title of the thesis - Department of Geology - Queen's University

Although principal stress axes can vary significantly from P-, B- and T-axes, similarities exist
between axis orientations.
The maximum principal stress orientation agrees with P-axis
orientations (Fig. 3.27A). The intermediate and minimum principal stresses also align with focal
mechanism T- and B-axes. The intermediate stress, however, corresponds to the orientation of the
T-axis, and the minimum principal stress with null axis (Fig. 3.27B, C).
A near-horizontal maximum principal stress and near-vertical minimum principal stress should, by
Andersonian fault theory (Anderson, 1951), produce reverse faults. New failures would be
expected to exhibit reverse-sense kinematics. Indeed shallow fractures were observed to have
reverse displacement along later fractures that intersect or displace shear zones and zones of high
strain (as discussed in Chapter 2). However, strike-slip mechanisms predominate, producing
mechanisms with different axis orientations. Strike-slip ruptures are also much more favourable
given the steep fault geometry and attitude with respect to the stress tensor. Strike slip failures
along steeply-dipping faults in Creighton Mine have been noted by Cochrane (1991).
Strike-slip failures in Sudbury mines in a reverse regime were also observed by McKinnon
(2006), who proposed that late stage faults subject to seismicity are in a state of critically stability.
Minor perturbations (e.g., mining-induced stress changes such as blasts) can cause instabilities
remote to openings. Given the variability in principal stress orientation and close proximity
between measured intermediate and minimum stress magnitudes, small perturbations (reduction in
sigma-2, increase in sigma 3 or both) could indeed cause strike-slip failure in an overall reverse,
pre-mining stress regime.
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