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Relative Paleointensity Records From Mafic Lava Sequences Erupted in Kauai (Hawaii) and North-central Nevada During Four Geomagnetic Polarity Reversals
  • Scott Bogue,
  • Yiming Zhang
Scott Bogue
Occidental College

Corresponding Author:[email protected]

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Yiming Zhang
Occidental College
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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.