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Geothermal Island-Arc Genesis at the Plate Margins On the Similarity of the two Geothermal Island-Arc Genesis in Southern Part of Kyushu, Japan and Sumatra, Indonesia

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Susumu N

ISHIMURA(NPO Think-tank Kyoto Institute of Natural History)

Abstract

Geothermal island-arc genesis are found in graben and caldera along the big-fault systems of Southern part of Kyushu, Japan and Sumatra, Indonesia. The magma of ignimbrite and welded tu#(‘Shirasu’) might be originated from the remelt of oceanic plate and/or crustal materials, when the front of Philippine and Indian Sea plates is subducted to nearly++*km in depth. These oceanic plates are subducted obliquely to trench axis.

The volcanism of these ignimbrite and Shirasu is so violent as to make a big caldera, of which Bouguer anomalies are about/*mgal lower than those of the surrounding area, and the big caldera has the diameter of+/%,*km which is disturbed the big-fault systems.

After these activities, the dacite and andesite volcanisms occur, which make the cen- tral cone of volcanoes. The magma of these volcanoes is quite di#erent from that of ignimbrite and welded tu#in the genesis. The magma must be originated from the partial- melt of the upper mantle under the volcanic belt of island arc. There are hot-springs within+/km distance from crater of volcanoes.

Acording to a plausible model for the migration of H^,O and the generation of basalt magma in subduction zones, the hydrated peridotite is formed by addition of slab-derived H^,O beneath the forearc region and should be dragged downward on the slab by the subduction of oceanic lithosphere. Amphibolite and chlorite in the dragged hydrated peridotite layer decomposes to release H^,O at around++*km just beneath a volcanic front.

When the front of migration of H^,O from the hydrated peridotite reaches the region with solidus temperature of hydrous peridotite. Partial melting takes place to produce initial magma. Successive addition of H^,O to the region expands the partial molten zone upwards in which the degree of partial melting increases upwards. From the top of the partially molten zone a mantle diapir starts rising to make a volcanism.

The di#erent two types of volcanic activities with geothermal activities are found at the volcanic belts in the dragged downward slabs of the around+/*and,/*km depths.

Key words : Island-arc, geothermal area, Hot Spring, Southern part of Kyusyu, Sumatra

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Fig.- Generalized geological map and Bouguer anomaly map of Southern Kyushu (Kobayashi,+322).

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Fig.1 The K^,OῌMgO diagrams of volcanic rocks from (a) southern part of Kyushu and (b) Sumatra (Nishimura,+32*).

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Fig.2 The model of geothermal genesis, in Kirishima area (Fujita,,**0).

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Fig.+* A plausible model for migration of H,O and the generation of basalt magams in subduction zones (modiﬁed from Tatsumi,+323).

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Fukao, Y., Maruyama, S., Obayashi, M. and Inoue, H. (+33.) : Geologic implication of the whole mantle P-wave tomography, Jour. Geological Society, Jap.,+**,.ῌ,-.

Hamilton, W. (+31.) : Earthquake Map of the Indonesian Region,+:/,***,***, USGS, Map I-21/-C.

Kobayashi, T. (+322) : Tectonic Setting and Major Pyroclastic Eruption in Southern Kyushu, Fig.++ A model of subduction system across Tohoku area, Japan (modiﬁed from Nishimura,+331),

using Plume Tectonics conducted by Fukaoet al.(+33.).

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A guide book for Sakurajima Volcano, Organizing Committee Kagoshima International Conference on Volcanoes,+ῌ0, July+3ῌ,-,+322, Kagoshima.

Nishimura, S. (+331) : Report of the Review of Topical Report on Volcanology of the Siting Studies at Muria Peninsula, Indonesia,-*p, International Atomic Energy Agency, Project INS/3/*+,, April,,,+331. Venna.

Nishimura, S. ed. (+32*) : Physical Geology of Indonesian Island Arcs.,,-3p, Kyoto Univ., Kyoto.

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Sidarto, T.C.A., Santosa, S. and Gunawan,W. (+33.) : Geology of the Kotaagung Quadarangle, Sumatera,+:,/*,***Explanatory note and geological map, Geological Research and De- velopment Centre.

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Tatsumi, Y. (+323) : Migration of ﬂuid phases and genesis of basalt magmas, J. Geophys., Res., 3.,.031ῌ.1*1.

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