Paleomagnetic secular variation of the Honolulu Volcanic Series (33–700 ka), O’ahu (Hawaii)

Abstract

Abstract Thirty years after the precursory study, [J. Geophys. Res. 13 (1972) 2129] 19 dated sites with ages between 33 and 700 ka from the Honolulu Volcanic Series (HVS; southeastern part of the island of O’ahu in Hawaii) have been revisited. A minimum of 16 samples per site have been studied using alternating field and thermal demagnetization. Both techniques yielded similar results. Coeval flows could be identified by relying on their KAr ages, similar rock magnetic parameters and identical directions. Reliable paleomagnetic directions have been obtained for all sites. Two site mean directions which deviate significantly from the geocentric axial dipole (GAD) correspond to periods of low dipole intensity and may thus reflect a larger proportion of non-dipole components. The average value of the site mean declinations is 358.6° while the mean inclination of 34.5° is 3.5° lower than the value of the geocentric dipole at the site. The north geographic pole is largely contained within the uncertainties surrounding the position of the mean virtual geomagnetic poles (VGP) latitude. The deviation of 2.3° in latitude is further reduced by 1° if we consider the existence of a standing small quadrupole component that would represent 5% of the dipole. The angular dispersion of the virtual geomagnetic poles (12°) is only 1.7° smaller than expected at the site latitude and thus not a firm indication of low secular variation. Thus, there is no significant difference with the global time-averaged field for the same period. Other recent results from the same island for an older time period display similar characteristics, but with reverse and normal polarities that are not perfectly antipodal. It is suggested that this could result from changes in the mean dipole field intensity during these time intervals. This points out to the importance of scrutinizing the entire field vector in any analysis of paleosecular changes. These results do not bring any evidence for a persistent field anomaly under Hawaii.