Paleomagnetism and magnetostratigraphy of Miocene volcanics in eastern Otago and Banks Peninsula, New Zealand

Abstract

Orientated samples were collected from 135 sites in 4 centres of Miocene alkali volcanic activity on the South Island of New Zealand: the Dunedin volcano, the Lyttelton volcano, the Akaroa volcano, and the Diamond Harbour Group. The natural remanent magnetisation (NRM) was measured using stepwise alternating field demagnetisation. From the results of at least three specimens a mean site NRM direction was calculated where possible. For each of the four centres a magnetostratigraphy has been determined. By comparing this with previously published K-Ar ages it has been possible to suggest correlations with the magnetic polarity timescale. The mean virtual geomagnetic poles (VGP) for the four centres, oldest to youngest, are: Dunedin volcano 86°.6N, 357°.0E; Lyttelton volcano 87°.7N, 086°.3E; Akaroa volcano 79°.7N, 137°.6E; and Diamond Harbour Group 68°.6N, 012°. 1E. Even after rotation of the VGPs back to their original positions, using hotspot-relative rotations, the two youngest VGPs deviate significantly from the geocentric axial dipole position. The irregular nature of volcanic activity has apparently caused a bias towards a particular direction.