K-Ar dating of clay-rich fault gouge, northern Sydney Basin H. Zwingmann^ 201 T. Wilson^ and R. Offler^ ^ CSIRO - Division of Petroleum Resources, PO Box 136, North Ryde, NSW, Australia 1670 2 present address : Centre of Excellence in Mass Spectrometry, Curtin University, WA, Australia 6102 3 Department of Geology, The University of Newcastle, Callaghan, NSW, Australia 2308 The clay minerals illite and illite-smectite ( I I S ) are commonly present in fault gouges. However, until recently, few attempts have been made to use these minerals to date fault movements. We present K-Ar data obtained from 1/S formed during fault movement, from fault zones located in the northern Sydney Basin. This study is the first of its type carried out in Australia. In the first part of the study, eleven samples were selected from two fault zones and host rocks located at Burwood Beach, SSW of Newcasde. The faults occur in siltstones and tuffs of the Lambton Subgroup of the Newcastle Coal Measures. They are subparallel, approximately 1.5 m apart, trend N-S and have steep E dips. A total displacement of 10 m occurred on the western fault and 3 m of vertical displacement on the eastern fault. 1cm wide, foliated gouge zones are developed along both fault surfaces representing 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. They were disaggregated using a repetitive freezethaw technique to separate <2 and 2-6 [im fractions. The mineralogy of the fractions was determined by X-ray diffraction (XRD) on air dried, glycolated and heated (550®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 afi:er addition of an ^^Ar spike. XRD analyses of samples indicate that 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 7090% iUite, suggesting temperatures <100®C during faulting. Twenty two K-Ar dates have been determined. The ages of samples in the gouge and damage zones range from 126.8 to 164.5 (n=9; x=l48.3 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 <2 |Llm fraction. Older ages of 272 -281.8 Ma and 237-244.9 Ma for the 26 and <2 [im fractions, have been obtained from undeformed host rock. Radiogenic ^^Ar ranges for these samples range between 59.28 and 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 <2 |im fraction of some samples. The 272-281 Ma K-Ar dates obtained from the 26 \Xm fractions of the host rocks are older than the age of the sequence in which the samples occur (245252 Ma; Roberts et al., 1996). This is due to the presence of detrital muscovite which is a common phase in these rocks. The younger ages of 237244.9 Ma are thought to reflect the time at which diagenesis occurred. The <2 \lm ages (122-150 Ma) obtained from the fault gouge date the last slip event occurring on the faults, which is possibly related to underplating and uplift of the Australian continent. Samples for the second part of the study were collected from drill core intersecting the Hunter Thrust, a major NW trending, shallow NE dipping fault zone separating the Permian sediments and coal measures of the Sydney basin from the Carboniferous, forearc basin sediments and volcanics of the southern New England Fold Belt. Zones in drill core from two drill holes (DDH BMP 106A and 127), showing litde fracturing to intense comminution, were sampled for analysis. XRD studies indicate that I/S and kaolinite to be the dominant phases in the fractions separated from the highly comminuted samples; chlorite-smectite is an additional phase in the host rocks (siltstone, tuff). Preliminary results obtained from the highly comminuted samples reveal a wide variation in ages in the different fractions. In 106A, the ages range from 227.0 Ma (<2 \im) to 189.3Ma (1-2 |im) to 174.3 Ma (<0.5 |im) and in 127 from 226.7 Ma (2-6 |im) to l60.2-176.4Ma (<2 |LLm). The older age obtained from the 2-6 |Llm fraction in 127 is attributed to die presence of detrital, 2M polymorph which XRD analysis showed to be present. However, this explanation is not applicable to 106A which contains <5% of the 2M polymorph. Particle size analyses are required to ascertain whether <2 |Hm fraction contains a higher percentage of coarser particles than the 1-2 |Xm SGTSG abstracts, February 2001