Directions and intensities of the Earth’s magnetic field during a reversal: results from the Permo-Triassic Siberian trap basalts, Russia

Abstract

An extensive palaeomagnetic study was carried out on an approximately 250 Ma old reversed to normal transition of the Earth’s magnetic field (EMF) recorded in 86 volcanic lava flows of the Siberian trap basalts, North Siberia, Russia. In addition to the investigation of the directional behaviour of the field (≈700 specimens) a total of 298 specimens was subjected to Thellier-type palaeointensity determinations. Adding several modifications to the original Thellier experiment, such as tests for MD tails and the additivity of partial thermoremanent magnetisation yields highly reliable palaeointensity estimates of the Late Permian/Early Triassic EMF. Transitional directions of the EMF were obtained from 20 flows. During the reversal a clustering of the virtual geomagnetic poles (VGPs) is observed (15 flows). Palaeointensity estimates suggest that this feature is not an artifact due to rapid flow emplacement since the directional cluster is associated with a well-defined increase in palaeointensity from 6 to 13 μT. Subsequently, the next VGPs move towards the pole position of normal polarity. Departing in a sudden movement from normal polarity the VGPs form a second directional cluster comprising the results of 14 flows. This feature is interpreted as a post-transitional excursion but lacks the characteristic intensity variation recorded during the first transitional cluster. The rest of the section (41 flows) is of normal polarity. The characteristic features of this reversal, low intensities and directional clustering during the reversal and an excursion shortly after the reversal, were also observed in records of polarity transitions of younger age. This suggests that the underlying reversal processes were similar. The mean virtual dipole moment calculated for the stable normal part of the studied section yields a rather low value of 2.2±0.9×10 22 Am 2. These findings confirm that the Mesozoic Dipole Low extends at least down to the Late Permian/Early Triassic. The geomagnetic VGP dispersion was calculated for the stable normal part of the section and yields values similar to those for the last 5 Ma. Considering that the intensity of the EMF was considerably higher in this time interval than in the Early Triassic, a direct relationship between intensity and secular variation seems unlikely.