Paleointensity at the Permian-Triassic boundary and in the Late Permian

The paleointensity H-an is determined from samples of Siberian traps collected in the Norilsk region and in the Bolshaya Nirunda River basin and dated at the Permian-Triassic boundary and the Lower Triassic (ages of 251-260 Ma). A Kazakhstan collection of basaltic andesites from two sections belonging to the Ural-Mongolian foldbelt and associated with the reversed-polarity Kiaman chron (ages of 260-275 Ma) is studied. The Thellier method and the check-point procedure were used for the determination of H-an. Small values of the virtual dipole moment (VDM) are shown to be characteristic of both the Late Permian and the Permian-Triassic boundary. New determinations obtained in our study imply that the transition from large values of the geomagnetic dipole in the Carboniferous to the small value characteristic of the Triassic and the entire Mesozoic took place within the Permian-Carboniferous reversed-polarity superchron in the interval 330-240 Ma. However, the spread in the VDM values of this period is too large to draw definite conclusions. Analysis of the presently available data on the paleointensity in the interval 330-240 Ma, i.e., inside and outside the Kiaman superchron, does not confirm the presence of a correlation between the VDM value and the reversal frequency, which agrees with results previously obtained for the Cretaceous superchron.