NEW PALEOMAGNETIC DATA FOR THE PERMIAN-TRIASSIC TRAP ROCKS OF SIBERIA AND THE PROBLEM OF A NON-DIPOLE GEOMAGNETIC FIELD AT THE PALEOZOIC-MESOZOIC BOUNDARY

Abstract

The thorough analysis of the available Permian-Triassic paleomagnetic data for the Siberian Platform and "Stable" Europe was carried out. Paleomagnetic poles, meeting to modern reliability criteria, were used to calculate the mean Permian-Triassic paleomagnetic poles of Siberia and Europe. The comparison of the resulting poles showed significant differences between them. Discussed in this paper are four potential factors that had caused the observed differences between the paleomagnetic poles of Siberia and Europe: (1) the large-scale relative movements of these cratons in post-Paleozoic time, (2) the different ages of the compared paleomagnetic poles, (3) the substantial contribution of non-dipole components to the geomagnetic field at the Paleozoic-Mesozoic boundary, and (4) the shallowing of the magnetic inclination in the European sedimentary rocks. Also discussed is the adequacy of the data selection. Arguments are advanced to prove that the possibility of the post-Paleozoic large-scale relative displacements of the cratonic blocks discussed as well as considerable age difference of their mean poles is unlikely. Also estimated were the input quadrupolar and octupolar sources in the total time-average geomagnetic field and also the values of the inclination shallowing factor, which might have explained the observed discordance of the Siberian and European poles. The best agreement of the European and Siberian paleomagnetic data was achieved for the octupolar coefficient g3 = - 10% or for the inclination shallowing factor f = 0.62. Our calculations showed that the statistically significant difference between the Siberian and European average poles can be removed assuming a very small value of the inclination shallowing corresponding to the f values of 0.9 to 0.95, potentially associated with some compaction of the studied sedimentary rocks. This gives grounds for interpreting the low inclinations in the European objects as the most probable source of the observed disagreement between the European and Siberian paleomagnetic data.