Australia: The Land Where Time Began

A biography of the Australian continent 

Sydney Basin – Constraining Timing of Brittle Deformation and Fault Gouge

In this paper Och, Offler & Zwingmann¹ report the results of structural and K-Ar studies of gouges in faults that trend N-S, NNE and E-W at 4 locations in the Sydney-Hunter region. The fault zones are manifest as joint swarms and zones that are highly brecciated and contain gouge with authigenic illite that results from the infiltration of fluid. Minor dip slip, accompanied strike-slip movement, on the NNE faults normal movement occurred, and on the N-S and E-W trending faults dip slip occurred. In this study Och, Offler & Zwingmann¹ analysed gouge from a NE-trending, steep, SE-dipping fault that exhibited dip-slip movement at Cut 10 on the Hunter Expressway, and from a E-W trending, steep south-dipping, normal fault at the West Side open Cut, Lake Macquarie. Ages that vary from 166-119 Ma for the <2 μm and finer fractions, and a mean age of about 120 Ma for the <0.4 μm fraction were found by K-Ar dating of illite and illite-smectite in fractions extracted from fault gouges in areas that were unaffected by thermal overprint. The ages obtained from fractions of similar size both for the gouge and for the host rocks are younger (134-76 Ma; av. <0.4 μm = 111 Ma), in the Sydney area and the Westside Open Cut coal mine, Lake Macquarie. Influence of a thermal overprint associated with subsurface magmas that were emplaced in the early stages of rifting of eastern Gondwana during the Early Cretaceous is indicated by the data.

Sources & Further reading

1. Och, D. J., R. Offler and H. Zwingmann (2014). "Constraining timing of brittle deformation and fault gouge formation in the Sydney Basin." Australian Journal of Earth Sciences 61(3): 337-350.