Journal of the Southern African Institute of Mining and Metallurgy
On-line version ISSN 2411-9717
The geometry and morphology of a set of low-angle fractures around a stope in a deep Witwatersrand gold mine are explained in terms of extension fractures forming under variable conditions of stress. Primary extension fractures (E1) form some distance ahead of an advancing stope along the σι, σ2 plane. With stope advance, a couple of these fractures end up in a stress regime conducive to transpressional shear and a secondary set of extension fractures (E2) is formed at a high angle to the primary fractures. i.e. at a low angle to the stope. As the E2 fractures are undermined, they migrate into a stress regime of transtensional shear and a third set of extension fractures (E3) may develop between E2 fractures. These have sigmoidal shapes, being parallel to the E2 fractures at the E2 discontinuity where σ3 is negative, and curved through the un-fractured rock between E2 fractures where σ3 is positive at the instant of fracturing. The fractures all display fractographic features characteristic of dynamic extension failure with striae indicative of the direction of rupture propagation and the local, instantaneous orientation of σ1.
Keywords : fracturing; extension fractures; low-angle fractures; fracture classification..