Abstract
Near-surface deformation related to neotectonics is accommodated by brittle faults.Displacement on the fault planes often results in the development of fault gouge composed ofrock fragments and authigenic illite. Previous studies highlighted the potential for determining theabsolute timing of brittle fault history using isotopic dating techniques (see summary, Zwingmann& Mancktelow 2004). The understanding of fault processes and the timing and extent of clay-richfault gouge formation is important for: (1) hydrocarbon exploration as faults may act as either aconduit or a seal for fluids and/or hydrocarbons; (2) civil engineering in the evaluation ofearthquake hazards and (3) ascertaining the suitability of sites for nuclear waste storage.K-Ar and preliminary 40Ar/39Ar data using micro-encapsulation of authigenic illite fromgouge developed in two different geological settings will be presented. The first study involvesgouge zones from the European Alps where the protolith consists of high grade metamorphic ormagmatic rocks (Zwingmann & Manckeltow, 2004). In these rocks a clear distinction is possiblebetween newly-grown illite, which is the only K-bearing mineral in the fine-grained fractionsseparated from the gouge, and the precursor assemblage. We dated fault movement on youngfaults related to the regional-scale Periadriatic Fault in the European Alps, using neocrystallineillite. The K-Ar ages are consistent both internally and with tightly defined field constraints.Average ages obtained from the < 2 micro m fractions that range from 5.2 to 21.5 Ma (2 sigma) arebracketed by AFTA and ZFTA ages from adjacent localities. This study provides absolute timeconstraints on the youngest, retrograde, neotectonic movements that occurred in the Alpineorogeny.The second study focuses on two fault zones from the northern Sydney Basin, Australia.The faults are hosted by Early Permian siltstones, tuffs and coal seams and foliated gouges,comminution and dilational breccias are developed within the fault zones. Clay minerals ofdetrital or authigenic origin are present within the sedimentary protolith and in the gouges. K-Arages obtained from samples in the gouge and tuffs in the damage zones range from 119 to 172 Marespectively. Illite ages of 196-245 Ma are thought to reflect the time at which diageneticsmectite-illite formed in the host rocks. 119-150 Ma ages from the <0.4 micro m fractions in the tuffsand gouges date the last slip event on the faults and can be related to the rifting of the easternmargin of the Australian continent. The data indicate the time at which diagenesis occurred in thehost rocks and slip movement on the faults and constrain the deformation events associated withthe rifting of the eastern margin of Gondwana during the Early Cretaceous.Zwingmann, H. & Mancktelow, N. 2004 Timing of Alpine fault gouges. EPSL, 223, 415-425.
