HR: 0830h
AN: GP31D-0775    [PDF]
TI: Detailed Directional Analyses of the Reunion Subchron (?) Recorded in the Koolau Volcanic Series, Oahu, Hawaii
AU: * Browne, E J
EM: browneej@hawaii.rr.com
AF: SOEST-HIGP, Paleomagnetics and Petrofabrics Laboratory, University of Hawaii, 1680 East West Rd., Honolulu, HI 96822 United States
AU: Herrero-Bervera, E
EM: herrero@soest.hawaii.edu
AF: SOEST-HIGP, Paleomagnetics and Petrofabrics Laboratory, University of Hawaii, 1680 East West Rd., Honolulu, HI 96822 United States
AB: In order to investigate the volcanic evolution of the Koolau Volcano (one of the volcanoes that make up the island of Oahu) and the short-term behavior of the geomagnetic field, we have sampled a long volcanic section located on the butressed flank of the Koolau Volcano within the Halawa Valley. Prior paleomagnetic and radiometric investigations of the Koolau Volcano have reported excursional directions within the Koolau Volcanic Series (Site F of Doell and Dalrymple, 1973). The composition of the (100 m) thick sequence of lava flows, easy access, and close geographical proximity to dated flows made these excellent candidates for a detailed paleomagnetic analysis. The flows sampled were bounded between the current K-Ar dates obtained from adjacent lava flows (1.9 ñ 0.1 Ma to 2.2 ñ 0.1 Ma). At least eight samples were taken of each analyzed flow. Specimens of each sample were stepwise demagnetized by both alternating field (5mT to 100mT) and thermal (from 28$^{o}$C to 575-650$^{o}$C) methods, and the mean directions were obtained by principal component analysis. All samples have indicated a strong and stable ChRM trending towards the origin based on no less than seven to nine steps, with thermal and AF results agreeing to a very high degree. Low field susceptibility versus temperature (k-T) analyses were conducted for individual lava flows, and the majority of them show reversible curves. Curie point determinations revealed a temperature close to or equal to 580$^{o}$C, indicative of almost pure magnetite for most of the flows. The mean directions of magnetization of the entire section sampled indicate that about 10 m of the section are characterized by excursional directions. The calculated corresponding VGPs are located at equatorial latitudes and on the central and south part of Africa with a few of the other ones close to the west part of Australia. These excursional directions place important constraints to the entire volcanic evolution of the Koolau edifice since they might correspond to the Reunion Subchron according to the reported K-Ar age determinations.
DE: 1513 Geomagnetic excursions
DE: 1520 Magnetostratigraphy
DE: 1522 Paleomagnetic secular variation
DE: 1530 Rapid time variations
DE: 1535 Reversals (process, timescale, magnetostratigraphy)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting