HR: 16:35h
AN: GP44A-02 INVITED     [Abstracts]
TI: Full Vector Analyses of the Short-Term Behavior Recorded in Long Volcanic Sequences in Hawaii, USA.
AU: * Herrero-Bervera, E
EM: herrero@soest.hawaii.edu
AF: SOEST-HIGP, Paleomagnetics and Petrofabrics Laboratory, University of Hawaii at Manoa, 1680 East West Rd., Honolulu, HI 96822, United States
AB: The Hawaiian volcanoes, in principle, offer the opportunity of observing the geomagnetic field behavior from present back to 5.72 Ma (from the Big Island of Hawaii to the island of Kauai). Thus, new paleomagnetic measurements coupled with radioisotopic dating are revolutionizing our understanding of the geodynamo by providing terrestrial lava records of the short-term behavior of the paleomagnetic field. As part of our investigations of some of these Hawaiian volcanoes, we have sampled long volcanic sequences of the Waianae, Koolau (island of O'ahu) and Mauna Loa (Big Island of Hawaii) volcanoes. These volcanic edifices have collapsed in the past leaving highly dissected lava sequences ideal for paleomagnetic sampling. We have sampled and studied in detail the directional characteristics and their respective absolute paleointensities of three successive reversals, namely, Gilbert-Gauss, Lower and Upper Mammoth polarity transitions recorded on Waianae lavas, Cryptochron C2r.2r-1 (ca. 2.514 +/- 0.030 Ma) as well as the Kaena Subchron recorded in the Koolau Volcano and the Laschamp and Pringle Falls excursions recorded on lavas from the Mauna Loa volcano (Big Island of Hawaii). The records of the three successive Gilbert-Gauss, Lower and Upper Mammoth reversals confirm that large oscillations of directions precede or follow the reversals, which reminds waveforms typical of paleosecular variation with their amplitude being considerably amplified by the decrease of the dipole. Determinations of absolute paleointensity were attempted on more than 750 samples. Special care was taken at selecting data obtained from segments covering more than 50% of the remanent magnetization of the samples. This procedure limited the success rate to 13% for the Waianae lavas, 70% for the Cryptochron flows and 25% for the Kaena samples but provided consistent and reliable paleointensities. In addition to other time intervals, the results document the field variations surrounding the five excursions and polarity transitions. For the 0.4 Ma record of the Waianae sections a period of weak field dominated before the reversal, then the transition was initiated by a transit from normal to reverse polarity and followed by a short restoration of field intensity in reverse polarity. A second episode of very weak field was accompanied by a return to positive inclinations before reaching the reverse polarity. The very strong and apparent rapid recovery of the dipole following completion of the reversal culminated at a value of 16 x 10²² Am² similarly to field intensities reported for the other detailed volcanic records of reversals studied so far. The asymmetry between the pre- and the post-reversal phases appears as a dominant characteristic and indicates the importance of field regeneration to initiate a new stable polarity interval. All transitional VGP's lie within the longitudinal bands of America, central Africa, western Europe and eastern Asia. This distribution does not support the hypothesis of a direct link with heterogeneities of the lower mantle underneath Americas and eastern Asia. Clusters of VGP's are observed in most records at various geographical locations without preference for specific longitudes, which in the present case most likely seems to result from intense volcanism during short time periods rather than from specific transitional states.
DE: 1513 Geomagnetic excursions
DE: 1520 Magnetostratigraphy
DE: 1521 Paleointensity
DE: 1522 Paleomagnetic secular variation
DE: 1535 Reversals: process, timescale, magnetostratigraphy
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Joint Assembly