HR: 0800h
AN: GP41E-01    [Abstracts]
TI: Matuyama-Brunhes reversal on Tjörnes Peninsula (Iceland) revisited
AU: * Camps, P C
EM: camps@gm.univ-montp2.fr
AF: Gésciences Montpellier, CNRS and Université Montpellier 2, cc 060, Montpellier, 34095, France
AU: Singer, B S
EM: bsinger@geology.wisc.edu
AF: Department of Geology and Geophysics, University of Wisconsin-Madison, Madison, WI 53018, United States
AU: Allen, B
EM: brianallen@wisc.edu
AF: Department of Geology and Geophysics, University of Wisconsin-Madison, Madison, WI 53018, United States
AU: Carvallo, C
EM: carvallo@impmc.jussieu.fr
AF: Institut de Minéralogie et de Physique de la matière condensée, Université Pierre et Marie Curie, Paris, 75015, France
AU: Goguitchaichvili, A
EM: avto@geofisica.unam.mx
AF: Dept de paleomagnetismo y geofisica nuclear, UNAM-Campus Morelia, Morelia, Mexico
AB: We report paleomagnetic investigations conducted on four overlapping stratigraphic sections (55 flows) through the lava pile in Tjornes peninsula (North Iceland). The purpose of the present study is to check the presence of intermediate directions previously interpreted as belonging to the Matuyama-Brunhes geomagnetic reversal, completing this record with Thellier-Thellier paleointensity determinations and ⁴⁰Ar/³⁹Ar radiometric dating. The directional results corroborate the findings by Kristjansson et al. (1988): a sequence of R- T-N polarity is found on each section. The polarity change is characterized by a jump from reverse VGPs to a quasi-cluster of transitional VGPs located over China, followed by a second jump to normal polarity. Reverse polarity flows present a magnetic mineralogy well suited for paleointensity determination. Of the 25 reverse flows, 20 yielded good quality paleointensity determinations. Unfortunately, the systematic presence of coarse multidomain (MD) titanomagnetite in the transitional and normal polarity flows did not enable us to obtain reliable paleointensities for none of these flows. The VDM calculated for the reverse polarity flows vary from 3.8 to 6.9 10²² Am² with an arithmetic mean value of 5.5±0.8 10²² Am². This value is identical to the mean VDM obtained for the 0.3-5 Ma time window and thus strengthens the conclusion that the recent geomagnetic field strength is anomalously high compared to that older than 0.3 Ma. ⁴⁰Ar/³⁹Ar isochron ages were obtained using the incremental heating technique on groundmass separates or phenocryst poor whole rock samples. Six of the transitionally magnetized lavas yielded isochron ages that are indistinguishable from one another at the 95% confidence level. The weighted average age is 862 ± 51 ka. This finding seems to support the hypothesis that a geomagnetic excursion has been recorded in Tjornes lava flows prior to the Matuyama-Brunhes reversal rather than the actual reversal.
DE: 1120 Isotopic disequilibrium dating
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 1513 Geomagnetic excursions
DE: 1521 Paleointensity
DE: 1535 Reversals: process, timescale, magnetostratigraphy
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Joint Assembly