Research on the geometry of shear fracture zones

The geometry of brittle shear fracture zones has been studied by the examination of some strike-slip faults and shear joints in China. Pinnate and en echelon deformational patterns are two fundamental arrangements of fractures within brittle shear fracture zones. The pinnate angle is usually less than 25° and the en echelon is more than 25°. The deformation in the “rock bridge,” the area between two secondary fractures, depends on the arrangements of secondary fractures and the sense of shear. Some tensile ruptures are formed at the terminations of both primary shear fracture zones and secondary fractures. The direction of a tensile rupture is commonly parallel to the orientation of the principal compression. Along the direction of propagation of the shear fracture, the horizontal displacement is large, and the attenuation of displacements is slow. The maximum horizontal displacement of the shear zone may be located in the region of the initial fracture. Under certain conditions the shear fracture zones may possess pivotal movement. This type of movement may be expressed by the behavior of the fault or by the geomorphology of the fault zone.