HR: 10:35h
AN: T32A-02    [Abstracts]
TI: Volcanic versus A-volcanic Accretion at a Melt-Poor Ultraslow-Spreading Ridge; New Insights from Magnetization of 0-30 Ma Seafloor at the Easternmost Southwest Indian Ridge
AU: * Sauter, D
EM: Daniel.Sauter@eost.u-strasbg.fr
AF: Institut de Physique du Globe, UMR7516 CNRS-ULP, 5 rue Descartes, Strasbourg, 67084 France
AU: Cannat, M
EM: cannat@ipgp.jussieu.fr
AF: Laboratoire de Geosciences Marines, CNRS-UMR7097, IPG, 4 place Jussieu, Paris, 75252 France
AU: Mendel, V
EM: veronique.mendel@eost.u-strasbg.fr
AF: Institut de Physique du Globe, UMR7516 CNRS-ULP, 5 rue Descartes, Strasbourg, 67084 France
AU: Patriat, P
EM: ppa@ccr.jussieu.fr
AF: Laboratoire de Geosciences Marines, CNRS-UMR7097, IPG, 4 place Jussieu, Paris, 75252 France
AB: Here we report on the first extensive off-axis data set covering ~200,000 km² in the easternmost part of the Southwest Indian Ridge. We collected gravimetric, magnetic and bathymetric data during the 'SWIR 61-65' cruise on board the french R/V Marion Dufresne between 61°E and 66°E in the most melt-poor section of the SWIR. The picking of the magnetic anomalies A2a, A3a, A5, A6, A6C, A8 and rarely A11 was used to compute the finite and instantaneous poles of rotation in the survey area. Spreading has been ultraslow (14 km/Myr) for the last 20 Myr (A6) with almost no asymmetry. Before 24 Ma (A6C) spreading was much faster (30 km/Myr) with little asymmetry (<7% on average). Spreading direction also changed 20 Ma ago: NS for the last 20 Myr and 8-16°N before. The RMBA map shows that past events of focused ridge magmatism (lasting <3Myrs) only occurred since 20 Ma suggesting that the focusing of magmatism is enhanced when spreading becomes ultraslow. The bathymetric data reveal that a significant proportion (~37%) of the seafloor generated in our study area shows no evidence for a volcanic upper crustal layer. This is very different from observations at faster spreading ridges where basalts are always present. Nevertheless, magnetic anomalies can be identified everywhere in the survey area (except in the finest crust areas). They have significant amplitudes in the a-volcanic domains but these amplitudes are not correlated with RMBA values except at the ridge axis. This suggests a complex source for marine magnetic anomalies with interplay of lavas, if any, gabbros and serpentinized peridotites.
DE: 1550 Spatial variations attributed to seafloor spreading (3005)
DE: 3035 Midocean ridge processes
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 3045 Seafloor morphology, geology, and geophysics
DE: 9340 Indian Ocean
SC: Tectonophysics [T]
MN: Fall Meeting 2005