[Abstract]

K-Ar dating of volcanic rocks from northern vicinity of Matsukawa geothermal field was carried out. Ages of 0.83+/-0.04 Ma and 0.58+/-0.10 Ma for Shimokura-Nakakura volcano and 0.72+/-0.03 Ma for 1470 m peak lava were obtained. The age of Shimokura-Nakakura volcano are correlated with the early stage of advanced argilllic alteration around Matsukawa geothermal field. The contemporaneity of hydrothermal and magma activities in the area implies the volcanic-hydrothermal activity in the core of Shimokura-Nakakura stratocone. The new dating of this study, together with the previously reported K-Ar ages and the geological studies, suggests distinct temporal change in alignments of volcanic centers in Sengan geothermal field at 0.6 Ma. The change was caused by outspread of volcanic activity from upheaval zone, which is bounded by reverse faults, to the outward zone. The volcanic alignment older than 0.6 Ma is located on the upheaval zone and alignment direction is transverse with regional maximum horizontal stress direction. On the other hand, younger volcanic alignments are located outside the upheaval zone and parallel to the regional stress direction, except for Akita Komagatake which is younger than 0.6 Ma but located on the upheaval zone.

e-mail:

ohbatu@mail.cc.tohoku.ac.j