CONTRASTING STYLES OF RIFTING - MODELS AND EXAMPLES FROM THE EASTERN CANADIAN MARGIN

We present the results of a dynamical model of lithospheric rifting and rupture which show that a wide range of crustal thinning patterns across rifted passive margins can be produced by varying the steady state geotherm, lithospheric composition (dry versus wet materials), and strain rate. The basic mechanism of continental rupture is assumed to be passive rifting and necking. We use a numerical thermomechanical model of lithosphere extension based on a finite element approach. When plasticity is significant (i.e., at lower temperatures or for ''harder' materials) deformation is unstable and thinning takes place abruptly, over a narrow area. Conversely, a progressive thinning across the margin is observed when creep is dominant (i.e., in warm or ductile conditions). Cooling and associated hardening of the thinned area can occur during extension and cause the locus of extension to migrate laterally. ID these circumstances, rupture is likely to take place asymmetrically along one edge of the thinned area, producing a narrow margin and a very wide conjugate. The eastern margins of Canada and their conjugates across the North Atlantic provide examples which cover this range of theoretical profiles. The crustal thinning patterns, inferred from deep seismic data, and the duration of rifting compare well with model results. We discuss also the constraints that these geodynamical models provide on such current issues as the seismic reflectivity of the lower crust, or the location of the ocean-continent boundary in wide areas supposedly underlain by 5-km thin continental crust.