Grenville Front and Rifting of the Canadian Shield

Abstract

RADIOMETRIC1, seismic2 and palaeomagnetic3,4 data suggest that the Grenville Structural Province was once displaced with respect to the rest of the Canadian Shield. The latter, referred to here as the Pre-Grenvillian Plate (Fig. 1), became stabilized about 1,800 m.y. BP during the Hudsonian orogeny and has not been subject to large-scale internal movements since then5. The palaeomagnetic evidence indicates, first, that a substantial part of the Province (the Grenville Plate) was once several thousand kilometres east-south-east of the Pre-Grenville Plate; second, that the Grenville Plate was brought into its present position with respect to the Pre-Grenville Plate as a result of motions that were predominantly dextral strike-slip and third, that these movements began about 1,300 m.y. ago or earlier and ceased about 1,100 m.y. BP, when the Grenville Plate became locked to the Pre-Grenville plate3. We now propose a model that accommodates both this sequence of events and the emplacement of three major contemporaneous suites of basic igneous rocks. We suggest that these three suites of igneous rocks were emplaced during successive tensional phases related to the waning stages of motion along a strike-slip plate juncture and not during the early stages of plate divergence with which such igneous rocks are commonly associated.