Relative Paleointensity Records From Mafic Lava Sequences Erupted in
Kauai (Hawaii) and North-central Nevada During Four Geomagnetic Polarity
Reversals
Abstract
New relative paleointensity results from mafic lava flows erupted in
quick succession during four Neogene geomagnetic reversals show that
volcanic transition zones can provide a favorable setting for the
pseudo-Thellier technique. The most compelling data are from a
normal-to-reverse (N-R) transition zone in the 4 my old basaltic lava
flows of the Olokele Formation on Kauai (Hawaii). Following stepwise
alternating field (AF) demagnetization of each sample’s natural remanent
magnetization, an anhysteretic remanent magnetization (ARM) was imparted
in a 50 uT bias field and then AF demagnetized. Very similar behavior of
the 35 mT to 90 mT coercivity fractions suggest that the NRM
(interpreted to be a primary thermoremanent magnetization (TRM)) and ARM
reside in the same subpopulation of magnetic grains. With the ARM
intensity serving as an analog for TRM susceptibility, the 35 mT-90 mT
TRM/ARM ratios provide a relative measure of the ancient field
intensity. Good correlation of flow-mean TRM/ARM ratios with a small set
of double-heating paleointensities from companion samples allows an
approximate scaling of the TRM/ARM ratios to absolute intensity. As the
geomagnetic polarity switched, the field strength apparently dropped by
91% compared to the maximum recorded by the flows. This decrease is
comparable to that observed in other paleomagnetic studies and numerical
simulations. During an interval of NE-down directional stasis recorded
by a succession of 10 flows in the lower half of the section, the
geomagnetic intensity rose to over 20 uT and then decreased to near 5
uT. This distinctive behavior could mark the waxing and waning of a
stationary, inward-directed flux patch on the core mantle boundary
centered NE of Kauai. Within-flow variability of relative paleointensity
was higher in two other Pliocene transition zones on Kauai (both R-N),
and in the R-N transition recorded by 15.2 my old lavas in the Sheep
Creek Range (Nevada). Nevertheless, all three records document a
reduction in the ancient geomagnetic field intensity similar in
magnitude to that seen in the Olokele flows. The relative uniformity of
magnetic mineralogy may help explain why the pseudo-Thellier technique
was able to capture the large transitional intensity changes recorded by
these rapidly-erupted flow sequences.