K-Ar dating of fluid flow and fault movement in clay rich gouge: an example from the northern Sydney Basis

Full text: The occurrence of synkinematic and authigenic clay minerals, in particular illite, is a common feature in fault gouges. Until recently very few attempts have been made to date fault gouges. We present preliminary age data for synkinematic illite growth in two fault zones of the northern Sydney Basin. This study represents the first of its kind in Australia. Eleven samples were selected for this study from two faults situated at Burwood Beach within the northern Sydney Basin. The faults are subparallel, approximately 1.5m apart and trend N-S with steep easterly dips. A total displacement of 10m is observed on the western fault, whilst 3m of vertical displacement has occurred on the eastern fault. 1cm wide foliated gouge zones are developed along both fault surfaces that represent the regions in which principal displacement has occurred. Samples were collected from the gouge zones and host siltstones and tuffs within and outside the damage zone originating from the Lambton Subgroup of the Newcastle Coal Measures. They were disaggregated using a repetitive freeze-thaw technique to separate <2 and 2-6mm fractions. The mineralogy of the fractions was determined by X-ray diffraction (XRD) on air dried, glycolated and heated (550 deg C) samples. K content was determined by atomic absorption. Argon was extracted from the separated mineral fractions by fusing samples within a vacuum line and analysed for isotopic composition by mass spectrometry after addition of an 38Ar spike. XRD analyses of samples indicate that illite/smectite (I/S) and kaolinite in varying proportions are present in the fractions from the fault gouges and host rocks in the damage zone. In most samples, the I/S contains 70-90% illite, suggesting temperatures <100 deg C during faulting. Twenty three K-Ar dates have been determined. The ages of samples in the gouge and damage zones from Burwood Beach, range from 126.8 to 164.5 (n=9; x=148.3 Ma; sn=10.5) for the 2-6mm fraction and 122.2 to 150.9 Ma (n=10; x=137.2 Ma; sn=7.8) for the <2mm fraction. Older ages of 272 -281.8 Ma and 237-244.9 Ma for the 2-6 and <2mm fractions, have been obtained from undeformed host rock. Radiogenic 40Ar ranges for these samples range between 59.28 to 98.1% indicating negligible atmospheric Ar contamination. The reliability of the ages is also confirmed by the agreement within 2s analytical limits for the duplicate analyses of the <2mm fraction of some samples. The 272-281 Ma K-Ar dates obtained from the 2-6mm fractions of the host rocks at Burwood Beach are older than the age of the sequence in which the samples occur (245-252 Ma; Roberts et al., 1996). This suggests that detrital mica is present in these samples consistent with petrographic studies. The younger ages of 237-244.9 Ma are thought to reflect the time at which diagenesis occurred. The <2mm ages (122- 150 Ma) obtained from the fault gouge is thought to reflect the last slip event occurring on the faults, which is possibly related to underplating and uplift of the Australian continent. This study highlights the potential and value of isotopic dating of synkinematic, diagenetic illite to determine upper crustal deformation events. Copyright (1999) Geological Society of Australia