Analogous Upper Proterozoic apparent polar wander loops

Abstract

The ∼1, 100 Myr Grenville mobile belt of the Laurentian (North American) Shield yields a record of uplift magnetisations defining a closed apparent polar wander (APW) loop1–4 nearly identical to the contemporaneous palaeomagnetic records from the Sveconorwegian belt of the Fennoscandian Shield and ∼1,100–800 Myr supracrustal successions of the African Shield. The palaeomagnetic data reported here show that these Shields were contiguous at this time4, and comparison with the pre-1,100 Myr palaeomagnetic record defines a relative rotation of the Fennoscandian Shield with respect to the other major shields at ∼1,100 Myr, although they remained in close proximity until late Precambrian times. Tectonic and palaeomagnetic correlations between the major shields suggest that this motion represented an early fragmentation of rigid sialic crust. The restriction of crustal mobility to long linear mobile belts and the progressive contraction of anorogenic magmatism both define a gradual consolidation of the Proterozoic crust as temperature gradients declined.