�@	Fundamental study for diversities of magma-hydrothermal system environments based on simplified-model numerical simulations
		Plinian fall deposits in the Fukushima-Tochigi area during 0.3-0.1 Ma: stratigraphy of marker tephra layers erupting from Numazawa, Hiuchigadake, Kinunuma, and Sunagohara volcanoes

Fundamental study for diversities of magma-hydrothermal system environments based on simplified-model numerical simulations

Hiroshi SHIGENO(Geothermal Research Department, GSJ)

1999

vol. 50 (12), P. 725-741

7 figs., 5 tables

Keywords: magma-hydrothermal system, geothermal resources, geothermal reservoir, cap rock, thermal history, modeling, numerical simulation, deep drilling, Deep-Seated Geothermal Resources Survey

Abstract: A numerical simulation method for magma-hydrothermal systems based on vertical one-dimensional transient thermal conduction models, in which high 'extended thermal conductivity' was assumed for geothermal reservoirs, was proposed for better understanding the macroscopic features of deep geothermal resources. This simplified method was applied to various hypothetical conditions, changing (1) depth of magma chamber top, (2) depths of geothermal reservoirs' bottom and top, (3) thickness of magma chamber, and (4) the 'extended thermal conductivity' of geothermal reservoirs. Their effects on temporal changes of underground temperature distributions were roughly calculated and evaluated. Deep magma chambers (depth of chamber top at 6 km down from the surface) are obviously less effective for heating the reservoirs distributed at 1 to 4 km depth levels than shallow magma chambers (chamber top at 4 km depth). For the former case, however, developments of thick deeper reservoir structures, especially with suitable cap rocks, enable formations of long-lasting high-temperature deep hydrothermal systems. This type of deep reservoirs could be more suitable for power generation exploitation in terms of reservoir temperature and size than the other type of deep reservoirs developed around shallow magma chambers.

Plinian fall deposits in the Fukushima-Tochigi area during 0.3-0.1 Ma: stratigraphy of marker tephra layers erupting from Numazawa, Hiuchigadake, Kinunuma, and Sunagohara volcanoes

Takahiro YAMAMOTO(Geology Department, GSJ)

1999

vol. 50 (12), P. 743-767

15 figs., 4 tables

Keywords: tephrostratigraphy, Numazawa volcano, Hiuchigadake volcano, Kinunuma volcano, Sunagohara caldera, Pleistocene

Abstract: This paper presents the stratigraphy, characteristics, distributions, and eruptive ages of marker tephra layers in the Fukushima-Tochigi area from 0.3 till 0.1 Ma. The Nikko-Mamiana tephra consists of scoria fall deposits (ca. 9 km3 in volume), erupting from the Nikko volcano group at about 0.10 Ma. The Numazawa-Shibahara tephra is made up of pumice fall deposits (ca. 4 km3 in volume), erupting from Numazawa volcano at about 0.12 Ma. The Hiuchigadake-Nanairi tephra is composed of pumice fall deposits (ca. 7 km3 in volume) and pyroclastic flow deposits (ca. 1 km3 in volume), erupting from Hiuchigadake volcano at 0.16-0.17 Ma. The Kinunuma-Kurodahara tephra consists of pumice fall deposits (ca. 5 km3 in volume) and mostly-welded pyroclastic flow deposits (less than 1 km3 in volume), erupting from Kinunuma monogenetic volcano at 0.20-0.24 Ma. The Sunagohara-Kubota tephra is made up of pumice fall deposits (ca. 4 km3 in volume) and pyroclastic flow deposits (more than 2 km3 in volume), erupting from Sunagohara caldera at about 0.22 Ma. Synchronized occurrences of Numazawa volcano and Sunagohara caldera in the Aizu district and Hiuchigadake and Kinunuma volcanoes in the Oze region suggest the volcanic fields in the southern part of the NE Honshu arc have expanded toward the back-arc side simultaneously since 0.30 Ma.