Earth Planets Space, Vol. 59 (No. 7), pp. 755-761, 2007
Nobutatsu Mochizuki1, Hideo Tsunakawa1, Hidetoshi Shibuya2, Takahiro Tagami3, Ayako Ozawa1, and Ian E. M. Smith4
1Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8551, Japan
2Department of Earth Sciences, Kumamoto University, Kurokami 2-39-1, Kumamoto 860-8555, Japan
3Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
4School of Geography, Geology and Environmental Sciences, University of Auckland, P.B. Box 92019, Auckland, New Zealand
(Received October 31, 2006; Revised February 12, 2007; Accepted February 17, 2007; Online published July 20, 2007)
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.
Key words: K-Ar age, paleointensity, Auckland volcanic field, Auckland excursion, Laschamp excursion, Mono Lake excursion.