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The occurrence of synkinematic and authigenic clay minerals is a common feature in fault gouges. Few attempts have been made to date fault gouges. We present the first age data in Australia for synkinematic illite–smectite growth in two fault zones of the northern Sydney Basin, NSW. The faults occur at Burwood Beach, NSW in the northern part of the Sydney Basin and are hosted by Early Permian siltstones, tuffs and coals of the Lambton Formation, Newcastle Coal Measures. The faults are 1.5m apart, show normal displacement and trend N–S with steep easterly dips. Foliated gouge zones, comminution and dilational breccias are developed along both fault surfaces. K–Ar ages extracted from samples in the gouge and tuffs in the damage zones are 172 (6–10μm) to 119Ma (<0.4μm), respectively. Older ages of 272–281Ma for the coarse fractions (>2μm), 237–245Ma for the <2μm fraction, 218Ma for the <0.4μm fraction and 196Ma for the <0.1 μm fraction have been obtained from siltstones within and outside the damage zone. We believe the younger ages of 196–237Ma indicate the time at which diagenetic illite–smectite formed and the 122–150Ma dates from the <2μm fraction represent the maximum age of gouge formation. The younger ages are thought to reflect the last slip event occurring on the faults, which is related to the rifting and dispersal of the eastern margin of the Australian continent. [Copyright &y& Elsevier]