HR: 0800h
AN: T21C-0547    [Abstracts]
TI: Electrical Conductivity Structure Beneath the Northwest Pacific Basin Revealed by Long-term Electromagnetic Observation at the Seafloor
AU: * Toh, H
EM: toh@sci.toyama-u.ac.jp
AF: Dept Earth Science, Toyama University, 3190, Gofuku, Toyama, 9308555 Japan
AU: Hamano, Y
EM: hamano@eps.s.u-tokyo.ac.jp
AF: Dept Earth & Planetary Science, University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo, 1130033 Japan
AB: A seafloor geoelectromagnetic observatory has been operated for more than three years since 1st of August, 2001. The site, NWP, is located at (41 07'03"N, 159 55'43"E, 5570m) in the northwest Pacific, which is over 1300 km away from any of existing land geomagnetic observatories. The seafloor observatory is measuring temporal variations of the Earth_fs electromagnetic field with one-minute interval in addition to very precise attitude data to recover the same reference frame as in land observatories. NWP was chosen not only for seaward extension of the land geomagnetic observatory network to increase our knowledge of the Earth_fs geomagnetic field in time and space, but also for looking deep into the Earth beneath the seafloor as old as 120 Ma. The long and simultaneous time-series of the seafloor electric and magnetic fields measured at NWP allow us to delineate the electrical conductivity structure all through the upper mantle using magnetotellurics. It is expected that the oceanic lithosphere just before subduction is well-developed, whose electrical structure may fall in an end member model of oceanic plates. This information is indispensable to understand the whole cycle of the seafloor process from its birth to subduction. The asthenospheric structure beneath the very thick lithosphere is also important in terms of small-scale convection and water content, both of which are key issues of present mantle dynamics. It is also noteworthy that Hirano et al. (2001) reported a very fresh (6 Ma) basalt sampled at the seaward edge of Japan Trench as old as 130 Ma. To understand this new type of intraplate volcanism, it is important to construct a standard model of the electrical conductivity structure in the northwest Pacific that constitutes the counterpart of Lizarralde et al's (1995) model in the northeast Pacific.
DE: 5109 Magnetic and electrical properties
DE: 7218 Lithosphere and upper mantle
DE: 3914 Electrical properties
DE: 3005 Geomagnetism (1550)
DE: 1515 Geomagnetic induction
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting