Geomagnetic fluctuations during a polarity transition

Abstract

Abstract. The extensive Roza Member of the Columbia River Basalt Group (Washington State) has intermediate paleomagnetic directions, bracketed by underlying normal and overlying reverse polarity flows. A consistent paleomagnetic direction was measured at 11 widely distributed outcrops; the average direction has a declination of 189 ø and an inclination of-5 ø, with greater variation in the inclination [Rietman, 1966]. In this study the Roza Member was sampled in two Pasco Basin drillcores, where it is a single cooling unit and its thickness exceeds 50 m. Excellent core recovery allowed uniform and dense sampling of the drillcores. During its protracted cooling, the Roza flow in the drillcores recorded part of a 15.5 Ma geomagnetic polarity transition. The inclination has symmetric, quasi-cyclic intraflow variation, while the declination is nearly constant, consistent with the results from the outcrops. Thermal models of the cooling flow provide the timing for remanence acquisition. The inclination is inferred to have progressed from 0 ø to -15 ø and back to -3øover a period of 15 to 60 years, at rates of 1.6 ø to 0.5ø/yr. Because the geomagnetic intensity was probably weak during the transition, these apparently high rates of change are not significantly different from present-day secular variation. These results agree with the hypothesis that normal secular variation persists through geomagnetic transitions. The lowamplitude quasi-cyclical fluctuations of the field over tens of years, recorded by Roza, suggest that the geomagnetic field reverses in discrete steps, and that more than 15-60 years were required to complete this reversal.

Palaeomagnetic secular variation as a function of intensity

“…In other words, angular secular variation is apparently inversely related to local eld strength. Insofar as polarity transitions are coincident with periods of weak eld intensity, our conclusion here is qualitatively consistent with that of Love (2000), who inspected a large number of directional lava data, with Aundunsson & Levi (1997), who examined a thick layer of slowly cooling basalt, and with (Valet et al . 1986), who examined sedimentary data.…”

Mono Lake or Laschamp geomagnetic event recorded from lava flows in Amsterdam Island (southeastern Indian Ocean)

“…In order to check whether the principal component was a robust estimate of the sample ChRM, we compared this direction with the fitting line constrained through the origin. When the angle between these two directions exceeds the MAD (Audunsson & Levi 1997) we concluded that the principal component was statistically different from the ChRM, and thus that the ChRM was not perfectly isolated. This method led to the rejection of only one sample (693) from further analysis, considering that no ChRM could be successfully determined for this sample.…”

Igneous Rock Associations 15. The Columbia River Basalt Group: A Flood Basalt Province in the Pacific Northwest, USA

“…Rietman 1966;Kienle 1971;Coe et al 1978;Choiniere and Swanson 1979;Magill et al 1982;Reidel et al 1984;Beeson et al 1985;USDOE 1988;Wells et al 1989;Hagstrum et al 2010). Some flows like the Roza even record a complete reversal in the magnetic field (Audunsson and Levi 1997) providing a constraint on the time it takes for a magnetic reversal to occur. Field determinations of the paleomagnetic polarity using a portable fluxgate magnetometer have proven to be an important criterion for identification and mapping flows, but these results must be viewed with caution.…”