Inclined K–Ar illite age spectra in brittle fault gouges: effects of fault reactivation and wall‐rock contamination |
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| Authors: | Espen Torgersen Giulio Viola Horst Zwingmann Iain H C Henderson |
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| Institution: | 1. Geological Survey of Norway, Trondheim, Norway;2. Department of Geology and Mineral Resources Engineering, Norwegian University of Science and Technology, Trondheim, Norway;3. CSIRO, Earth Science and Resource Engineering, Bentley, WA, Australia;4. School of Earth and Environment, The University of Western Australia, Crawley, WA, Australia;5. Department of Applied Geology, Curtin University, Bentley, WA, Australia |
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| Abstract: | K–Ar clay fraction ages of brittle faults often vary with grain size, decreasing in the finer size fractions, producing an inclined age–grain‐size spectrum. K–Ar ages and mineralogical characterization of gouges from two normal faults in the Kongsberg silver mines, southern Norway, suggest that inclined spectra derived from brittle fault rocks reflect the mixing of inherited components with authigenic mineral phases. The ages of the coarsest and finest fractions constrain faulting at c. 260–270 Ma and reactivation around 200–210 Ma, respectively. This study demonstrates how wall‐rock contamination influences the K–Ar age of the coarsest size fractions and that authigenic illite and K‐feldspar can crystallize synkinematically under equivalent conditions and thus yield the same K–Ar ages. |
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