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A Refined ~4.5 Ma Paleomagnetic Pole From Basaltic Lava Flow Sequences on Kauai, Hawaii
  • Scott Bogue,
  • Yiming Zhang
Scott Bogue
Occidental College

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Yiming Zhang
University of California Berkeley
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Laboratory reanalysis of paleomagnetic samples from two sets of superposed ~ 4.5 m.y. old lava flows (N=64) collected on Kauai (Hawaii) in the 1960s, when combined with more recently published results from two additional flow sequences (N=59) on the island, yields a mean paleomagnetic pole that differs significantly from the spin axis. The new laboratory work was motivated by the fact the 60% of the published flow mean directions on the old collection derived from measurements of natural remanent magnetization without any demagnetization treatment. In our reanalysis, we subjected all specimens to stepwise alternating field (AF) demagnetization (13 steps; 90 mT peak AF) and modern line-fit analysis. As has been found in many restudies of this sort on young, well-behaved basaltic lavas, modern demagnetization technique produced little change in the flow-mean directions (an average change of just over 2° in this case.) To minimize the bias introduced when rapidly erupted flow sequences sample a single geomagnetic field direction, we thinned the combined data by grouping superposed flows with similar paleomagnetic directions. The procedure requires choosing a minimum angle between directions that will be considered independent samples of the ancient geomagnetic field. Previous studies have commonly used a 6° threshold. We found, however, that much larger values (10° to 16°) were needed to eliminate significant serial correlation (measured in several ways) in the thinned directional sequence. Our conclusion is that the 123 lava flows in this study provide only 33 independent estimate of the ancient field direction. The mean normal polarity paleomagnetic pole resulting from the analysis (Lon=076.8, Lat=82.2, N=13) is almost perfectly antiparallel (179°) to the mean reverse polarity pole (Lon=258.4, Lat=-83.2, N=20). The grand mean of 33 normal and reverse polarity data is Lon=077.3 Lat=82.8 k=31.8 α95=4.5°. The VGP dispersion (without correction for within-site dispersion, of order 0.5° for these very well-behaved paleomagnetic recorders) is 14.3° about the mean pole and 15.9° about the spin axis, comparable to that found in other analyses of Hawaiian paleomagnetic data.