New Late Eocene pole for Australia, time‐averaging of remanence directions, and palaeogeographic reference systems

SUMMARY A new Late Eocene (˜36.5 Ma) South palaeomagnetic pole based, on 33 sampling levels in the Browns Greek Formation of the Otway Basin, south‐eastern Australia, lies at 65.5°S, 112.5°E with A 95 = 2.5°. This pole fills a 30 Myr gap in the Palaeogene part of the Australian apparent polar wander pole path, and appears well suited for calibrating the path because of a well‐defined age and evidence that the sequence was magnetized during a depositional period long enough to thoroughly average out secular variations. The remanence seems to have been acquired during postdepositional re‐working of the sediment. The pole agrees with one biostratigraphically dated and three undated poles that constrain the Palaeogene trajectory. All Palaeogene poles on the path lie west of an alternative apparent polar wander path. The latter is modelled by low‐order curve fitting of much more widely scattered poles from basalts in which the scatter appears to be due to incomplete time‐averaging of remanence directions. The new pole also reinforces a previously observed systematic westward displacement, by a few degrees, of the early Tertiary palaeomagnetic poles relative to hotspot‐derived poles. This displacement may be caused by true polar wander and/or departures from the geocentric axial dipole model. It again highlights the approximateness of the available palaeogeographic reference systems.