Further K-Ar dating and paleomagnetic study of the Auckland geomagnetic excursions

Nobutatsu Mochizuki,Takahiro Tagami,Ian E M Smith,Hideo Tsunakawa,Hidetoshi Shibuya,Ayako Ozawa

doi:10.1186/bf03352738

Nobutatsu Mochizuki, Takahiro Tagami + Show 4 more

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https://doi.org/10.1186/bf03352738

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Abstract

Abstract Three different excursional paleomagnetic directions were reported from eight volcanoes of the Auckland volcanic field in New Zealand: north-down (ND) directions obtained from five volcanoes, west-up (WU) from two volcanoes, and south-up (SU) from one volcano. K-Ar ages have been reported for two of these volcanoes: 27±5 (1σ) ka for the Wiri volcano of the ND group and 55±5 ka for the Hampton Park volcano of the WU group. In the present study, we have carried out further K-Ar age determinations on three other volcanoes and obtained reliable ages for two of them: 30±5 ka for the Puketutu volcano of the ND group and 50±6 ka for the McLennan Hills volcano of the SU group. The age of Puketutu agrees well with that of Wiri, and these two ages give a weighted mean age of 29±3 (1σ) ka for the ND group. The age of the ND group is distinguishable from those of the SU and WU groups at the 2σ level, confirming that excursions occurred at two different times separated by a few tens of thousands of years. The age of the SU group is indistinguishable from that of the WU group, and a weighted mean age of 53±4 ka can be calculated for this combined group (SU-WU group). The age of the ND group and that of the SU-WU group are distinguishable from the latest age estimate of the Laschamp excursion. Overall, these age data from volcanic rocks show that at least three excursions occurred between approximately 30 and 60 ka. These three excursions are likely to be confined in the weak dipole interval of 20–70 ka, and all of these excursions yield particularly low virtual dipole moments (VDMs) of 2×1022 A m2 or less. Since it is suggested that the larger virtual geomagnetic pole (VGP) deviations from the geographic pole are related to the lower VDMs, the excursional fields possibly have resulted from a significantly reduced dipole field and comparable non-dipole components.

Highlights

Geomagnetic excursions are characterized by a swing of the paleomagnetic field direction that is larger than secular variation but distinct from polarity reversals
These two distinct excursions are considered to have occurred at 30 and 40 ka, respectively, a few researchers have noted that the Mono Lake excursion at the original locality may be a record of the Laschamp excursion (Kent et al, 2002; Zimmerman et al, 2006)
We have newly obtained K-Ar ages of two volcanoes recording the Auckland geomagnetic excursions

Summary

Introduction

Geomagnetic excursions are characterized by a swing of the paleomagnetic field direction that is larger than secular variation but distinct from polarity reversals. Recent geomagnetic excursions are generally identified by a virtual geomagnetic pole (VGP) departure from the geographic pole and its return to the original polarity. Mono Lake excursion identified from lake sediments in western North America (Denham and Cox, 1971), dated at approximately 30 ka (Benson et al, 2003). These two distinct excursions are considered to have occurred at 30 and 40 ka, respectively, a few researchers have noted that the Mono Lake excursion at the original locality may be a record of the Laschamp excursion (Kent et al, 2002; Zimmerman et al, 2006). A lack of paleomagnetic data, including paleointensities and radiometric ages, is the major obstacle to establishing a correlation (or making a distinction) between those young excursions and defining the geomagnetic field during the geomagnetic excursion

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