TERRAPUB Earth, Planets and Space

Earth Planets Space, Vol. 57 (No. 12), pp. e21-e24, 2005


Gravity and density variations of the tilted Tottabetsu plutonic complex, Hokkaido, northern Japan: implications for subsurface intrusive structure and pluton development

Hiroyuki Kamiyama1, Akihiko Yamamoto2, Takeshi Hasegawa3, Takanori Kajiwara1, and Toru Mogi1

1Institute of Seismology and Volcanology, Graduate School of Science, Hokkaido University, Sapporo, Japan
2Department of Earth Sciences, Faculty of Science, Ehime University, Matsuyama, Japan
3Department of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, Sapporo, Japan

(Received September 9, 2005; Revised November 9, 2005; Accepted November 10, 2005)

Abstract: An exposed cross section of the tilted Tottabetsu plutonic complex allows direct evaluation of its original 2-D cross-sectional shape and pretilting vertical density variations in both the pluton and the country rocks, which serves as a strong constraint in gravity modeling that complements information on the 'missing' pretilting horizontal dimension of this tilted pluton. The pluton is stratified with the uppermost thin granitic unit (~1-km thick) and the underlying thick gabbro-diorite units ( ~9-km thick) that preserve a stratigraphic record of numerous hotter replenishments in the form of alternation of originally horizontal mafic sheets and cumulate layers. Both the pluton and the country rocks show systematic density increase with pretilting crustal depth, but density contrast of the pluton with the country rocks varies between each unit. The 2-D cross-sectional shape and gravity analysis revealed that the pluton had a vertically-elongated shape with vertical side walls before tilting. The vertical side walls, together with the stack of the originally horizontal sheets and cumulate layers, suggests that the pluton grew only vertically by piston mechanism. The very thick, exposed cross section provides unequivocal evidence for development of such a pluton with this unusual shape and mass distribution, which has been inferred elsewhere only by some geophysical studies.
Key words: tilted pluton, pluton emplacement, gravity anomaly, rock density.

Corresponding author E-mail: kami@eos.hokudai.ac.jp

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