Localization and fault growth in layered brittle‐ductile systems: Implications for deformations of the continental lithosphere

Abstract

We present experiments and numerical simulations dealing with the growth of faults in thin brittle/ductile systems to understand deformation modes in the continental lithosphere. Experiments were uniaxial shortening of layers of dry sand and silicone putties of various viscous resistances. For large strength ratios between the brittle and ductile layers (R>5-10), the deformation localizes into two shear bands; the fault 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 with a trivial dimension of 2, and grows continuously during deformation. 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 investigated by means of electrical analog simulations. A fuse network, which represents an elasto-brittle layer, is coupled with a capacitor layer which models strain-rate dependent fluids. An AC potential is applied 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 obtained marking a transition between a localization mode where the density of burned fuses decreases as the system size increases, and a delocalization mode where the density of burned fuses remains constant with increasing system size. The scaling dependency of the fracture process, as well as the critical frequency, are consistent with experimental results. Available information on the rheology of the continental lithosphere shows that this mechanical transition is bracketed by the possible range of brittle-toductile strength ratios. (-50-100 km) is a good candidate for this scale [England and Jackson, 1989]. At lengths larger than •,, the instabilities, such as stress enhancement around crack tips, are considered mechanically negligible. Definition of the homogenization scale requires a detailed analysis of the deformation pattern which is not possible with available data. An attempt to examine •, was made by Davy et al. [1990] by assuming scaling laws for fault lengths and densities; however their results lead to the question of whether such a homogenization scale exists.Key observations that may resolve this debate concern the degree to which the lithosphere deforms homogeneously or within localized zones over several thousand kilometers. Such observations are possible where horizontal motions involve large continental blocks, as displayed by the India-Asia collision where the eastern boundary (Pacific border) is made up of oceanic subduction zones and may be considered as a free boundary [Tapponnier and Molnar, 1976]. The deformation localization in large wrench faults, as proposed by Tapponnier and coauthors, is certainly too a simplification of the deformation and distributed deformation has been record...

Generalized statistical mechanics approaches to earthquakes and tectonics

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Effects of Hydrothermal Cooling and Magma Injection on Mid-Ocean Ridge Temperature Structure, Deformation, and Axial Morphology

“…Because of the difficulties in directly measuring subsurface thermal structure, researchers frequently rely on observables such as axial topography and abyssal hill fabric to infer thermal conditions near oceanic spreading centers. Fault evolution is highly sensitive to rheological heterogeneities within the brittle lithosphere and the underlying ductile asthenosphere [Davy et al, 1995;Heimpel and Olsen, 1996;Cowie, 1998]. Thus, by incorporating experimentally determined rheological laws in thermo-mechanical models of lithospheric stretching, the observed fault patterns can be used to place important constraints on axial thermal structure [e.g., Sleep and Rosendahl, 1979;Phipps Morgan et al, 1987;Lin and Parmentier, 1989;Chen and Morgan, 1990a;Shaw and Lin, 1996;Poliakov and Buck, 1998].…”

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“…Davy et al [1995] and Bonnet [1997] show that the ratio of the ductile strength over the brittle strength could explain the transition from a localized damage mode (low ductile/ brittle strength ratio) towards homogeneous shortening (high ductile/brittle strength ratio). The ductile strength therefore apparently defines a change in the brittle deformation mode.…”