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

Abstract

Polymodal fault systems are sets of faults in three or more orientations that formed in relation to a common state of strain. The importance of polymodal fault systems was first demonstrated by Oertel (1965) in a series of remarkable experiments on clay deformed in a three-dimensional strain state, including measurements of both displacements and stress. He outlined a theoretical framework for the development of four sets of faults, which were related to the principal strain rate axes by symmetry arguments and tensor geometry. Oertel''s work was followed up a decade later by Reches (1978Reches ( , 1983, Aydin and Reches (1982) and Reches and Dieterich (1983), who produced field and experimental evidence for polymodal fault sets, and analysed the strain associated with their formation. Krantz (1988) documented polymodal faults and showed how strain states could be inferred from field measurements by the "Odd Axis" method. Subsequently, Healy et al. (2006a, b) drew attention to the existence of non-Andersonian, polymodal fault sets, and proposed a model for their formation based in crack tip interactions. Fig. 1 displays the differences between conjugate faults predicted by Andersonian fault theory, and a polymodal fault geometry.Scaling, kinematics and evolution of a polymodal fault system: Hail Creek Mine, NE Australia AbstractWe analyse a system of normal faults that cut cuts sandstone , siltstone , mudstone, coal, and tuff at Hail Creek Coal Mine in the Bowen Basin, NE Australia. Our detailed mapping utilized 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.Keywords: Normal fault; Orthorhombic fault; Polymodal fault; Displacement--length scaling; Coal...

Interacting Intraplate Fault Systems in Australia: The 2012 Thorpdale, Victoria, Seismic Sequences

“…Fault 2 aligns with this secondary network of faults well. If a more recent date than middle Eocene (e.g., Etheridge et al, ) can be established for these faults, a polymodal mechanism cannot be ruled out given that observations of polymodal faults have been made in NE Australia (Carvell et al, ).…”

Late Oligocene to Pliocene Extension in the Maltese Islands and Implications for Geodynamics of the Pantelleria Rift and Pelagian Platform

“…The formation of two coeval fault sets oblique to the main extension direction can be explained with a three-dimensional deformation where all the principal strain axes are different from zero (Krantz, 1988;Reches, 1978Reches, , 1983Reches & Dieterich, 1983). These kind of fault systems, sometimes called orthorhombic systems (Krantz, 1988;Reches, 1978Reches, , 1983Reches & Dieterich, 1983), have been already described in different areas of the world in extensional tectonic settings (Aydin & Reches, 1982;Bistacchi & Massironi, 2000;Carvell et al, 2014;Collanega et al, 2017;Franceschi et al, 2014;Krantz, 1988;Miller et al, 2007).…”

Development of a Brittle Triaxial Deformation Zone in the Upper Crust: The Case of the Southern Mesa Central of Mexico

“…Polymodal fault systems are compound of three or more fault sets forming and slipping simultaneously (Healy et al, 2015;Peacock et al, 2016), and it is well established that these fault sets form under triaxial deformation fields (Healy et al, 2015;Oertel, 1965;Reches, 1978Reches, , 1983. These kinds of fault systems have been reported in the literature, both in experimental models and field cases (Aydin & Reches, 1982;Beacom et al, 1999;Bertini et al, 1985;Carvell et al, 2014;Donath, 1962;Ghaffari et al, 2014;Jamison & Stearns, 1982;Koestler & Ehrmann, 1991;Krantz, 1988Krantz, , 1989McCormack & McClay, 2018;Miller et al, 2007;Peacock & Sanderson, 1992;Reches & Dieterich, 1983;, including the fault arrays with orthorhombic symmetry described by Oertel (1965) and Reches (1978Reches ( , 1983. (Bergh et al, 2007;Peacock, 2004) or a perturbation of the local stress field induced by reactivated preexisting faults (e.g., Deng et al, 2020;Peacock & Sanderson, 1994).…”

“…Since the pioneering work of Oertel (1965), a few theoretical and experimental studies have been published regarding fault systems formed under triaxial deformation. Most of the geometric, dynamic, and kinematic models have been proposed for orthorhombic fault systems from centimetric to kilometric scales (e.g., Aydin & Reches, 1982; Cai, 2019; Carvell et al, 2014; Healy et al, 2006a, 2006b; Krantz, 1988, 1989; McCormack & McClay, 2018; Reches, 1978). Krantz (1989) proposed the graphical “odd‐axis method” to relate the fault‐slip data with the strain state.…”