A Refined ~4.5 Ma Paleomagnetic Pole From Basaltic Lava
Flow Sequences on Kauai, Hawaii
Abstract
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.