Volume loss and deformation around conjugate fractures: comparison between a natural example and analogue experiments

In well-stratified sedimentary rocks, measurement of the bedding offsets along fractures gives a value for the relative displacement at each location. To restore the undeformed geometry of the layers, the offset along each fracture must be cancelled. Restoration of the geometry enables the location of places where shortening is concentrated to be established, namely at the fracture intersection and at fracture tips. Slow displacement during deformation is demonstrated by the growth of fibres on the fault surfaces. Pressure solution inside the layers is responsible for the internal strain with associated volume loss. This mechanism is compatible with low strain rates giving aseismic displacements along the fractures. To model rock rheology dominated by pressure solution, a viscous material (paraffin wax) has been employed in the analogue model. Volume loss in nature is represented by area changes during the analogue experiments. In the experiments, shortening is concentrated around the fractures in exactly the same geometrical positions as in nature. When the amount of displacement is different on each of the two fractures, one of the fractures is observed to offset its conjugate fracture. When this is the case, relative timing of displacement along conjugate fractures cannot be determined without ambiguity from the observation of fracture offsets.