Modes of continental lithospheric extension: Experimental verification of strain localization processes

Three modes of continental lithospheric extension, narrow rift, wide rift and core complex, were studied by means of analogue experiments. The experimental method is based on models which simulate an idealized lithospheric strength profile and are scaled for gravity, Various styles of lithospheric extension were induced by systematic variation of brittle upper crustal thicknesses. Narrow rifting occurs during stable lithospheric extension, when the resistance of the brittle upper crust dominates the deformation process, Wide rifting results from unstable lithospheric extension, when the brittle upper crust is thin and ductile flow in the lower crust allows expansion of the deformation zone. Periodic instabilities with two wavelengths dominate wide rifting. Core complex mode extension is transitional between unstable and stable lithospheric extension. It begins as a distributed zone of unstable extension, due to thin upper brittle crust, but lower crustal flow induced by the gravity force causes a relocalization of strain into narrow zones. Transitions between the three modes of extension are characterized in terms of competition between the strength of lithosphere and gravity forces, as well as between stable and unstable deformation processes.