HR: 1330h
AN: GP13B-10    [Abstracts]
TI: Early Miocene R-N Reversal Recorded in Lavas From Queensland, Australia: Evidence for Stationary Mantle-held Flux Over the Past 21 Ma
AU: * Hoffman, K A
EM: khoffman@calpoly.edu
AF: Cal Poly State University, Physics Department, San Luis Obispo, CA 93407 United States
AU: Camps, P
EM: Pierre.Camps@dstu.univ-montp2.fr
AF: Université Montpellier II, Lab Tectonophysique, ISTEEM, Case 049, Cedex 5, Montpellier, France
AU: Hansen, L N
EM: larhansen@hotmail.com
AF: Cal Poly State University, Physics Department, San Luis Obispo, CA 93407 United States
AU: Johnson, K
EM: kjohns03@calpoly.edu
AF: Cal Poly State University, Physics Department, San Luis Obispo, CA 93407 United States
AU: Carvallo, C
EM: ccar@soc.soton.ac.uk
AF: University of Southampton, School of Ocean and Earth Science, Southampton, SO143ZH United Kingdom
AB: The contention of a long-lived transitional field VGP cluster patch near Australia suggests that mantle-held flux at the core surface below this region may be largely responsible. To shed more light on this claim we have been investigating transitional field records from Australasia, the region closest to such a flux concentration and, hence, likely to be most affected by it during field reversal. Here we present paleodirectional data associated with a late Miocene (about 21 Ma) reverse-to-normal polarity transition recorded in a continuous sequence of some 30 flows in southeastern Queensland. The lavas record magnetic remanence that is either remarkably stable to demagnetization by alternating field, or displays unidirectional behavior with little or no viscous overprint. FORC diagrams confirm the presence of a high coercivity component, most likely hematite, in all samples studied. There is strong microscopic evidence that the high coercivity component was produced by the oxidation of ilmenite. This oxidation is one that occurs at high temperature. Samples which contain a significant fraction of titanomagnetite render transitional vector directions wholly compatible with those found in the most stable samples, some of which display almost no titanomagnetite grains under microscopic examination. Hence, the natural remanence can be considered to have been entirely recorded during the initial cooling of the lavas. The R-N transitional VGP path is dominated by two clusters, the first, off the west coast of Australia; the second, within Siberia. Each cluster is associated with several sequential lavas with no recorded movement of the VGP between them. The geographical location of each of these clusters correlates with a vertical field concentration as seen at Earth's surface in the modern-day field after removal of the axial dipole term (i.e. the NAD-field). This finding provides evidence that the lower-most mantle has kept the associated flux concentrations essentially stationary for at least the past 21 Ma. In contrast, the published Liverpool Volcano Oligocene (about 34 Ma) R-N reversal, recorded in detail by lavas in nearby New South Wales some 13 Myr prior to the Queensland record, is dominated by two Southern Hemisphere VGP clusters, each located to the east of a modern-day NAD-field flux concentration. Such a comparison raises the question as to whether mantle-held flux features at the top of the outer core may intermittently drift relative to Earth's surface.
DE: 1507 Core processes (8115)
DE: 1519 Magnetic mineralogy and petrology
DE: 1535 Reversals (process, timescale, magnetostratigraphy)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2005 Joint Assembly