Fig.1 Topographic map of the Sengan Geothermal Area, Northeast Japan (Okuma, 1998)
Contour interval is 100 m. The small and large boxes bounded by black solid lines show the close-up and the detail study area of Akita-Yakeyama volcano, respectively. The box bounded by blue solid lines show the detailed study area of Iwate-san volcano.
Fig.2 Geological map of the Sengan Geothermal Area modified from Research group for the geologic map of Sengan Geothermal Area (1985) and Suto (1992) (Okuma, 1998)
Younger Volcanic Rocks 1: Matsukawa Andesite, Younger Volcanic Rocks 2: Otsuki-yama volcanics, Younger Volcanic Rocks 3: Hachimantai volcanics and Kayo-dake volcanics, Younger Volcanic Rocks 4: Nanashigure-yama volcanics, Takakura-yama volcanics, Moriyoshi-yama volcanics, Akita-Yakeyama volcanics, Iwate-san volcanics and Akita-Komagatake volcanics.
Recently, detailed geologic maps
have been compiled for the Sengan Geothermal Area (Research group
for the geological map of Sengan Geothermal Area, 1985) and for
Akita-Yakeyama volcano and its surrounding areas (Suto, 1992),
respectively. These maps show that the geothermal area is covered
widely by Quaternary volcanic rocks with outcrops of Pleistocene
ones (Fig. 2). Drilling showed that the surface volcanic rocks
are underlain by Neogene volcanic and sedimentary rocks in the
Sengan Geothermal Area. The absence of pre-Neogene rocks inside
the geothermal area and the presence of Paleozoic sedimentary
rocks and Cretaceous granitic rocks outside the area suggest the
existence of a large tectonic subsidence beneath the surface volcanic
rocks.
Fig.3 Total intensity aeromagnetic anomaly (IGRF residuals) map of the Sengan Geothermal Area (Okuma, 1998).
Contour interval is 25 nT. The altitude of datum plane is 1,800 m above sea level. See also Fig.1.
In conjunction with various geophysical
surveys, an aeromagnetic survey for geothermal exploration was
conducted in the Sengan Geothermal Area (MITI, 1978). Okuma and
Suto (1986) recompiled aeromagnetic anomaly maps from the original
anomaly data. The observed magnetic anomalies (Fig. 3) are influenced
strongly by the effect of topographic relief. The volcanic rocks
which compose the terrain have their magnetic properties, strong
enough to cause intensive magnetic anomalies. Magnetic highs and
lows lie in the southern and northern flanks of many volcanoes:
Iwate-san volcano, Takakura-yama volcano, Akita-Komagatake volcano,
Maemori-yama volcano, Nanashigure-yama volcano and Moriyoshi-yama
volcano, etc. On the contrary, such simple relationship between
the topography and magnetic anomalies has not been observed for
Akita-Yakeyama volcano, Hachimantai volcano, Kayo-dake volcano,
etc.
To remove the effects of magnetic terrain, an apparent magnetization intensity mapping method (Okuma et al., 1994; Nakatsuka, 1995) was adopted to the magnetic anomalies of the Sengan Geothermal Area. These methods were useful for revealing concealed old volcanic edifices on Izu-Oshima Island (Okuma et al., 1994) and the quadratic distribution pattern of magnetization intensity along the Tanna fault, presumably caused by movements of the strike-slip fault (Nakatsuka, 1995).
Fig. 4 Magnetic model employed for an apparent magnetization intensity mapping
Mij: Magnetization of a prism at ij-th point, Tij: Synthetic magnetic anomaly at ij-th point
ZijU: Top depth of a prism at ij-th point, ZijL: Bottom depth of a prism at ij-th point
A magnetic model composed of finite rectangle prisms that corresponds to the volcanic terrain (Fig. 4) was assumed. Each prism is magnetized uniformly in a direction of the present Earth's magnetic field. Magnetization intensities vary only laterally among the prisms. The top depth of the model corresponds to the ground surface, while the bottom depth corresponds to a flat surface, though it can be assumed to be of any shape. A flat surface of 5 km below sea level was used for the bottom depth of the magnetic model, taking account of a local shallow Curie depth (< 8 km) (Okubo et al., 1989). A magnetization intensity of a prism at ij-th point can be inverted from the observed magnetic anomaly by employing the conjugate gradient method (Nakatsuka, 1995) instead of solving simultaneous linear equations directly.
Fig.5 Magnetization intensity map of the Sengan Geothermal Area with a topographic shading (Okuma, 1998).
Contour interval is 0.2 A/m. The areas bounded by solid lines indicate hydrothermal altered areas. Solid and open circles locate volcanic rocks which are normally and reversely magnetized, respectively. Paleomagnetic data (Suto, 1985; Suto, 1987; Suto and Mukoyama, 1987; Suto, 1992) which have stable magnetization intensities (³1.0 A/m ) with magnetic pole latitudes (³ |±50°|) were plotted. See also Fig. 1.
(1) Magnetization highs lie obviously
on Quaternary volcanoes such as Maemori-yama volcano, Iwate-san
volcano, Obuka-dake volcano, Takakura-yama volcano and Akita-Komagatake
volcano (Fig. 5). This result clearly shows that each edifice
is roughly magnetized in a direction of the present earthÕs magnetic
field.
(2) On the other hand, obvious magnetization lows lie on Kurasawa-yama
and Kayo-dake volcano. Paleomagnetic dada indicate that normally
magnetized volcanic rocks are distributed in high magnetization
areas, whereas reversely magnetized rocks are in low magnetization
areas. This relationship is much clearer than that of Okuma and
Suto (1987).
(3) Magnetization lows with small amplitudes, or rather to say
weak magnetization areas, correspond to hydrothermally altered
areas mapped by Research group for the geological map of Sengan
Geothermal Area (1985); weak magnetizations lie on and around
the summit of Akita-Yakeyama volcano, between Sumikawa and Onuma,
in Matsukawa and Kakkonda, between Takakura-yama and Akita-Komagatake
volcanoes, etc. It is implied that hydrothermal alterations caused
a loss of a large amount of magnetic minerals in volcanic rocks.
Recently, young granitic intrusions have been found in Kakkonda
area, one of weak magnetization areas, by drilling. A temperature
of the bodies exceeded 600 ¡C at the bottom of a drill hole close
to the Kakkonda geothermal power plant. On the basis of the results,
the apparent magnetization intensity method is useful to estimate
the location of hydrothermal areas in Quaternary volcanic areas.
[Iwate-san
volcano]
[Akita-Yakeyama volcano]
To Okuma's Home | GSJ Home | Geoscience Page |