Does the Permo-Triassic geomagnetic dipole low exist?
Résumé
Studies of absolute paleointensity have been conducted on specimens from Permo-Triassic sills from the eastern (areas of the kimberlite pipes Sytikanskaya, Yubileinaya and Aikhal) and northwestern (extrusions near Noril'sk city) parts of the Siberian platform. A total of 341 specimens have been subjected to a modified Thellier-Thellier technique which included partial thermoremanent magnetization checks and multidomain tail checks. The paleomagnetic directions derived from the present set of experiments (D = 107.5°, I = 75.5°, k = 82.8, α95 = 5.3°, N = 10 sills, for the eastern localities and D = 95.1°, I = 71.8°, k = 35.2, α95 = 15.7°, N = 4 outcrops, for the northwestern locality) agree with the directions from previous studies. The specimens from the northwestern sites (Noril'sk) did not provide any suitable result of absolute paleointensity. The mean virtual dipole moment obtained from only the eastern localities has a value close to 6.01 ± 1.45 × 1022 Am2 which is over 50% higher than the results obtained by Heunemann et al. (2004) and Shcherbakova et al. (2005) on northwestern specimens. The discrepancy could possibly be explained by the presence of multidomain grains in northwestern specimens, where previous studies were undertaken, which would reduce the paleointensity estimates at high temperatures. However, the present results lead us to conclude that: (1) the geomagnetic field intensity at the Permo-Triassic boundary was very close to the observed present-day values and as a consequence a dipole low is not characteristic during this period of time; (2) the Mesozoic dipole low did not extend to the Permo-Triassic boundary, contrary to previous studies.