Control of the rheological structure of the lithosphere on the inward migration of tectonic activity during continental rifting.

Analogue models are used to analyse the parameters controlling the evolution of extensional deformation in continental rifts. Models are deformed in a centrifuge and simulate the continental lithosphere floating and extending above a low-viscosity asthenosphere. Model results reproduce the typical evolution of deformation during continental narrow rifting, with early activation of large boundary faults and basin subsidence, followed by their abandonment and localization of tectonic activity in internal faults near the centre of the rift. The experiments document the strong influence exerted by the thickness of both brittle and ductile crustal layers and syn-rift sediment accumulation on the evolution of deformation, namely on the amount of bulk extension preceding inward fault migration. Thin upper and/or lower crust and absent or low syn-rift sedimentation promote a rapid abandonment of boundary faults and a transition to in-rift fault development for low amounts of extension; conversely, thick upper and/or lower crust and high syn-rift sediment accumulation favour prolonged slip on boundary faults and delayed development of internal faulting. The experimental results suggest that the inward migration of faulting during extension of continental lithosphere results from the interplay between the ductile stresses acting at the base of the upper crust and the total resistance of this brittle layer.