Fig. 4

Formation of conjugate arrays of expansion structures. Analogy between a the Anderson model of conjugate faulting and b a schematic representation of the conjugate arrays of expansion structures and delaminations we observe in biotite. In both cases structures (faults in a expansion structures in b) form in two principal orientations, which are up to 60° apart and display opposite shear sense. The obtuse angle between the two orientations is bisected by σ₃ in a and the c-axis in b. In both cases this is the orientation along which expansion (or extension) occurs. In this way the diamond-shaped expansional arrays are crystallographically controlled as is evidenced by the fact that they have different orientations in different grains, being always aligned with the c-axis of each grain. Conjugate delamination arrays represent the most efficient way to relieve stored grain-scale c-axis strain energy, just as conjugate normal faults result from extension on a larger scale. The orientations of principal stresses in this figure are therefore representative of the time when stored elastic strain energy is released (during thinning for TEM), not necessarily the time during which strain was being accumulated (deformation within the viscous shear zone)