(a)Shear bands (solid lines)with conjugate angles of 109°and 110°in a roof support pillar at depth of 1000 m, Boulby mine, Cleveland, UK.Vertical broken lines indicate grooves made by excavator shovel, which are offset along yield bands (after Watterson, 1999)^[1].(b)Conjugate shear bands obtained in sandbox analogue modeling of extensional structures using a sand layer 3 cm thick extended at constant rate about 5 ×10^-3 cm sec by a stretchable basement to a total extension of 46%(the length ratio of analogue modeling is designed to be about 10^-5, implying that 1 cm in the model simulates 1km in nature).The conjugate shear bands with conjugate angles of 109°in the top view of sandbox merge as the amount of extension exceeds 20%(after Bahroudi et al., 2003)^[3].(c)Rose diagram showing the orientations of conjugate shear bands measured at 15°intervals, in the later Archaean granite-greenstone terrain of the Western Superior Province, Canada, with a conjugate angle of 109°(after Park, 1981)^[4].

Diagrams showing the hypothesis of maximum lateral displacement rate (MLDR) in the cases of (a) compressive and (b)tensile driving stress applied to a solid.The MLDR hypothesis implies the fastest way for the blocks divided by conjugate shear bands to be laterally extruded from the deformation domain in the case of compressive driving stress, known as extrusion, or to be laterally fed into the deformation domain in the case of tensile driving stress, known as necking.

A shear band model to calculate the lateral displacement rate of a homogeneous and isotropic solid deformed by (a)compressive driving stress and (b)tensile driving stress (σ).

Variations of the half conjugate angle of shear bands, θ, with the power-law index, n, in an isotropic solid

A sketch map shows three different types of tectonic stresses, which generate four different types of structures. σ stands for the tectonic stress, v stands for the lateral displacement rate