Rotation, offset and separation of oblique-fracture (rhombic) boudins: theory and experiments under layer-normal compression

Abstract

Boudinage structures are generally produced by extension fracturing of competent rocks normal to layering. Boudin-like structures may also develop by shear fracturing of competent rocks oblique to layering. In such structures boudins of rhomboid shape are rotated and offset with respect to each other. It is usually considered that shear fracture boudins do not get separated until a critical rotation is attained, when they can just touch each other. However, the experiments under layer-normal compression and the theoretical analysis of the present study indicate that the layer-segments produced by a set of parallel shear fractures may undergo rotation with separation or rotation with interfacial slip depending upon their geometry. The thickness to length ratio (G_T) of layer-segments and the orientation of fractures (Φ) are the parameters that could control the rate of rotation versus rate of displacement of layer-segments.