HR: 15:10h
AN: T33G-07    [Abstracts]
TI: Crustal Accretion in the Parece Vela Backarc Basin: The world's Fastest Ultraslow-Spreading Ridge
AU: * Ohara, Y
EM: ohara@jodc.go.jp
AF: Hydrographic and Oceanographic Department of Japan, Tsukiji, Tokyo, 104-0045 Japan
AU: Okino, K
EM: okino@ori.u-tokyo.ac.jp
AF: Ocean Research Institute, University of Tokyo, Nakano, Tokyo, 164-8639 Japan
AU: Snow, J E
EM: jesnow@uh.edu
AF: Geosciences Department, University of Houston, Calhoun, Houston, TX 77204-5007 United States
AU: Hellebrand, E
EM: ehelle@mpch-mainz.mpg.de
AF: Max-Planck Institute for Chemistry, Postfach 3060, Mainz, D-55020 Germany
AU: Harigane, Y
EM: r0435018@ipc.shizuoka.ac.jp
AF: Institute of Geosciences, Shizuoka University, Oya, Shizuoka, 422-8529 Japan
AU: Michibayashi, K
EM: sekmich@ipc.shizuoka.ac.jp
AF: Institute of Geosciences, Shizuoka University, Oya, Shizuoka, 422-8529 Japan
AU: Ishizuka, O
EM: o-ishizuka@aist.go.jp
AF: Geological Survey of Japan/AIST, Higashi, Tsukuba, 305-8567 Japan
AB: The seafloor formed over the Miocene in the northern Parece Vela Basin (PVB) in the Philippine Sea is characterized by rugged topography, indicating a weak magma supply and mantle exposure in an extinct backarc basin. PVB was a physically fast/intermediate spreading system (full-spreading rate: 8.8-7.0 cm/y), although many tectono-magmatic features resemble those of slow to ultraslow spreading systems. These features include: (1) Development of oceanic core complexes at many segments (2) Occurrence of the world_fs largest oceanic core complex, Godzilla Mullion (3) Very large water depth at segment ends (maximum water depth ca. 7500 m) (4) Exposure of mantle peridotite at segment centers (5) Presence of fertile lherzolite (spinel Cr/Al+Cr ratio ca. 0.17) In order to see the change in crustal accretion mode along the entire length of the PVB spreading center, we have mapped and sampled the southern PVB with the recent three cruises (YK03-09-2, KH03-03, and KH05-01-5). The seafloor in the southern PVB is also characterized by the closely-spaced, NNE-SSW trending fracture zones as in the northern PVB, although there are no apparent rift axes in the south. Instead, there exist possible pull-apart basins (maximum water depth ca. 7200 m), indicating spreading was oblique there. The closely-spaced fracture zones and oblique spreading of PVB is a possible result of complex evolutionary history of the basin, including genetic relationships with the Yap arc system. Two prominent oceanic core complexes with continuous corrugation are recognized, and one of them extends ca. 90 km, almost as large as Godzilla Mullion. Dredging on these oceanic core complexes yielded gabbros and fault rocks. The observations seen in the entire PVB imply that the morphologic and petrologic characteristics of ridges we normally assume to be a direct function of spreading rate are in fact solely a function of magmatic supply. We propose that the lower magma supply in PVB was due to the thick lithosphere possibly caused by closely-spaced fracture zones and oblique spreading.
DE: 3001 Back-arc basin processes
DE: 3035 Midocean ridge processes
DE: 3039 Oceanic transform and fracture zone processes
DE: 3614 Mid-oceanic ridge processes (1032, 8416)
DE: 3621 Mantle processes (1038)
SC: Tectonophysics [T]
MN: Fall Meeting 2005