Normal stress dependent rupture morphology in direct shear tests on sandstone with applications to some natural fault surface features

Abstract

It is important for geologists to be able to identify the kind of natural fault features which can be formed by neorupture in the upper crust and also the controlling formation conditions. This paper describes a set of shear box tests on intact sandstone specimens which concentrated on the sequential development of elementary fractures during loading and the surface morphology. The role of stress concentration in initiation and propagation is emphasized. Although both parts of the specimen tended to rotate relatively during shearing, inducing an inhomogenous stress/strain distribution, it is shown that an increasing normal stress led to an increasing striated/non-striated surface ratio, while isolated shear surfaces (facing the movement of the missing block) tended to be replaced by numerous tight (Riedel-type) shears dipping in the sense of movement of the missing block; rupture was also less brutal with increasing normal stress. The resulting features seem homologous with already described fault plane structures corresponding to small displacements; the formation conditions of these structures are also discussed.