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

Laboratory experiments on analogue models of the lithosphere are useful too ls to study tectonic processes and, in particular, to test physical hypothe ses. They complement numerical modelling because the inherent limitations o f each method are different. The basic principles of the method are recalle d with particular application to models simulating the brittle-ductile laye ring of continental crust and lithosphere using sand and silicone putties t o simulate the frictional and viscous behaviour of rocks. A selection of ex periments is used to examine the role of rheology on the development of cru stal-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 tha t has a four layer-type of strength profile with the greatest strength loca ted in the sub-Moho mantle. It gives birth to narrow rifts, from continenta l rifts to passive margins. Spreading occurs preferentially in a thickened lithosphere whose strength profile, after thermal relaxation, exhibits maxi mum strength at the base of upper brittle crust. It gives birth to wide rif ts, 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 extens ion but are anomalies in wide rifts.