in diameter, which are distributed along the Shear Zone. Each basin is tilted northeastwards. The eastern margin of each basin is bounded by the western marginal fault of the Tanakura Shear Zone, while the northern margin of each is bounded by a fault nearly perpendicular to the Tanakura Shear Zone. On the northern side of the fault bounding the northern margin of each basin, a granitic mass is always exposed. This systematic distribution pattern of faults and masses of granitic rocks was an important factor in locating the Neogene basins. The basins are filled with andesitic volcanic sandstone, conglomerates, sandstone, andesitic volcanic breccia, black siltstone, breccia and fine sandstone ranging from middle Early Miocene to early Middle Miocene in age. As the total thickness for individual basins ranges from 1000m to 3500m, these basins may have subsided very rapidly. The sediments were deposited in the narrow bay stretching north to south along the west side of the Tanakura Shear Zone. Changes of lithofacies and thickness of sediments reflect the tectonic movement of the Tanakura Shear Zone and its adjacent area during deposition. Rapid subsidence of the basins occurred as if it would have avoided the simultaneity each other among the adjacent basins, so that as a whole these basins subsided in differential movement. During this period, the surveyed area had been influenced by a tensional stress field, but after the retreat of the manine environment from this area, movement along the Tanakura Shear Zone was right lateral on a small scale. In the late Middle Miocene, when the area west of the Tanakura Shear Zone was emergent the area east of the Shear Zone began to subside. Sediments in this area are mainly composed of sandstone and sandy mudstone which were deposited in a narrow bay which developed along the eastern margin of the Tanakura Shear Zone. The sedimentary rocks of this narrow basin are less thick and less deformed with respect to those west of the Shear Zone. Invasions of the sea, from both north and south, took place during different stages to the west and east of the Tanakura Shear Zone, but were limited by the Shear Zone. Pliocene transgression, although less pronounced than the former two, also came from both north and south simultaneously, and tuffaceous sandstone and mudstone were deposited in northern and southern parts of the area. The Pliocene formations formed during this transgression are, however, scarcely affected by the movement of the Tanakura Shear Zone. Therefore, the Tanakura Shear Zone which originated during the Late Cretaceous was active during in the Miocene. but thereafter has not revealed anv important movement.