Interpretation of a set of faults across the hinge and a limb of a large-scale flexure in the Mount Isa district, Queensland, Australia, in terms of fractures related to the folding process

A revised interpretation of a number of faults across the hinge and western limb of a large-scale anticlinal flexure in the Mount Isa district has been made in terms of the faults following earlier-formed be joints. Such joints often develop in weakly or moderately folded competent sediments, as a result of either tensile stresses that were active at a late stage during folding or the influence of residual stresses generated during tectonic uplift. The joints are oriented such that on a stereographic projection their poles plot parallel to the a axis of a fabric cross and at 90° to the fold axis (b). bc joints are thus approximately normal to bedding and contain the fold axis, and hence they fan around the axial plane of the fold containing them. Across the hinge and western limb of a steeply N-plunging large-scale F₂ flexure in the Mount Isa district, a number of faults at high angles to bedding fan about the axial plane. Making use of the fold geometry and local bedding orientation it is possible to predict the orientation of ideal bc fractures at locations within the fold. These predictions fit well with the observed fault pattern. The movement on the faults, although apparently complex, appears consistent with continued shortening perpendicular to an axial-plane cleavage during the D₂ deformation or as part of a later D₂ deformation.