HR: 15:06h
AN: GP33B-06 INVITED     [Abstracts]
TI: Origin of deep-sea magnetic lineations: magnetized oceanic crust or serpentinized exhumed mantle
AU: Lin, J
EM: jylin@ifremer.fr
AF: Ifremer Centre de Brest, B.P. 70, Plouzane, 29280 France
AU: * Sibuet, J
EM: jcsibuet@ifremer.fr
AF: Ifremer Centre de Brest, B.P. 70, Plouzane, 29280 France
AU: Srivastava, S
EM: shirisrivastava@hfx.eastlink.ca
AF: Geological Survey of Canada, Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, N.S., B2Y 4A2 Canada
AU: Manatschal, G
EM:
AF: CGS-EOST, Universite Louis Pasteur, 1 rue Blessig, Strasbourg, 67084 France
AB: Since a long time, it has been established that sea-floor spreading at mid-oceanic ridges gives rise to oceanic crust at medium or high spreading rates (magmatic segments) and mostly exhumed mantle at slow and ultraslow spreading rates (amagmatic segments). Whatever the extension rates, the nature of the crust and the process of crustal magnetization are, magnetic lineations appear to be symmetrical or roughly symmetrical allowing to determine spreading rates and to convincely reconstruct the positions of plates in the past. Thus, the origin of magnetic anomalies was never a real concern for the Ridge community. A few observations can be made concerning the emplacement of crustal material at mid- oceanic ridges by using constraints established in the Iberia/Newfoundland transitional crusts and at or close to mid-oceanic ridges: 1) The contribution of abyssal peridotites to marine magnetic anomalies becomes significant when they are affected by a larger than 75 per cent degree of serpentinization (NRM values larger than 5 A/m). Such highly serpentinized peridotites are characterized by Vp seimic velocities of 5-5.5 km/s and down to 1.5-2 km in the Mid-Atlantic ridge axial discontinuity at 35 °N where serpentinized peridotites have been dredged. 2) Magnetic anomalies, resembling those generated by seafloor spreading processes, have been observed at the base of several passive continental margins where mantle is exhumed as serpentinized peridotites. In the Iberia Abyssal Plain, ages of gabbroic intrusions within the exhumed mantle are identical to ages of basement obtained from modeling of magnetic anomalies as seafloor spreading anomalies. We suggest that these anomalies are caused by the magnetization of titano-magnetite grains formed during the serpentinization of peridotites, which starts at lower crustal level during mantle exhumation and continues until most of the exhumed mantle reaches the sea bottom. Thus, serpentinization and magnetization of exhumed mantle take place in time and space resembling a process of formation of seafloor spreading crust at mid-oceanic ridges formed at medium to high spreading rates. This also explains why seafloor spreading type anomalies are usually observed across slow and ultraslow spreading ridges, which mostly contain exhumed mantle material. Thus, sequences of magnetic anomalies can provide informations concerning the timing of the emplacement of crust, but generally not on its nature (oceanic crust versus exhumed mantle).
DE: 1517 Magnetic anomalies: modeling and interpretation
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005