LOCALIZATION AND FAULT GROWTH IN LAYERED BRITTLE-DUCTILE SYSTEMS - IMPLICATIONS FOR DEFORMATIONS OF THE CONTINENTAL LITHOSPHERE

We present experiments and numerical simulations dealing with the grow th of faults in thin brittle/ductile systems to understand deformation modes in the continental lithosphere. Experiments were uniaxial short ening of layers of dry sand and silicone putties of various viscous re sistances, For large strength ratios between the brittle and ductile l ayers (R>5-10), the deformation localizes into two shear bands; the fa ult pattern is created before reaching 10% shortening, and has fractal dimensions varying between 1.6 and 1.8. For small strength ratios (R< 5-10), deformation never localizes; the fault pattern is homogeneous w ith a trivial dimension of 2, and grows continuously during deformatio n. The transition between localized and homogeneous deformation occurs when the mechanical resistance of brittle layers is 5-10 times larger than the resistance of ductile layers. This transition was also inves tigated by means of electrical analog simulations. A fuse network, whi ch represents an elasto-brittle layer, is coupled with a capacitor lay er which models strain-rate dependent fluids, An AC potential is appli ed and the fuses progressively burned out until they form a connected network. The AC-potential frequency, f, is a tuning parameter similar to the applied strain rate in experiments. A critical frequency is obt ained marking a transition between a localization mode where the densi ty of burned fuses decreases as the system size increases, and a deloc alization mode where the density of burned fuses remains constant with increasing system size. The scaling dependency of the fracture proces s, as well as the critical frequency, are consistent with experimental results. Available information on the theology of the continental lit hosphere shows that this mechanical transition is bracketed by the pos sible range of brittle-to-ductile strength ratios.