• Tectonic studies at the S.W end of the Aar Massif, Switzerland

      Taylor, Christopher (University of BedfordshireUniversity of Bedfordshire, 1976-09)
      Comparisons are drawn of the effects of the Alpine orogeny on Mesozoic sediments and the underlying Hercynian massif basement rocks at the SW end of the Aar Massif. The study has also brought a clearer understanding of the pre-Permian geological history of the massif. The main lithological units in basement and cover are briefly described, and their mapping on 1 : 10,000 scale has shown large and medium scale structures. Deformation sequences have been established for the basement (D1b to D6b) and for the cover (D1c to D3c) and there is a clear correlation of Alpine st ructures (D1c to D3c equivalent to D4b to D6b). In the basement the late-Hercynian deformation of Upper Carboniferous metasediments is recognizable. These sediments were deposited on the older Altkristallin gneiss complex which had previously suffered the main Hercynian deformation and metamorphism. The trend of Alpine structures is subparallel to the trend of late-Hercynian structures, producing difficulties in discernment, and consolidating the regional NE-SW trend of basement structures. Alpine deformation was fairly simple, comprising one main deformation, followed by two weak phases producing only localized small-scale structures. Quantitative strain analysis for the main Alpine deformation investigates local situations and compares different types of strain marker. Incremental strains are recorded in curved pressure shadow quartz fibres in both basement and cover. These indicate early extension directions steeply plunging SE, later rotating through gentle plunge E, to subhorizontal NE and gentle plunge SW. Deformed Carboniferous pebbles, and cover ooids, intraclasts, faecal pellets, and ammonites have been analyzed, but all except faecal pellets suffer from the presence of initial preferred orientations and ductility contrasts with their matrix materials. Illite crystallinity studies show that the pelitic rocks investigated (basement and cover) reached a degree of Alpine metamorphism just inside low-grade as defined by WINKLER (1974) though the presence of graphite may have allowed this grade to be attained at relatively low temperatures. Further studies show this to have been a Barrovian-type syntectonic metamorphism. Generally similar finite Alpine deformation of basement and cover may be inferred from similar development of structures and similar quantitative strain estimates, but the very varied compositions and textures of rocks will have required somewhat different deformation mechanisms, and varied strain distribution.