HR: 1340h
AN: GP43A-0923    [Abstracts]
TI: The 2.7 Ga Pilbara Drilling Project, Western Australia: Paleomagnetic Results
AU: * Poitou, C
EM: poitou@ipgp.jussieu.fr
AF: Equipe de paleomagnetisme, Institut de Physique du Globe de Paris, 4 place Jussieu, Paris Cedex 05, 75252, France
AU: Besse, J
EM: besse@ipgp.jussieu.fr
AF: Equipe de paleomagnetisme, Institut de Physique du Globe de Paris, 4 place Jussieu, Paris Cedex 05, 75252, France
AU: Valet, J
EM: valet@ipgp.jussieu.fr
AF: Equipe de paleomagnetisme, Institut de Physique du Globe de Paris, 4 place Jussieu, Paris Cedex 05, 75252, France
AU: Philippot, P
EM: philippo@ipgp.jussieu.fr
AF: Equipe geobiosphere actuelle et primitive, Institut de Physique du Globe de Paris, 4 places Jussieu, Paris Cedex 05, 75252, France
AB: The Pilbara Drilling Project diamond drillhole intersects subaerial basaltic flows (Maddina Formation, 2715 Ma), stromatolitic carbonates, interbedded black shale (Meentheena Member, Tumbiana Formation) and underlying volcaniclastic sandstones (2719 Ma). Three to four 15 mm diameter and 13 mm long tiny cylindrical specimens were sampled every metre downcore and subjected to thermal and alternating field demagnetization. The demagnetization results of the upper Maddina basalts (0-40 m depth) revealed one or two distinct magnetization components. The high temperature component was observed for more than 50% of the samples and could not be isolated before 550°C. It shows normal polarity and is always almost antiparallel to the low temperature component of reverse polarity. Alternating field demagnetization displayed similar characteristics with opposite directions for the low reverse and high normal coercivity components. We thus consider that the high unblocking temperature and high coercivity grains carry the same normal characteristic component. In contrast to the basaltic flows, the Tuffaceous materials (98-104m depth) show a unique component of reverse polarity which could either reflect thermo-chemico-viscous overprint or an actual reverse polarity before emplacement of the basaltic flows. Paleomagnetic studies on interbedded sedimentary materials and complementary rock magnetic experiments are under way to discriminate between the two hypotheses. If the second scenario had to be retained we would have identified the second oldest known reversal of the Earth's magnetic field (first one was presented in Strik et al., doi:10.1029\slash2003JB002475, 2003). Other implications of the present study would concern the characteristics of the paleosecular variation during Precambrian time but also lateral drift of continental plates. This would also constrain the apparent polar wander path of the Pilbara Craton and the origin of plate velocity which is suggested to be 10 times faster than now.
DE: 1520 Magnetostratigraphy
DE: 1525 Paleomagnetism applied to tectonics: regional, global
DE: 1533 Remagnetization
DE: 1535 Reversals: process, timescale, magnetostratigraphy
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting