THE EASTERN END OF THE GRENVILLE OROGEN MODELLED IN TERMS OF INDENTOR TECTONICS, AND BROADER IMPLICATIONS.

English Abstract:

Contrary to traditional depictions showing the Grenville Province as continuing its northeast trend out into the Atlantic Ocean and linking up with the Sveconorwegian front in Scandinavia, the Grenville orogen (1085-985 Ma) ends in eastern Labrador and the Sveconorwegian rocks of Scandinavia belong to a separate orogen, or sub-orogen. This conclusion is a consequence of acceptance of an indentor tectonic model for the easternmost Grenville Province. The basis for this model is recognition of a fundamental change from northeast geochronological, structural, metamorphic and geophysical trends, which prevail throughout much of the Grenville Province, to southeast trends in eastern Labrador. These contrasts in trend are related, respectively, to a northwest-verging frontal –thrust ramp tectonics and a southeast trending, dextral strike-slip lateral ramp regime, the junction between the two being an indentor corner. As part of the model, it is also argued that allochthonous terranes in the western part of the eastern Grenville Province were displaced 150 km farther northwest, relative to their eastern counterparts during the late Grenvillian (after 1020 Ma). After restoration by ‘removal’ of the northwest displacement, a simple and elegant pattern emerges which is analogous, in some ways, to that seen in the Himalayan-Tibetan tectonic system, and which can be convincingly modelled utilizing channel-flow concepts. A major contrast, however, is that, whereas India is being subducted under pre-India Asia, in indenting crust in the eastern Grenville Province was thrust over pre-Grenvillian Laurentia. This contrast has major implications at the syntaxial corner, which, in the Himalayas, is a region of constriction and vertical escape, but, in eastern Labrador, was less constraining horizontally, and is envisaged as a region of tectonic spillover.