Chapter 14 A Grenvillian Model of Proterozoic Plate Tectonics

Abstract

Abstract Uniformitarian and non-uniformitarian models proposed so far to explain the evolution of the Grenville Province have not proven entirely acceptable. Pre-Grenvillian evolution involved early Proterozoic reworking of a sialic crust, in part of Archaean age, and intrusion around 1500 Ma ago of a suite of anorthosites and associated felsic plutons. A major structural zone, the Chibougamau—Gatineau lineament, marks the western limit of large anorthosite plutons and may be a long-lived line of crustal weakness. The Grenville Group of marbles, pelitic rocks and metavolcanics dating from 1250–1300 Ma ago occupies part of the southern Grenville Province. It includes some of the few rocks that did not reach at least amphibolite facies of metamorphism. Geochronological data show two peaks of activity, at 1100 ± 50 Ma and 950 ± 50 Ma. In North America, the first corresponds to a tectonic disturbance that only affected parts of the central Grenville Province. The second was felt in all the province but was of lesser intensity. Similar conditions appear to have existed in east Greenland and in southern Scandinavia. In the foreland of the province, basaltic material filled dykes and grabens prior to its deformation, but this activity peaked during orogeny, around 1150 Ma ago. Characteristics that are typical of Phanerozoic belts formed according to present-day plate tectonics have not been found in the Grenville Belt. It is argued that Proterozoic (and Grenvillian) tectonics involved intra-plate deformation but not subduction. Trends of dyke swarms, grabens, major shear-zones and folds are used to deduce the probable orientation of principal stress-directions at the time they formed. Stress-field orientations seem to have been similar inside and outside the Grenville Province. Palaeomagnetic hairpins faithfully match the structural evolution of the province. The drift that they indicate for eastern North America between 1200 Ma and 900 Ma ago may suggest motions of the continent inside a constant stress-field, or at least a stress-field independent of the drift of the continent. The stresses that were necessary for intracratonic deformation of the Grenville Belt could only build up because the lack of subduction caused plates to be locked together in “plate-jams”. At present, the particular geometry of plates in regions surrounding Tibet keeps forcing the Indian craton into Asia, causing a localized plate-jam not entirely unlike those that accompanied the formation of the Grenville Province.