Pressure solution in sandstone from the Olympic Accretionary Wedge, Olympic Mountains, NW Washington State, USA. The horizontal dimension is ~ 10 mm (photomicrograph by M.T Brandon). Laboratory meassurements of steady-state rock creep and field observations in metamorphic terrains indicate that ductile rocks may not be well-approximated by the homogeneous, isotropic, incompressible, and linear viscous fluids of uniform viscosity described in the previous chapter. In this chapter, we consider: (i) rheological nonlinearity; (ii) dilatation at a macroscopic scale mediated by diffusional transport; (iii) bulk properties of a composite material; and (iv) instability in the deformation of an anisotropic material. All such deviations from the homogeneous Newtonian viscous fluid lead to effects that produce observable features in the structures and internal fabrics of deformed rocks, and they are of substantial interest in application to the interpretation of field data.
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