Origin, damping, and pattern of development of faults in granite

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Abstract
Observations of brittle fractures in the granite of the Massif de la Borne as well as in other rocks allow us to discuss the different hypotheses regarding the origin of faults. It seems that all faults originate by shear along a preexisting plane which can be an important fracture (joint, former fault) or a microflaw (pore, grain boundary). The shear on the plane gives rise to an array of en echelon cracks that makes the rock less resistant and permits the lengthening of the fault. We also found an arrangement of “horsetail” fractures damping the movements on the faults. We then propose a pattern for the development of the faults. They originate on preexisting plane and propagate by initiating en echelon cracks. The movements are damped by horsetails which permit, by faults connection in relays, the creation of larger faults.