|Budget Amount *help
¥1,000,000 (Direct Cost : ¥1,000,000)
Fiscal Year 1998 : ¥500,000 (Direct Cost : ¥500,000)
Fiscal Year 1997 : ¥500,000 (Direct Cost : ¥500,000)
The purpose of this project was to reexamine Tertiary tectonics of the Japan arc, particularly, when the Japan Sea opened.
(1) Paleostress measurement from fault-slip data
Mesoscale faults can constrain paleostress. We collected data from the Boso area, and examined methods to infer paleostresses (Mino and Yamaji, in press).
(2) Stratigraphy and tectonics of the Kuma area
There are few data available in Setouchi region to constrain tectonics accompanied by the Japan Sea opening. So, we studied the Kuma Group, Ehime Prefecture, and revealed that the group is composed by Eocene and Lower Miocene strata (Narita et al., in press). And, the group is found to be covered with clinounconformity by the Ishizuchi Group. Extensional synsedimentary tectonics was suggested from the lithology and geologic structures of the Kuma Group.
(3) Paleomagnetic rotation
When the Japan Sea opened, NE Japan rotated counterclockwise. However, data indicating the opposite sense of rotation were reported from the northern part of Niigata Prefecture. We made geologic field survey and dense paleomagnetic sampling. Consequently, it was found that the area was broken into blocks showing differential rotations (Yamaji et al., 1999). In addition, a large dextral strike-slip fault that accommodated differential rotations was discovered.
(4) Plate kinematics
When the Japan Sea opened, the Japan arc experienced a sequence of drastic tectonic events, e.g., rapid vertical movements, expansion of volcanism, paleomagnetic rotations, variation of stress field. It is shown that a hypothetical microplate that was detached from the Pacific Plate can account for the events (Yamaji and Yoshida, 1998).