Dating of shallow faults: New constraints from the AlpTransit tunnel site (Switzerland)

Abstract

Research Article| June 01, 2010 Dating of shallow faults: New constraints from the AlpTransit tunnel site (Switzerland) Horst Zwingmann; Horst Zwingmann 1CSIRO Earth Science and Resource Engineering, PO Box 1130, Bentley, WA 6102, Australia, and School of Earth and Environment, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia Search for other works by this author on: GSW Google Scholar Neil Mancktelow; Neil Mancktelow 2Department of Earth Sciences, ETH-Zurich, CH-8092 Zurich, Switzerland Search for other works by this author on: GSW Google Scholar Marco Antognini; Marco Antognini 3Museo Cantonale di Storia Naturale, Viale C. Cattaneo 4, CH-6900 Lugano, Switzerland Search for other works by this author on: GSW Google Scholar Raffaele Lucchini Raffaele Lucchini 4Geolog.ch SA, Viale Stazione 16a, CH-6501-Bellinzona, Switzerland Search for other works by this author on: GSW Google Scholar Geology (2010) 38 (6): 487–490. https://doi.org/10.1130/G30785.1 Article history received: 16 Oct 2009 rev-recd: 21 Dec 2009 accepted: 30 Dec 2009 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Horst Zwingmann, Neil Mancktelow, Marco Antognini, Raffaele Lucchini; Dating of shallow faults: New constraints from the AlpTransit tunnel site (Switzerland). Geology 2010;; 38 (6): 487–490. doi: https://doi.org/10.1130/G30785.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Methods for dating shallow faults in the Earth's crust are still evolving and their reliability remains controversial. Based on results from well-preserved fault gouge samples from the AlpTransit deep tunnel site in Switzerland, we propose a simplified and readily applicable approach to investigate and date clay-rich gouge zones. We collected 10 samples covering a north-south section of 15 km. All sampled faults are developed in orthogneiss host rock, have a rather consistent approximately northwest-southeast strike, are mainly normal, and are related to northeast-southwest orogen-parallel extension during Alpine exhumation. Fine-grained clay separates from the fault gouges consist of illite 1M/1Md and 2M1 polytypes, smectite and chlorite, with minor amounts of feldspar in some sample fractions. The K-Ar ages for <2 μm illite fractions range between 7.1 and 9.5 Ma (mean 8.5 ± 1.4 [2σ] Ma) and for <0.1 μm range between 3.9 and 7.2 Ma (mean 6.0 ± 2.1 [2σ] Ma); ages consistently decrease with grain size. These ages are close to published apatite fission track ages of ca. 6 Ma from the immediate vicinity, consistent with illite stability in fault gouges generally occurring at temperatures slightly above the partial annealing zone of apatite. The influence on authigenic illite gouge ages of potential contamination by fine-grained cataclastic protolith was evaluated by dating coarse K-feldspar fractions from host-rock clasts in two samples. The K-feldspar ages are significantly older, ca. 13.5 Ma. However, measured illite K-Ar ages are quite constant and do not correlate with amount of K-feldspar impurity, which suggests that fine-grained cataclastic feldspar grains have isotopically reequilibrated, presumably due to fluid-rock interaction within the fault zone. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.