Constraining timing of brittle deformation and fault gouge formation in the Sydney Basin

Abstract

Structural and K–Ar dating studies of gouge in N–S, NNE and E–W-trending faults in four locations in the Sydney–Hunter region are reported. The fault zones are manifest as joint swarms and highly brecciated zones containing gouge with authigenic illite produced as a result of fluid infiltration. Strike-slip movement accompanied by minor dip-slip, normal movement occurred on the NNE faults, with dip slip on N–S and E–W-trending faults. In this study, gouge from a NE-trending, steep, SE-dipping fault showing dip-slip movement at Cut 10, on the Hunter Expressway and from an E–W-trending, steep south-dipping, normal fault at the Westside Open Cut, Lake Macquarie have been analysed. K–Ar dating of illite and illite–smectite in fractions extracted from fault gouges in areas unaffected by a thermal overprint reveals ages varying from 166 to 119 Ma for the <2 μm and finer fractions, and a mean age of ca 120 Ma for the <0.4 μm fraction. In the Sydney area and the Westside Open Cut coal mine, Lake Macquarie, the ages obtained from similar size fractions both for the gouge and for the host rocks are younger (134–76 Ma; av. <0.4 μm = 111 Ma). The data indicate influence of a thermal overprint associated with subsurface magmas emplaced during the early stages of the rifting of eastern Gondwana during the early Cretaceous.

本文记录了对悉尼- 猎人区四个地点的N-S、 NNE和E -W向断层内断层泥的结构和K-Ar年龄所进行的研究。该断裂带表现为连接群和角砾度高的带，由于流体渗透的结果，具有带自生伊利石的断层泥。走滑运动伴有轻微的倾滑，正向运动发生在北北东断层上，上倾滑发生在Q1N-S和E -W向断层上。在这项研究中，我们对猎人高速公路上第十切口处显示倾滑运动的北东向、陡、东南倾断层中的断层泥，以及对Lake Macquarie的Westside露天矿的东- 西向陡峭南倾的正断层产生的断层泥进行了分析。对在未经受热叠印的区域的断层泥中提取的伊利石和伊利石- 蒙脱石碎片进行了 K-Ar年龄测试，其结果是：小于2微米和更微小的碎粒的年龄范围是116至119Ma；小于0.4微米的碎粒的平均年龄为120Ma左右。在悉尼地区和Lake Macquarie的Westside露天煤矿，对断层泥和主岩中类似粒级碎粒进行年龄测试的结果较为年轻些（134-76Ma; av. <0.4微米1/4 111Ma）。这些数据说明，早白垩世时冈瓦纳东部在开裂早期受到由于地下岩浆而导致的热叠加影响。

KEY WORDS:

• K–Ar dating
• gouge
• faults
• joints
• dykes
• eastern Australia
• Sydney–Hunter region

ACKNOWLEDGEMENTS

We thank Andrew Todd, CSIRO Petroleum, for analytical assistance, and Jenny Zobec, for assistance with the X-ray diffraction analysis. We thank Peter Chatburn and Vince Urbano, from the Hunter Expressway Alliance, for funding this study. We are also grateful to Neil Rae, Operations Manager, Westside Mine, Xstrata Coal, for giving us permission to use fault and dyke data collected during the operation of the mine, and to Yanyan Sun, for the preparation of the thin-sections.