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Narrow rifts versus wide rifts: inferences for the mechanics of rifting from laboratory experiments

Laboratory experiments on analogue models of the lithosphere are useful tools to study tectonic processes and, in particular, to test physical hypotheses. They complement numerical modelling because the inherent limitations of each method are different. The basic principles of the method are recalled with particular application to models simulating the brittle–ductile layering of continental crust and lithosphere using sand and silicone putties to simulate the frictional and viscous behaviour of rocks. A selection of experiments is used to examine the role of rheology on the development of crustal–scale extensional structures: continental rifts, passive margins, wide extended domains, and core complexes. The difference between narrow rifts and wide rifts is attributed to the type of mechanical instability that can develop for a given type of lithospheric strength profile: namely necking versus spreading. Necking occurs preferentially in a stable lithosphere that has a four–layer–type of strength profile with the greatest strength located in the sub–Moho mantle. It gives birth to narrow rifts, from continental rifts to passive margins. Spreading occurs preferentially in a thickened lithosphere whose strength profile, after thermal relaxation, exhibits maximum strength at the base of upper brittle crust. It gives birth to wide rifts, such as the Basin and Range of the western United States or the Aegean. Core complexes are not considered to represent a particular mode of extension but are anomalies in wide rifts.