An analysis of anisotropy of rocks containing shape fabrics of rigid inclusions

This paper presents a theoretical basis for estimation of mechanical anisot ropy in homogeneous rocks containing shape fabrics of rigid inclusions. The analysis is based on two types of viscous models: one containing linear fa brics of prolate (a > b = c) inclusions (cf. L-tectonite) and the other con taining planar fabrics of oblate (a < b = c) inclusions (cf. S-tectonite). Models show contrasting bulk viscosities in stretching (normal viscosity) a nd shearing (shear viscosity) parallel to the fabric. The axial ratio R (= a/b) and the volume concentration (rho(v)) of rigid inclusions appear to be the principal parameters in determining the viscosity contrast. In anisotr opic models with linear fabrics, normal viscosity (eta(p)) increases monoto nically with increase in R, whereas shear viscosity (eta(s)) increases to a maximum, and then drops down to a near-stationary value. In anisotropic mo dels with planar fabrics, the normal viscosity increases little with increa sing flatness of inclusions, but the variation assumes a steep gradient whe n the latter is large. Shear viscosity, on the other hand, is relatively le ss sensitive to the shape of inclusions. The ratio of normal and shear visc osities, conventionally described as anisotropy factor delta, in both the m odels is always greater than I, indicating that normal viscosity will be es sentially greater than shear viscosity, irrespective of the axial ratio of inclusions forming the fabric. Models with a linear fabric show contrasting normal viscosities in pure shear how along and across the linear fabric. T he anisotropy is expressed by the ratio of longitudinal and transverse norm al Viscosities (anisotropic factor sigma). It is revealed that the transver se viscosity is essentially less than the longitudinal viscosity, as observ ed in test models. (C) 2000 Elsevier Science Ltd. All rights reserved.