Reactivated faults and transfer zones in the Southern Coalfield, Sydney Basin, Australia

The Permo-Triassic Sydney Basin, which includes the Southern Coalfield to the north and northwest of Wollongong, contains important coal deposits. The most important deformation events in the Southern Coalfield were a north-northeast extension event, characterised by normal faults, followed by approximate northwest-contraction, characterised by strike-slip faults which are conjugate about northwest-southeast. The largest faults in the Southern Coalfield are west-northwest- to northwest-striking normal faults with displacements of up 90 m. The faults are typically segmented, with relay ramps and other transfer zones transferring displacement between overstepping faults. Transfer zones are areas of reorientated bedding which typically allow high displacement gradients to develop near fault tips. Fault interaction may thus cause considerable variability in throw-distance (t-x) profiles and displacement gradients. The transfer zones were initiated during extension, but were reactivated during contraction as strike-slip relay ramps, which are areas of reorientated bedding accommodating vertical separation of beds. North-northwest-striking sinistral faults and northeast-striking thrusts developed in the contractional oversteps between dextrally reactivated normal faults. Greater complexity becomes apparent in the Southern Coalfield as the observations become more detailed. A possible reason for this is that transfer zones can be accommodated by networks of smaller faults.