Persistent anomalous inclinations recorded in the Koolau volcanic series on the island of Oahu (Hawaii, USA) between 1.8 and 2.6 Ma

Ten sites of basaltic lava flows from the Koolau series (island of Oahu, Hawaii) were selected to improve the determination of the Matuyama time-averaged geomagnetic field between 1.8 and 2.6 Ma. Identical primary directions with reverse polarity carried by magnetite were isolated by alternating-field and thermal demagnetization in 13–16 specimens from each site. The final mean direction points south while the inclination is 7° lower than the inclination of the geocentric axial dipole (GAD). This offset is reduced after correcting the site latitude for the Pacific plate motion, which has been neglected so far in most studies. The mean corrected direction is almost perfectly antipodal to the Brunhes mean direction derived from the Honolulu series in the same island. Despite this correction, the inclination remains 3.6° larger than the inclination of the axial dipole and cannot be accounted for by a small axial quadrupole which would reverse with the dipole. The mean inclinations of other Hawaiian records (mostly younger than 1 Ma) involving long sequences of lava flows exhibit similar deviations from the GAD which are too large to be accounted for by a small axial quadropole. Tilting of all sections is difficult to defend for so many localities and persistant secondary components have not been detected. Thus these inclination anomalies would reflect the presence of a long-term standing component under Hawaii but crustal magnetization linked to the volcanic edifice cannot be completely ruled out.