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

We present the results of a dynamical model of lithospheric rifting an d rupture which show that a wide range of crustal thinning patterns ac ross rifted passive margins can be produced by varying the steady stat e geotherm, lithospheric composition (dry versus wet materials), and s train rate. The basic mechanism of continental rupture is assumed to b e passive rifting and necking. We use a numerical thermomechanical mod el of lithosphere extension based on a finite element approach. When p lasticity 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 mar gin is observed when creep is dominant (i.e., in warm or ductile condi tions). Cooling and associated hardening of the thinned area can occur during extension and cause the locus of extension to migrate laterall y. ID these circumstances, rupture is likely to take place asymmetrica lly along one edge of the thinned area, producing a narrow margin and a very wide conjugate. The eastern margins of Canada and their conjuga tes 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 mode ls provide on such current issues as the seismic reflectivity of the l ower crust, or the location of the ocean-continent boundary in wide ar eas supposedly underlain by 5-km thin continental crust.