Paleomagnetism of the Lower Cambrian from the Lower Lena River Valley: Constraints on the Apparent Polar Wander Path from the Siberian Platform and the Anomalous Behavior of the Geomagnetic Field at the Beginning of the Phanerozoic

Detailed paleomagnetic studies of Lower Cambrian reference sections of the Siberian platform were performed; the sections are located within the Chekurovka and Bulkur anticlines in the northeasternmost part of the Siberian platform. The bulk of rocks composing the sections studied (including volcanic flows of basic composition) is shown to have been completely remagnetized by the Mesozoic geomagnetic field. The overprinting time coincides (with certainty in the Chekurovka section and with a high probability in the Bulkur section) with the period of fold-thrust deformations that took place in the study area at the end of the Cretaceous and were associated with the formation of the Kharaulakh segment of the Verkhoyansk fold area. Two possible interpretations of the inferred paleomagnetic directions are discussed. It is shown that, regardless of the inter- pretation variant, the resulting paleomagnetic poles are well consistent with the apparent polar wander path (APWP) proposed by Besse and Courtillot (1) for stable Europe. The Late Jurassic pole from the Konder Massif (new data) and the average Permo-Triassic pole from the Siberian platform (37) also lie in the close vicinity of the segments of this curve of the respective ages. This result implies that, at least from the end of the Paleozoic, the North Eurasian plate has behaved like a rigid body (within an accuracy of the paleomagnetic method) and the European APWP can be used as a reference path for the Siberian platform. Based on the results of the paleomagnetic studies, ancient components having the Khramov and directions were identified in rocks of the Upper Vendian-Lower Cambrian part of the Chekurovka section, stratigraphically located below volcanic flow. The existence of the Kirschvink direction being questioned by some researchers, this result is independent evidence for its reality and is thereby of great importance, because the Kirschvink direction and the related pole form a cornerstone of the inertial interchange true polar wander hypothesis (13), implying a substantial (by about 90 ° ) and rapid (over 20-25 Myr) displacement of the Earth's rotation axis rel- ative to the Earth's surface at the beginning of the Phanerozoic.