Scaling, kinematics and evolution of a polymodal fault system: Hail Creek Mine, NE Australia

• Coal seams are cut by normal faults in an orthorhombic network.
• The polymodal faults have typical displacement–length ratios for normal faults.
• Faults are kinematically coherent in groups.
• Faults evolved by growth of variably orientated components.

We analyse a system of normal faults that cuts sandstone, siltstone, mudstone, coal, and tuff at Hail Creek Coal Mine in the Bowen Basin, NE Australia. Our detailed mapping utilised the dense borehole network and strip mining operations. The fault surfaces have complex geometries, yet the components of the individual faults show similar orientation variability to the whole fault system. The faults and their components dip to the SE, NW, NNW, and SSE with an orthorhombic symmetry that we refer to as polymodal. There are multiple displacement peaks, with complementary changes on adjacent faults. This observation suggests kinematic coherence between neighbouring faults. Twin displacement peaks on some faults suggest that segment linkage occurred on a scale of hundreds of m. These polymodal faults follow the same displacement–length scaling laws as other normal faults. Fault dip is affected by lithology, with steeper dips in more competent (sandstone) beds. An ‘odd–axis’ construction using whole fault planes suggests that they formed in a triaxial strain state (three different principal strains) with vertical shortening, and horizontal extension along principal directions of 148° and 058°. Odd–axis constructions using individual fault components, as opposed to whole faults, give similar principal strain orientations and maximum strain ratios. The variable component orientations, and the consistency of fault kinematics on different scales, suggest that the faults evolved by the propagation or linkage of smaller components with variable orientations, within the same bulk strain state.

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