Flanking structure development in anisotropic viscous rock

Flanking structures typically occur in foliated or banded rock, and this marked planar structure implies a significant mechanical anisotropy, which has not been considered in previous mechanical models of flanking structure development. Both analytical and numerical techniques are used to investigate the influence of anisotropic viscosity on flanking structure formation. Reorientation of the principal stress axes in anisotropic materials can cause the sense of shear along a fracture in certain orientations to be opposite to that expected in an isotropic material. Applying the principle of stress reorientation in anisotropic rocks to natural flanking structures allows a qualitative estimation of the degree of anisotropy during flanking structure formation. It is shown that a strong foliation or banding does not necessarily imply a strong mechanical anisotropy, which calls for caution in inferring mechanical anisotropy directly from the structure of rocks without additional rheological information.