Dating brittle deformation in the Archean Yilgarn Craton, Western Australia

Major deformation throughout the Archean Yilgarn Craton has mostly been interpreted to be Neoarchean (Blewett and Czarnota, 2007). The timing of the deformation events of the brittle/ductile deformation generally relies on dating of cross-cutting intrusions or unconformities. Proterozoic overprinting and reactivation of Archean structures in the north-western part of the Yilgarn Craton has previously been dated from direct dating of the structures and fabrics from the Narryer Terrane(Spaggiari et al., 2008). However, the brittle deformation that postdates Neoarchean brittle-ductile structures in the Yilgarn Craton have received little attention to date. In the centre of the Yilgarn Craton, the Eastern Goldfields present a well developed network of E-W trending of normal brittle faults and fractures. Typically these structures are interpreted to have developed in result of a late Neoarchean tectonic relaxation following the main Yilgarn wide E-W contraction (Blewett and Czarnota, 2007). Poorly preserved and weathered faulted rocks in the subsurface environment preclude direct dating of fault gouge. However, exposure from the underground Agnew mine, in the Agnew Wiluna greenstone belt, recently provided access to fresh fault gouge material suitable for analysis. The clay gouge was characterized by SEM, TEM and XRD methods prior to age dating indicating an authigenic origin (Zwingmann et al., 2010). K-Ar illite age data of a whole rock sample split yielded an age of 1148 ± 23 Ma, which is within error close to the <2 micron clay fraction yielding an age of 1094 ± 22 Ma (Mesoproterozoic-Stenian). Our result is the first documentation of the age of the brittle deformation that affects the Yilgarn Craton. This age is within error of the Gilles event which is an extension event that affected the whole Australian continent and is responsible for the emplacement of the Warakurna Large Igneous Province and related dolerite dykes in the Yilgarn Craton (Evins et al., 2010). Blewett, R., and Czarnota, K., 2007, A new integrated tectonic framework of the Eastern Goldfields superterrane, Kalgoorlie '07, Volume Record 2007/14, p. 33-38. Evins, P., Smithies, R., Howard, H., Kirkland, C., Wingate, M., and Bodorkos, S., 2010, Redefining the Giles event within the setting of the 1120-1020 Ma Ngaanyatjarra rift, West Musgrave province, central Australia THE Record 2010/6: Perth, Geological Survey of Western Australia. Spaggiari, C.V., Wartho, J.-A., and Wilde, S.A., 2008, Proterozoic deformation in the northwest of the Archean Yilgarn Craton, Western Australia: Precambrian Research, v. 162, p. 354-384. Zwingmann, H., Mancktelow, N., Antognini, M. and Lucchini, R., Dating of shallow faults - new constraints from the AlpTransit tunnel site (Switzerland). Geology, 2010. 38 (6) p. 487-490; doi10.1130/G30785.1