Paleomagnetic studies at Geophysics Division, 1980-1983

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Mumme, T.C.; Walcott, R.I. 1985 Paleomagnetic studies at Geophysics Division, 1980-1983. [s.l.]: [s.n.]. Report / Geophysics Division 204 62 p.

Abstract: The remanent magnetisation of 610 cores of sedimentary rocks of Lower Neogene, Paleogene and Mesozoic age from several regions in New Zealand were measured before and after demagnetisation (both thermal and alternating field). Various storage tests on samples indicated that those stored unshielded in the laboratory acquired a viscous remanent magnetisation (VRM) parallel to the direction of the Earth's magnetic field. In general this is easily removed during AF or thermal demagnetisation. However it was apparent that many samples possessed a hard secondary magnetisation acquired at the outcrops which was difficult to remove on demagnetisation and so masked any primary magnetisation still present in these rocks. An apparent paleomagnetic reversal at a Hicks Bay locality proved to be a weathering effect. Stable primary magnetisation was recognised from only 11 of the 52 localities samples. Paleomagnetic data from eastern NZ localities gave declination anomalies of 51 degree (Rakauroa) and 99 degrees (Deadman Stream) and indicate rates of clockwise rotation of 2 and 5 degrees/Ma respectively. The higher rates in the north-eastern part of the South Island are consistent with present day rotation rates computed from geodetic data for that area. Paleomagnetic data from a limited number of western NZ localities of ages (20-40 Ma) tend to support the hypothesis of a clockwise bending of that region during the Paleogene. However primary magnetisation needs to be recognised from several more localities before reliable rates of rotation can be established. (auth)