EROSIONAL FORCING OF BASIN DYNAMICS: NEW ASPECTS OF SYN- AND POST-RIFT EVOLUTION

Abstract

We revisit a number of important topics associated with the problem of interactions between surface and subsurface processes during syn- and post-rift evolution. To demonstrate the importance of these interactions and to verify a number of earlier ideas on rift evolution, we use a fully coupled three-fold mechanical behaviour, surface processes, heat transport numerical model, which combines brittle-elastic-ductile rheology, fault localization, erosion and sedimentation mechanisms. The model simulates fault formation causing brittle strain localization. Fault distribution and evolution are thus outputs of the model, allowing for new, geologically sensible constraints on the results. The numerical algorithm accounts for 'true' surface erosion/sedimentation, that is, the numerical elements are eliminated (eroded) and created (sedimented) with respective changes in properties. The results show that sedimentation in the basin and erosion on the rift flanks strongly control the mode of extension. In particular, active erosion/sedimentation on the synrift phase results in more pronounced thinning and widening of the basin, so that the apparent coefficients of extension increase by a factor of 1.5-2. Surface loading/unloading results in lithospheric flexure. Flexural stresses in places of maximum bending exceed lithospheric strength and create zones of localized weakening that partly or completely compensate strengthening due to cooling in the post-rift phase, when the subsidence rates also accelerate. Erosional unloading on the rift shoulders has the opposite effect, producing local strengthening and flexural rebound. Pressure gradients induced by subsidence/rebound result in lower crustal flow that controls 20-30% of subsidence rates, stability of the rift shoulders and drives some post-rift extension or compression. By taking account for the intermediate and lower crustal rheology, new explanations for some synrift phase effects such as polyphase subsidence of the basement provoked by crustal flow and 'switching' of the level of necking from one competent lithospheric level to another are suggested. Syn- and post-rift stagnation, upward and downward accelerations find a natural explanation within our model without the necessity to invoke external mechanisms.