Contrasting styles of rifting: Models and examples from the Eastern Canadian Margin

Abstract

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. In 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. controlled the distribution of deformation across and along the margin. WIDE VERSUS NARROW BASINS: CONTROLLING FACTORS Let us review the processes and parameters that may influence the crustal thinning pattern and margin width. Most authors have emphasized the importance of one factor while overlooking the others. Indeed, the number of parameters involved in any modeling of lithospheric rifting and the complexity of its theology make it very difficult to examine the effect of each parameter individually and systematically, in a dynamical model. The most extensively discussed factor has been the effect of heat diffusion during extension. England [1983] first pointed out that cooling during extension may cause the vertically averaged strength of the lithosphere to increase because of the temperature dependence of rock rheology. This strengthening will eventually inhibit further deformation, thus limiting the extensional strain attainable in a given area. This happens when the replacement of crust by stronger mantle dominates weakening of the lithosphere due to thinning. Such Planet. Sci. Lett., 40, 25-32, 1978. Nichols, B.C., and G. Bassi, Use of ADINA to model large deformation of the Earth's lithosphere, Comoeut. Struct., 32 (3/4), 761-777, 1989. Pinheiro, L.M., R.B. Whitmarsh and P.R. Miles, The ocean-continent boundary off the western continental margin of Iberia. II, Crustal structure in the Tagus Abyssal Plain, Geophys. J. Int., in press, 1993....

Extensional basins of the former Soviet Union — structure, basin formation mechanisms and subsidence history

“…However, such obser-vations can probably be explained if more complex non-associative plastic flow law for mantle rocks are adopted (Nikitin and Ryzhak, 1977;Nikolaevskii, 1983) or when softening effects of the mantle superplasticity are taken into account (Ranalli and Murphy, 1987;Bassi et al, 1993).…”

The role of pre-existing faults in basin evolution: constraints from 2D finite element and 3D flexure models

“…Brace & Kohlstedt 1980), numerical modelling studies on rift dynamics confirm that rheology strongly controls the distribution of rift activity and vice versa (e.g. Bassi et al 1993). The actual trigger of rift localization can be small perturbations in an initially homogeneous material configuration and homogeneous thermal state of the lithosphere (e.g.…”

“…The latter found that rift induced heating weakens the lithosphere, whereas crustal thinning has an opposite effect. The relative contributions heavily depend on the rift velocity, the distribution of crustal and subcrustal deformation, and the initial thermal and material state of the lithosphere (Buck 1991;Bassi et al 1993). Thermo-mechanical models for a single extension phase, show that predicted styles of extension are in good agreement with observations on lithospheric scales (Buck 1991;Bassi et al 1993).…”

Role of pre-rift rheology in kinematics of extensional basin formation: constraints from thermomechanical models of Mediterranean and intracratonic basins

“…These models have addressed the role of strain rate variations and the theological properties of the lithosphere. The role of the initial pre-rift lithospheric configuration (Bassi et al, 1993) appears vital in this respect. Questions relating to the relative contributions made by pure shear/simple shear, the role of the crust in the continuum sense or the influence of faults (Boutilier and Keen, 1994) are open to exploration by these models.…”

“…Dunbar and Sawyer, 1989;Buck, 1991;Bassi et al, 1993;Govers and Wortel, 1993). These models have addressed the role of strain rate variations and the theological properties of the lithosphere.…”