Australia: The Land Where Time Began

A biography of the Australian continent 

Gondwana Breakup Implications – K-Ar Dating of Fault Gouge in the Northern Sydney Basin, NSW, Australia

It is common for synkinematic and authigenic clays to be present in fault gauges. The dating of fault gouges has not often been attempted. In this paper Zwingmann et al. present the first data for the growth of synkinematic illite-smectite for 2 fault zones in the northern Sydney Basin, New South Wales, at Burwood Beach that are hosted by siltstones, tuffs and coals of Early Permian age in the Lambton Formation, Newcastle Coal Measures. The faults are 1.5 m apart, showing normal displacement and trend N-S with steep easterly dips. Along both fault surfaces are developed foliated gouge zones, comminutional and dilational breccias. Samples from the gouge and tuffs in the damage zones have given K-Ar ages of 172 (6-10 μm) to 119 Ma (<0.4 μm), respectively. Older ages of 272–281 Ma for the coarse fractions (>2 μm), 237–245 Ma for the <2 μm fraction, 218 Ma for the <0.4 μm fraction and 196 Ma for the <0.1 μm fraction were obtained from siltstones within and outside the damage zone. Zwingmann et al. suggest the younger ages of 237-196 Ma indicate the time of formation of the illite-smectite and the 150-122 Ma from the <2 μm fraction represent the maximum age for the formation of the gouge. The younger ages are believed to reflect the last slip event which occurred on the faults, which is related to the rifting and dispersal of the Australian continent’s eastern margin.

Sources & Further reading

1. Zwingmann, H., R. Offler, T. Wilson and S. F. Cox (2004). "K–Ar dating of fault gouge in the northern Sydney Basin, NSW, Australia—implications for the breakup of Gondwana." Journal of Structural Geology 26(12): 2285-2295.