Chapter 25 Palaeomagnetism of the Baltic shield Implications for Precambrian Tectonics

Abstract

Abstract Palaeomagnetic data of the Precambrian (age 2700–800 Ma) rocks from the Baltic (Fennoscandian) Shield are reviewed. In the light of new palaeomagnetic pole positions and radiometric age data, a revised apparent polar wander path (APWP) is presented for the Baltic Shield. Apart from the Sveconorwegian (1000–800 Ma ago) results there are no major discrepancies in the pole positions of widely separated rock units, suggesting that the Baltic Shield has behaved as a coherent unit since early Precambrian times. The shape of the APW path of the Baltic Shield differs from that of the Laurentian Shield, indicating independent drift of the shields at least during part of the Precambrian. The dissimilar palaeolatitude curves of the two shields support this idea. However, during the Svecokarelian/Hudsonian orogeny (age 1900 Ma) and during the Jotnian/MacKenzie magmatic interval (1300–1200 Ma ago), the two shields had similar palaeolatitudes and may have been in juxtaposition. A collision of the two shields of these times could have triggered the Svecokarelian/Hudsonian orogenies 1900 Ma ago and the global rifting and magmatic episode (Jotnian/MacKenzie) about 1250 Ma ago. An intimate connection between the Shields was evident also during the Grenville/Sveconorwegian orogeny. This is suggested by palaeomagnetic and geological data. The APW speed (c. 0.35°/Ma) of the Baltic Shield during the Precambrian is significantly lower than that of the Laurentian Shield (0.50° /Ma). Remanent magnetization with “normal” polarity is dominant in the Baltic Shield.