Anatomy of a mining-induced fault zone

Abstract

A mining-induced fault zone that formed in the East Rand Proprietary Mine, Boksburg, South Africa, during the sloping of a large, highly stressed pillar is associated with two large seismic events that occurred within 17 days of each other; each event caused appreciable rock-burst damage in the neighboring stopes. The zone consists essentially of two high-angle normal faults intersecting at about 30°, across which shear displacements of as much as 10 cm were measured. In detail, the fault zone comprises two approximately planar shear zones, each of which strikes subparallel to the stope working faces at the time of the seismic events, and which encompass areas of at least 500 and 600 m 2 , respectively. The shear zones are themselves made up of smaller, en echelon shear planes, which are connected by subsidiary conjugate shears and extension fractures. These shear planes become diffuse and are replaced by extension and bedding-plane fractures at the boundaries of the fault zone. Microscopically, the shear planes are seen to contain a layer of finely comminuted gouge material within which fluxion structure may be developed. The wall rock adjacent to the gouge is a microbreccia formed by shattered quartz grains and larger areas of rock, cut by conjugate shear and extension fractures across which some comminution has also occurred. The microscopic observations suggest the following sequence of events resulting in the formation of the fault zone: (1) development of extension cracks in the region of planes of maximum shearing stress, (2) coalescence of extension cracks to form conjugate shear planes, and (3) rapid movement along the more favorably oriented of these shear planes to form the major gouge zone and feather fractures. Dynamic analyses of the attitudes of the microfractures and macrofractures define the stress field operating at the time of the seismic event. There is a striking similarity between the fault zones and natural faults, both in macroscopic and microscopic form and in the development of cataclastic rock types.