Paleomagnetism of the Ordovician section of the Mishina Gora (Pskov oblast): a record of the ancient geomagnetic field and restrictions on the age of the Mishinogorsk explosive structure

Abstract

Paleomagnetic studies of the Ordovician rocks of the Mishina Gora explosive structure (Gdovsky district, Pskov region) have been carried out. Based on the results of thermal magnetic cleaning, two stable magnetization components S and P have been identified. These components have similar unblocking temperature ranges (~200–600°C), but their mean directions are statistically different the difference of their mean directions are statistically significant. Refional fold–test indicates the postfolding age of the component S and prefolding age of the component P. Prefolding age of the latter is supported by the positive result of the local fold-test which has been carried out in the scale of the studied section. Comparison of the paleomagnetic pole calculated from the S component with the Apparent Polar Wander Path for the East European Platform definitely indicates the Late Permian-Early Triassic age of this component, which, in combination with the available geological observations, determines the time range of the formation of the Mishina Gora structure as ~370-250 million years ago. The ancient component of the magnetization P has the same direction as the components, which have been identified earlier in a number of coeval sections of the North-West of Russia, Estonia, and Scandinavia and which have been considered to be primary. The corresponding paleomagnetic poles are widely used in Paleozoic geodynamic and tectonic paleoreconstructions with the participation of the Baltic paleocontinent. A positive fold test result for the P component validates the use of these poles for paleoreconstructions. For the stratigraphic interval, which includes the upper Floian, Dapinian, and lower 2/3 of the Darrivilian stages, a magnetostratigraphic section has been constructed containing a single magnetozone corresponding to the reverse polarity of the geomagnetic field. Thus, the magnetostratigraphic data do not support the hypothesis of the existence of intervals of normal polarity within the Ordovician geomagnetic superchron of reverse polarity (Moyero superchron).