RELATIVE IMPORTANCE OF STRAIN-RATE AND RHEOLOGY FOR THE MODE OF CONTINENTAL EXTENSION

Both extension rate and theology have been shown to influence the style of continental rifting. So far, their effect on the dynamics of extension has been discussed separately in the literature, leading to diverging conclusions. The rate of extension mostly affects the dynamics of rifting through the occurrence of syn-rift cooling at low extension rates. This cooling results in an increase of strength if the lithosphere has a temperature-dependent theology. England (1983) first suggested that this effect will limit the amount of extension that an area may undergo. Kusznir & Park (1987) argued more generally that it will induce a widening of the rift, and concluded that extension rate is the major control on wide versus narrow rifting. Later studies (Bassi 1991; Bassi, Keen and Potter 1993), however, have demonstrated the importance of theological layering within the lithosphere for the mode of continental extension by showing that a plasticity-dominated theology will lead to rapid and narrow necking while a creep-dominated theology results in wider rifts and margins. Gravitational stresses arising from density anomalies due to lithosphere thinning also affect the style of rifting (Buck 1991). In this paper, we examine the combined effect of these different parameters in order to reconcile earlier studies and assess the importance of syn-rift cooling for the process of lithospheric necking. The approach, introduced in a previous study (Bassi 1991), uses a finite-element technique to simulate extension of a vertical cross-section of the lithosphere. The effect of cooling on the necking pattern depends strongly on the theology of the thinned lithosphere when it starts to cool. If heat diffusion always induces strengthening and delays rupture in time, it only modifies significantly the geometry of the rift when the upper mantle is weak and viscous. In this case, the locus of maximum strain rate migrates laterally once the central area starts to cool and harden, as hypothesized by Kusznir and Park (1987); eventually, strain concentrates at the transition between deformed and undeformed lithosphere. We predict that this pattern will induce asymmetric breakup and produce a very narrow margin and a wide, uniformly thinned, conjugate margin. In all other cases, i.e. when the upper mantle is initially in the low-plasticity regime, or if it reaches the yield stress during extension, the rate of extension appears to have little effect on the rifting pattern.