HR: 0800h
AN: V41B-1439    [Abstracts]
TI: Development of a cylindrical pluton: combined field and gravity evidence from an exposed cross section of the tilted Miocene Tottabetsu plutonic complex, Hokkaido, Japan
AU: * Kamiyama, H
EM: kami@eos.hokudai.ac.jp
AF: ISV Hokkaido Univ., N10W8 Kita-ku, Sapporo, 060-0810 Japan
AU: Yamamoto, A
EM: star@sci.ehime-u.ac.jp
AF: Department of Earth Sciences, Ehime Univ., 2-5, Bunkyo-cho, Matsuyama, 790-8577 Japan
AU: Hasegawa, T
EM: pasegawa@ep.sci.hokudai.ac.jp
AF: Department of Earth and Planetary Sciences, Hokkaido Univ., N10W8 Kita-ku, Sapporo, 060-0810 Japan
AU: Kajiwara, T
EM: kajitaka@eos.hokudai.ac.jp
AF: ISV Hokkaido Univ., N10W8 Kita-ku, Sapporo, 060-0810 Japan
AU: Mogi, T
EM: tmogi@eos.hokudai.ac.jp
AF: ISV Hokkaido Univ., N10W8 Kita-ku, Sapporo, 060-0810 Japan
AB: Construction of a pluton is a multi-process geological event including emplacement, growth and solidification of magma bodies within the Earth's crust. Important information on the mechanism by which plutons are constructed is preserved in their shapes and internal structures. However, exposures of the overwhelming majority of plutons only provide subhorizontal sections, so information on the vertical variations in shape, lithology and internal structures is severely limited. An exposed cross section of the tilted Miocene Tottabetsu plutonic complex allows not only direct observation of its 2-D cross-sectional shape, but also direct evaluation of paleovertical density variations in both the pluton and country rocks, which serve as a strong constraint in gravity modeling that complements information on the missing paleohorizontal dimension of this tilted pluton. The pluton is stratified with a thin granitic unit (~1-km thick) at the top and the underlying thick gabbro-diorite units (~9-km thick) that preserve a stratigraphic record of crystallization and recurrent replenishments in the form of alternation of originally horizontal mafic sheets and cumulate layers. The 2-D cross-sectional shape and the gravity analysis revealed that the pluton had a cylindrical form (10-km thick and 8-km wide) with vertical side walls before tilting. Construction by gradual vertical stacking of the horizontal sheets, together with the cylindrical shape, suggests that the pluton grew only vertically by piston mechanism. Apart from the contact aureole, the roof country rocks are nearly non-metamorphic, contradicting what is expected from ~10-km upward displacement of originally deep-seated country rocks by roof lifting. Thus, it is suggested that space for the successive batches of injected magmas was created mostly by floor depression rather than by roof lifting. The very thick, cross-sectional exposure of the Tottabetsu plutonic complex 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.
DE: 0905 Continental structures (8109, 8110)
DE: 3618 Magma chamber processes (1036)
DE: 3642 Intrusive structures and rocks
DE: 8035 Pluton emplacement
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005