HR: 0800h
AN: T41A-1170    [Abstracts]
TI: Atlantis Bank as a Key to Understanding the Nature of the Moho and Crust-Mantle Boundary
AU: * Matsumoto, T
EM: tak@sci.u-ryukyu.ac.jp
AF: University of the Ryukyus, 1 Senbaru, Nishihara-cho, Okinawa, 903-0213 Japan
AU: Arai, S
EM: ultrasa@kenroku.kanazawa-u.ac.jp
AF: Kanazawa University, Kakuma-machi, Kanazawa, 920-1192 Japan
AU: Dick, H J
EM: hdick@whoi.edu
AF: Woods Hole Oceanographic Institution, 360 Woods Hole Rd., Woods Hole, MA 02543 United States
AB: Atlantis Bank on the Southwest Indian Ridge is a key site for the study of the oceanic lower crust and mantle. It exposes an eroded oceanic core complex lying along the western flank of the Atlantis II transform. The complex exposes of huge 400+ km2 gabbroic massif that is at least 1500-m thick at ODP Site 735. Mantle peridotites are exposed on the lower slopes of the transform wall that flanks the core complex, where they locally appear to underlie the gabbro body. During the SHINKAI 6500 cruises in 1998 and 2002, seafloor gravimetry was carried out at 16 stations on the Bank to detect its precise density structure. The average density of the basement rocks on the western slope of the bank was estimated as 3.0 g/cc (lower part), 3.8 g/cc (middle part) and 3.1 g/cc (upper part), corresponding to mantle peridotite, oxide Fe-Ti gabbro and peridotite/oxide olivine gabbro collected at these sites respectively. Average density of basement rock on the eastern slope was estimated as 3.1 g/cc (lower part) and 2.6 g/cc (middle part). The southwestern slope of the bank is characterised by exposure of unaltered layered gabbro underlain by serpentinized peridotite according to observations and sampling during the 2002 survey. Average bedrock density at this site is 2.5g/cc, extremely low considering the exposure of lower crust and upper mantle rocks there. According to the seismic survey crossing the bank (Muller, et al., 1997, 2000), a discontinuity at 4 km below seafloor at the foot of the bank and at 6 km bsf at the summit was identified. P wave velocity contrast indicates that the boundary is equivalent to Moho. Their result also shows that the southwestern slope corresponds to the Layer-2 (Layer-1 is missing), and the estimated P wave velocity of 3.6 km/s is in good agreement with the basement rock density derived from seafloor gravimetry. The Moho is not necessarily identical to the petrological crust-mantle boundary and it is possible that it is the boundary between serpentinized and non-serpentinized peridotite as shown in 'Hess model', and as also suggested by Muller, et al. (1997). This hypothesis will be tested in the future IODP riser drilling.
DE: 9340 Indian Ocean
DE: 8100 TECTONOPHYSICS
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8122 Dynamics, gravity and tectonics
DE: 3035 Midocean ridge processes
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting