Japan Geoscience Union Meeting 2016

Presentation information


Symbol S (Solid Earth Sciences) » S-CG Complex & General

[S-CG63] Dynamics in mobile belts

Mon. May 23, 2016 10:45 AM - 12:15 PM A08 (APA HOTEL&RESORT TOKYO BAY MAKUHARI)

Convener:*Yukitoshi Fukahata(Disaster Prevention Research Institute, Kyoto University), Norio Shigematsu(Research Institute of Earthquake and Volcano Geology, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology), Aitaro Kato(Graduate School of Environmental Studies, Nagoya University), Hikaru Iwamori(Geochemical Evolution Research Program, Japan Agency for Marine-Earth Science and Technology), Yasutaka Ikeda(Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo), Toru Takeshita(Department of Natural History Sciences, Graduate School of Science, Hokkaido University), Chair:Hitomi Nakamura(Department of Solid Earth Geochemistry, Japan Agency for Marine-Earth Science and Technology), Toru Takeshita(Department of Natural History Sciences, Graduate School of Science, Hokkaido University)

12:00 PM - 12:15 PM

[SCG63-12] K-Ar dating of fault movement in clay rich gouge: an example from the Alpine Fault at Gaunt Creek and Waikukupa River, South Island, New Zealand

*Horst Zwingmann1, Martin Timmerman2, Masafumi Sudo2, Roland Oberhänsli2, Virginia Toy3 (1.Department of Geology and Mineralogy, Kyoto University, 606-8502 Kyoto, Japan, 2.Institute of Earth and Environmental Science, University of Potsdam, D-14476 Potsdam, Germany, 3.Department of Geology, University of Otago, Dunedin 9054, New Zealand)

Keywords:fault gouge, Illite K-Ar dating, New Zealand

The occurrence of synkinematic and authigenic clay minerals, in particular illite, is a common feature in neotectonic fault gouges. Numerous attempts have been made to date fault gouges [see summary in Zwingmann et al., 2010]. We present new age data for synkinematic illite growth in two fault gouges from surface exposures of the Alpine Fault at Gaunt Creek and Waikukupa River, South Island, New Zealand. The Alpine Fault in the South Island of New Zealand marks the Australian-Pacific plate boundary. An amphibolite-facies mid-crustal ductile shear zone (mylonite series rocks) in the Pacific Plate hanging wall is exhumed along a current brittle fault marked by cataclasite and fault gouge.

Size separation combined with mineral characterization (SEM, TEM, XRD, LPS) enables to identify suitable samples for isotopic dating. Investigations of two <2 micron illite gouge separates from fault gouge samples collected from surface exposures at Gaunt Creek and Waikukupa River yield K-Ar ages of resp. 4.1 ± 0.4 and 1.9 ± 0.2 Ma, corresponding to the late Pliocene. K-Ar illite ages are consistent with well-defined field constraints and within error similar to c. 1 to 2.5 Ma 40Ar/39Ar ages for micas from hanging wall metapelites and amphibolites and to published K-Ar mica and near-zero apatite fission track ages. The corresponding illite and mica ages suggest that hanging wall rocks were rapidly exhumed and cooled c. 1 – 4 Ma ago with coeval exhumation resulting to extensive hydration in the brittle part of the Alpine Fault documented by illite authigenesis. Argon diffusion modeling supports the cooling timeframe. The ages of fault gouge illite provide absolute time constraints on the youngest, retrograde, neotectonic movements on this part of the Australian-Pacific plate boundary. This study highlights the potential of isotopic dating of synkinematic illite to determine upper crustal deformation events.

Zwingmann et al. 2010. Geology, v. 38, no 6, 487-490; doi10.1130/G30785.1