Abstract

Ordovician palaeomagnetic data from the upper reaches of the Lena River, Southern Siberia, confirm and refine the earlier reported data sets. Ordovician palaeomagnetic poles from Siberia define a systematic southwesterly apparent polar wander (APW) trend during Ordovician times (mean south poles: 500 Ma: 42°N, 310°E; 467 Ma: 27°N, 314°E; 460 Ma: 23°N, 313°E; 448 Ma: 22°N, 301°E and 437 Ma: 0N, 290°E). A primary or early magnetization age is verified by the reversal stratigraphy.

Siberia was geographically inverted at low southerly latitudes during the Latest Cambrian and Early Ordovician and drifted slowly northward and across the equator at an average palaeo-latitudinal velocity of c. 5cm/year. In Early Ordovician times, Avalonia and the European Massifs (e.g. Armorica and Bohemia) were located together with Gondwana in high southerly latitudes, Laurentia was positioned in equatorial latitudes whereas Baltica was located at intermediate southerly latitudes. Siberia was probably located north of Baltica in latest Cambrian–Early Ordovician times. Subduction-related, eclogite-facies metamorphism in latest Cambrian–Early Ordovician time in the Scandinavian Caledonides occurred in an ocean-continent transition zone marginal to Baltica but facing northern Siberia, and thus throws doubt on traditional Baltic-Laurentian correlations during this particular time period. With Baltica rotating counterclockwise during the Ordovician, the plate scenario allows for a Siberian source for Late Ordovician sedimentation in some areas of maritime Laurentia and perhaps even northern Norway. It also helps to explain the imposition of a deep-seated, sinistral strike-slip, fault regime between the obliquely converging Baltica and Laurentia, a transcurrent system which may have led to the permissive ascent of calc-alkaline granitoid magmas in favourable sites prior to the main stages of Scandinavian orogenic deformation.

Recent proposals that Laurentia formed a conjugate margin to the South American part of Gondwana during Ordovician times are permissible from palaeomagnetic data, but a tight continental fit during the entire Ordovician is contradicted by biogeographic data. The tight palaeomagnetic fit could perhaps be an artefact of inaccuracies in the palaeomagnetic record for Gondwana.

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