Faults

Abstract

Where rocks have undergone observable displacement along a macroscopic shear or fracture plane in the earth they are said to have been faulted. The word fault refers both to the plane itself, which is called the fault plane, and to the displacements that have gone on along it. In conformity with the ideas on plastic slip and shear-fracturing expressed in Chapter III, fault planes may be shear-fractures along which the internal cohesion of the rocks has been destroyed, or slip surfaces formed during plastic yielding and across which cohesion was maintained during the movements. Again, under certain conditions of rupturing, the fault plane may be a tension crack, as is suggested for some large fault troughs and perhaps for ring-fractures associated with igneous activity. The majority of fault planes are, however, either shear-fractures or slip planes, and may be said to be shear planes, the geometry of which conforms with the principles of stress-strain relationships already discussed. The stress-conditions that induce faulting in relatively homogeneous rocks do, however, vary widely, and faulting is also strongly influenced by inhomogeneities, particularly where strata of markedly different physical properties are interbedded.