HR: 16:15h
AN: U34A-02    [Abstracts]
TI: Longitude through time
AU: * Torsvik, T H
EM: trond.torsvik@ngu.no
AF: Center for Geodynamics, NGU, Leiv Eiriksonsvei 39, Trondheim, NO-7491, Norway
AU: Steinberger, B
EM: bernhard.steinberger@ngu.no
AF: Center for Geodynamics, NGU, Leiv Eiriksonsvei 39, Trondheim, NO-7491, Norway
AU: Cocks, R L
EM: r.cocks@nhm.ac.uk
AF: The Natural History Museum, Cromwell Road, London, SW7 5BD, United Kingdom
AB: Earth scientists today have had no objective method of calculating what the palaeolongitudes of tectonic plates and other geological units were in the long eons prior to the oldest known hotspot trails, which are only of Cretaceous age (ca. 130 Ma). Before this time, palaeomagnetism is the only method by which to position plates quantitatively on the globe. Palaeomagnetic studies only directly yield latitudes and plate rotations, but the longitude uncertainty can be minimized by selecting an appropriate reference plate: if one can determine which plate has moved least, then it should be used as the reference plate. Africa has been nearly surrounded by mid- ocean ridges since the break-up of Pangea, and thus the ridge push forces should have roughly cancelled each other out. Moving hotspot-based plate motion models show minimal longitudinal motion for Africa (<10 degrees) for the past 130 million years, confirming the lack of significant longitudinal motion inferred from consideration of the plate driving forces. It is uncertain whether the 'zero-longitude' assumption about Africa holds before Pangea's break-up, but in the absence of better reference points, we have regarded zero longitudinal average motion for Africa as the best assumption. With this approach we have been able to demonstrate that virtually all Large Igneous Province (LIPs) for the last 300 million years project radially onto the edges of the African and Pacific Large Low Shear Velocity Province (LLSVPs) near the core-mantle-boundary (CMB). The LIPs must for this reason be derived from mantle plumes, and CMB heterogeneities must have remained quite stationary since the formation of Pangea. LIPs have erupted since Archean times and there is no reason to preclude that they were all derived from LLSVPs in the deep mantle. That inspired us to consider whether older LIP events would yield similar results. We attempt to reconstruct Gondwana in longitude in Cambrian times based on the substantial Antrim plateau volcanics (Australia), a LIP of ca. 510 Ma age along the Gondwanan margin. If the LIP was formed at the margin of the Africa or Pacific LLSVPs and they have remained the same throughout Earth's history there are six possible marginal sites on the CMB from which to choose, but three sites that do not position the long- lived subduction margin of Gondwana (e.g. South America, East Antarctica and East Australia) above regions of high seismic velocity (the subduction graveyards) can be eliminated. If, as recently postulated, there have only been one LLSVP (or upwelling zone) in Pre-Pangean time (Pacific LLSVP) that reduces longitude choices to two possible marginal sites on the CMB.
DE: 1527 Paleomagnetism applied to geologic processes
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8121 Dynamics: convection currents, and mantle plumes
DE: 8137 Hotspots, large igneous provinces, and flood basalt volcanism
DE: 8157 Plate motions: past (3040)
SC: Union [U]
MN: 2007 Fall Meeting