FACTORS CONTROLLING THE STYLE OF CONTINENTAL RIFTING - INSIGHTS FROM NUMERICAL MODELING

Passive continental margins, similarly formed by lithospheric stretching and rifting, can differ significantly in terms of width, ranging from 50 to 400 km, or subsidence history. Our interest lies in determining what properties of the lithosphere and what stress and strain boundary conditions control this regional variability and the siting of rupture. Continental breakup occurs probably as the result of a stretching instability which develops in a zone of material or geometrical heterogeneity. To analyze the factors controlling these instabilities, we use a numerical thermo-mechanical model of lithosphere extension based on a finite-element program. The particular program was chosen because it combines constitutive models appropriate to our knowledge of lithosphere rheology (elasticity, plasticity and nonlinear, temperature-dependent creep) with a kinematic formulation for large inelastic strain. We choose to weaken the plate by locally increasing the crustal thickness by a very small amount and examine how this "defect" or "perturbation" acts to localize the deformation. We compare the geometry and timing of necking for relatively "wet" versus "dry" rheology, "cold" versus "warm" geotherm and "fast" versus "slow" strain-rate. We examine as well how the width and amplitude of the perturbation itself affects the deformation history. Since a velocity boundary condition is applied, the required extensional forces are evaluated and compared to estimates of plate driving forces. Conditions that favor deformation by plasticity rather than creep ("cold" geotherms, "dry" rheologies) lead to a more localized extension and rapid rupture but the forces involved (> 10(13) N/m) are larger than typical plate driving forces. Increasing the amplitude of the defect, i.e. crustal thickness contrasts, accelerates necking as well. Cooling of the thinned area becomes significant at low extension rates. Depending on the initial rheology, cooling simply delays the moment when deformation becomes unstable or, conversely, inhibits necking.