抄録
Geology and fractures in the Takinoue geothermal area, Iwate Prefecture, Northeast Honshu, Japan are investigated to clarify the process of development of fractures and the associated stress field from the structural geological point of view. From the above, the relationship between fracture systems and paths of geothermal water is discussed. The conclusions are as follows: (1) The Torigoenotaki Dacite as well as the Matsuzawa dacitic rock body is regarded as the lava and intrusive member of the Minamishirasawa Formation. (2) The tuffaceous member of the Minamishirasawa Formation is newly defined as the Kitashirasawa Formation. (3) The hydrothermal veins and most minor faults are classified into four systems on the basis of their dominant orientations, that is, NNW-SSE to NW-SE strike and moderate dip system (NW-moderate dip system), NW-SE strike and steep dip system (NW-steep dip system), NE-SW to E-W strike and steep dip system (NE to E-W system) and horizontal system. (4) The NE to E-W system consists of veins and faults with lateral and normal displacement, both of which were formed by the stress field with horizontal NW-SE minimum principal stress axis (F3).(5) The NW-moderate dip system and NW-steep dip system consist of veins and faults with lateral to normal displacement, both of which were formed by the stress field with horizontal NNE-SSW to ENE-WSW minimum axis (F2). (6) The oldest stress field obtained from the fracture analysis in this study is the tensional stress field with horizontal NNESSW to NE-SW minimum axis (F1), which existed before the tilting of the Miocene Yamatsuda Formation. Thereafter, the F2 stress field continued at least after the deposition of the younger volcanic products (the Omatsukura volcanic products). The latest stress field is F3, which continues at least after 0.2Ma in the studied area to the present. (7) The present paths of geothermal water are controlled by the fractures belonging to the NE to E-W system.
