Ductile shear zones as counterflow boundaries in pseudoplastic fluids

Ductile shear zones with different styles are attributed to rocks having deformed as pseudoplastic power law fluids with different exponents n>1 of the stress sensitivity of the strain rate. Profiles of velocity and accumulated displacements recorded by passive markers in pseudoplastic fluids flowing steadily and continuously along no-slip boundaries have robust geometries distinctive of the n-value. What have previously been considered as single shear zones are forward modelled as pairs of back-to-back boundary shears coupled across individual counterflow boundaries. cement gradients in natural shear zones are shown to fit theoretical displacement curves for steady flows of pseudoplastic fluids along no-slip boundaries in a variety of rock types of different age, environment and scale. General fits of these curves indicate that strain was homogeneous along natural counterflow boundaries, so that specific values or ranges of n-value can be assigned to the rocks when they sheared whatever deformations and deformation mechanisms were involved. Fits that are only local indicate inhomogeneous strains along natural boundary shears and require more detailed analysis.