HR: 1340h
AN: GP23A-0024    [Abstracts]
TI: Marine Magnetic Anomaly and Magnetization of Oceanic Plate around the Japan Trench in the Northwestern Pacific
AU: * Fujiwara, T
EM: toshi@jamstec.go.jp
AF: JAMSTEC, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Obi, A
EM: obi@ori.u-tokyo.ac.jp
AF: ORI, Univ. Tokyo, 1-15-1 Minamidai, Nakano-ku, Tokyo, 164-8639 Japan
AU: Noda, Y
EM: c1000sakura@nenv.k.u-tokyo.ac.jp
AF: Univ. Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656 Japan
AU: Kido, Y
EM: ykido@jamstec.go.jp
AF: JAMSTEC, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Nakanishi, M
EM: nakanisi@earth.s.chiba-u.ac.jp
AF: Chiba Univ., 1-33 Yayoi-cho, Inage-ku, Chiba, 263-8522 Japan
AU: Hirano, N
EM: nhirano@geo.titech.ac.jp
AF: Tokyo Inst. Tech., 2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8551 Japan
AU: Abe, N
EM: abenatsu@jamstec.go.jp
AF: JAMSTEC, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Ogawa, Y
EM: yogawa@arsia.geo.tsukuba.ac.jp
AF: Univ. Tsukuba, 1-1-1 Tennoudai, Tsukuba, 305-8572 Japan
AB: We have newly collected dense magnetic data around the Japan Trench in the northwestern Pacific. We present characteristics of the complied magnetic anomaly and crustal magnetization variation. The Pacific Plate in the study area has a series of parallel magnetic anomalies (Japanese Lineation Set), identified as chron M14-M7 (140-127 Ma). These anomalies are well lineated, in the direction of WSW-ENE, and have high-amplitudes of ~500-1000 nT, peak-to-trough, in the seaward slope of the trench. The northwestern margin of the Pacific Plate is being subducted beneath the northern Japanese Islands at the Japan Trench. The amplitudes of anomalies gradually decrease towards the land from the trench axis. To correct for effects of seafloor topography, crustal magnetization was calculated. Densely distributed seismic survey profiles enable us to constrain the depth of subducting oceanic crust, and to correct for the depth effects. The resultant crustal magnetization indicates along-lineation variation with a period of ~50-200 km. Wakes of the higher magnetization, sub-perpendicular to the magnetic lineation, could suggest non-transform discontinuities that originate at a mid-ocean ridge. The low magnetization appears on the seaward trench slope where horst-graben structure is developed. This result may suggest that the lower magnetization relates to formation of the horst-graben structure associated with plate bending and normal faulting. The across-lineation variation may indicate temporal variation of the Cretaceous field intensity. The ~135 Ma crust has higher magnetization and decreases towards ~130 Ma. The magnetization gradually decreases as the plate subduction proceeded. The apparent decay could reflect destruction and mechanical disorganization and/or chemical demagnetization of the topmost part of the oceanic crust along the plate boundary.
DE: 1550 Spatial variations attributed to seafloor spreading (3005)
DE: 3005 Marine magnetics and paleomagnetics (1550)
DE: 3045 Seafloor morphology, geology, and geophysics
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 3075 Submarine tectonics and volcanism
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005