The Steens Mountain (Oregon) geomagnetic polarity transition: 1. Directional history, duration of episodes, and rock magnetism

Abstract

The thick sequence of Miocene lava flows exposed on Steens Mountain in southeastern Oregon is well known for containing a detailed record of a reversed‐to‐normal geomagnetic polarity transition. Paleomagnetic samples were obtained from the sequence for a combined study of the directional and intensity variations recorded; the paleointensity study is reported in a companion paper. This effort has resulted in the first detailed history of total geomagnetic field behavior during a reversal of polarity. A comparison of the directional variation history of the reversed and normal polarity intervals on either side of the transition with the Holocene record has allowed an estimate of the duration of these periods to be made. These time estimates were then used to calculate accumulation rates for the volcanic sequence and thereby provide a means for estimating time periods within the transition itself. The polarity transition was found to consist of two phases, each with quite different characteristics. At the onset of the first phase, a one‐third decrease in magnetic field intensity may have preceded the first intermediate field directions by about 600 years. Changes in field direction were confined near the local north‐south vertical plane when the actual reversal in direction occurred and normal polarity directions may have been attained within 550±150 years. The end of the first phase of the transition was marked by a brief (possibly 100–300 years) period with normal polarity and a pretransitional intensity which suggests a quasi‐normal dipole field structure existed during this interval. The second phase of the transition was characterized by a return to very low field intensities with the changes in direction describing a long counterclockwise loop in contrast to the earlier narrowly constrained changes. This second phase lasted 2900±300 years, and both normal directions and intensities were recovered at the same time. Both directional and intensity data document very erratic geomagnetic field behavior during the polarity transition. Changes in magnetic field direction were variable and occurred either (1) in a regular, progressive manner, (2) with sudden, extremely rapid angular changes (58°±21°/year), or (3) with little or no movement for periods of the order of 600±200 years. Changes in magnetic intensity occurred in a like manner and were sometimes correlated with changes in direction, but during other periods both directional and intensity changes occurred independently. Directional changes following the polarity transition occurred in a seemingly normal manner, although intensity fluctuations attest to some instability of the newly reestablished dipole.

A reversal of the Earth's magnetic field recorded in mid‐Miocene lava flows of Gran Canaria: Paleodirections

“…Assuming that the secular variation rate during the mid-Miocene was similar to that during the Holocene and that there are no major time gaps in the El Paso section, we can use the cumulative angular VGP path and the average angular change of the VGP for southern Europe (5°/100 years according to Mankinen et al [1985]) for the calculation of the time interval and the accumulation rate of the lavas. This procedure was conducted for several directional intervals (Table 2).…”

“…C-TP1 to C-TP6 R 24.7 4.9 3.2 500 C-TP10 to C-TP24 R 123.4 10.3 1.3 2500 C-TP29 to C-TP42 R 140.4 11.7 1.7 2800 C-TP48 to C-TP61 I 86.4 7.9 2.3 1730 C-TP82 to C-TP89 N 41.2 5.9 4.5 820 a Accumulation rates of lavas and covered time intervals of five directional periods are during the EL Paso sequence. The accumulation rates were calculated using the Holocene angular change rate of the VGP track for southern Europe [Mankinen et al, 1985]. Additionally, the cumulative VGP path and the average path lenght between individual flows are given.…”

“…[21] Another possibilty for estimating the time interval of the succession and the polarity transition is the usage of the Holocene average angular change of the VGP track [Mankinen et al, 1985]. Assuming that the secular variation rate during the mid-Miocene was similar to that during the Holocene and that there are no major time gaps in the El Paso section, we can use the cumulative angular VGP path and the average angular change of the VGP for southern Europe (5°/100 years according to Mankinen et al [1985]) for the calculation of the time interval and the accumulation rate of the lavas.…”

K-Ar Ages and Paleosecular Variation of Direction and Intensity from Quaternary Lava Sequences in the Ruapehu Volcano, New Zealand.

“…Such paleomagnetic records from lava sequences are usually very reliable owing to the stable nature of the remanence of volcanic rocks, and absolute paleointensities can often be recovered also. Good examples are the studies of the transitional field as illustrated in the studies of the Steens Mountain lavas in Oregon (Mankinen et al, 1985). Even if the lava accumulation rate is low and random, the dataset may still give a good opportunity to study the statistical features of the geomagnetic field as shown in the studies of the Icelandic lavas (e.g., Kristjansson, 1985).…”

Paleomagnetic directional groups and paleointensity from the flood basalt in the Tarim large igneous province: implications for eruption frequency

“…Paleomagnetic secular variation (PSV) analysis has been employed to constrain the eruption frequency down to the millennium or century timescale, and to identify the eruption pulses in some LIPs (Makinen et al 1985;Riisager et al 2003;Knight et al 2004;Chenet et al 2008;Pavlov et al 2011;Courtillot and Fluteau 2014). The geomagnetic field exhibits secular variation, both in direction and intensity, in timescales of decades to a century.…”