HR: 12:05h
AN: T52B-08    [Abstracts]
TI: Shearing within in situ lower crust during progressive retrogression: a structural study of the Godzilla Mullion, Parece Vela Basin spreading ridge (Parece Vela Rift), Philippine Sea
AU: * Harigane, Y
EM: f5644006@ipc.shizuoka.ac.jp
AF: aInstitute of Geosciences, Shizuoka University, 836 Ohya, Shizuoka, 422-8529, Japan
AU: Michibayashi, K
EM: sekmich@ipc.shizuoka.ac.jp
AF: aInstitute of Geosciences, Shizuoka University, 836 Ohya, Shizuoka, 422-8529, Japan
AU: Ohara, Y
EM: ohara@jodc.go.jp
AF: Hydrographic and Oceanographic Department of Japan, 5-3-1, Tsukiji, Chuo-ku, Tokyo, 104-0045, Japan
AB: Microstructural and petrological analyses of gabbroic rocks sampled from the Godzilla Mullion, located along the Parece Vela Basin spreading ridge (Parece Vela Rift), Philippine Sea, reveal the development of a detachment fault at depth as part of an oceanic core complex. Microstructures indicative of intense deformation are observed within samples of gabbro dredged from the breakaway of the mullion (dredge site D6), at the site of its initiation. The sizes of recrystallized grains, nature of crystal-preferred orientation, and chemical composition all vary systematically with respect to the temperature of deformation. The sizes of dynamically recrystallized grains of plagioclase can be divided into three types: coarse (80–130 µm), medium (25 µm), and fine (8 µm). Furthermore, although the chemical composition of plagioclase porphyroclasts is consistently An 40–50 among all grain sizes, the compositions of dynamically recrystallized grains vary with size, being An 40–50 for the coarse type, An 30–40 for the medium type, and An 20–30 for the fine type. Given that the chemical composition of plagioclase potentially results from a change in temperature during dynamic recrystallization, this finding suggests that the microstructural development of plagioclase occurred under increasing stress during uplift-related cooling of the gabbro body. Plagioclase crystal-preferred orientations (CPOs) shows a (010)[100] pattern for coarse and medium grains, but a largely random pattern for fine grains. This indicates that grain-size-sensitive creep became progressively dominant in the fine type, thereby leading to strain softening and localization during cooling. Although the chemical composition of hornblende varies from pargastic-hornblende to actinolite and tremolite within each of the gabbro samples, the pattern of variation is similar among the three sample types. However, hornblende in the coarse type shows no evidence of deformation, whereas hornblende in the medium and fine types is plastically deformed. Consequently, we propose that a primary shear zone developed at the breakaway under high temperatures (>700�E#8249;C) and at depth under anhydrous conditions. The shear zone subsequently evolved during progressive retrogression in association with hydration of the shear zone, eventually resulting in the development of the detachment fault that gave rise to the Godzilla Mullion.
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 3035 Midocean ridge processes
DE: 8150 Plate boundary: general (3040)
DE: 8416 Mid-oceanic ridge processes (1032, 3614)
DE: 8424 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8135)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting