Evidence for the existence of a transitional geomagnetic field recorded in a Proterozoic lava succession

Abstract

SUMMARY An anomalous palaeomagnetic field direction is recognized in a sequence of rapidly extruded subaerial basaltic lava flows within the Mid-Proterozoic (c. 1300 Ma) Gardar lava succession, South Greenland. Thermal demagnetization and component analysis link this direction to a thermoremanent magnetization (TRM) acquired during primary cooling. The direction, which bears no resemblance to any other palaeomagnetic direction witnessed within the Group, is recorded by seven successive flows which appear to have been extruded and magnetized over a short period of time during a geomagnetic polarity transition. Palaeointensity experiments employing the modified Thellier technique indicate that this transitional direction was acquired in a field with only 25 per cent of the magnitude of the stable polarity field. The behaviour of the field direction during and after this transition reflects that witnessed in studies of much younger transitional sequences, whereas that for the field intensity differs fundamentally from consistent observations from younger lava successions. These results provide a significant contribution to our knowledge of the ancient geodynamo and may well document the oldest transitional field identified by a combined palaeomagnetic and palaeointensity study and the only one reported from the Precambrian era.