HR: 0800h
AN: GP11B-0828    [Abstracts]
TI: Paleomagnetic Directions and 40Ar/39Ar Ages From Tatara-San Pedro Volcanic Complex, Chilean Andes: Lava Record of a Matuyama-Brunhes Precursor?
AU: * Brown, L
EM: lbrown@geo.umass.edu
AF: Dept Geosciences, Univ Massachusetts, Amherst, MA 01003
AU: Singer, B
EM: bsinger@geology.wisc.edu
AF: Dept Geology and Geophysics, Univ Wisconsin, Madison, WI 53706
AU: Pickens, J
EM: jim.c.pickens@exxonmobil.com
AF: Dept Geosciences, Univ Massachusetts, Amherst, MA 01003
AU: Jicha, B
EM: bjicha@geology.wisc.edu
AF: Dept Geology and Geophysics, Univ Wisconsin, Madison, WI 53706
AB: Lava flows in Quebrada Turbia, Tatara San Pedro Volcanic Complex, central Chile (36S, 289E), preserve a detailed record of what was presumed to be the Matuyama-Brunhes geomagnetic reversal. Sampling in the west wall of the valley yields reverse and transitional polarity flows; two km to the north, another section yields 17 transitional flows from the same sequence overlain by flows with normal polarity. 40Ar/39Ar incremental heating experiments on lavas of the two sections provide 9 independent age determinations and yield a weighted mean of 792 ka for the magnetic transition. Alternating field and thermal demagnetization, rock magnetic analyses, and petrographic observations indicate primary magnetization carried by titanomagnetite. The polarity change is characterized by a jump from reverse poles to poles clustered over Australia, followed by a jump to normal polarity latitudes. Magnetization of these lavas is consistent with either a brief period when the field was dominated by a subequatorial dipole, or a more complex non-dipolar field that may reflect the influence of a long-lived regional lower mantle control over a weakened dynamo. The Quebrada Turbia lavas are ca. 16 kyrs older than those recording the reversal at Haleakala volcano, Maui. Moreover, the 792 ka age of the Chilean lavas is older than most astronomical estimates for the Matuyama-Brunhes reversal suggesting that this section may record a precursor to the actual field reversal, seen in low paleointensities from more than a dozen marine sediment cores.
DE: 9360 South America
DE: 1035 Geochronology
DE: 1535 Reversals (process, timescale, magnetostratigraphy)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting