The Arosa zone in Eastern Switzerland: oceanic, sedimentary burial, accretional and orogenic very low- to low grade patt
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The Arosa zone in Eastern Switzerland: oceanic, sedimentary burial, accretional and orogenic very low- to low grade patterns in a tectono-metamorphic me´lange Rafael Ferreiro Ma¨hlmann • Matthias Giger
Received: 30 November 2011 / Accepted: 14 May 2012 / Published online: 27 October 2012 Ó Swiss Geological Society 2012
Abstract In the area of Arosa–Davos–Klosters (Eastern Switzerland) the different tectonic elements of the Arosa zone me´lange e.g. the Austroalpine fragments, the sedimentary cover of South Penninic ophiolite fragments, as well as the matrix (oceanic sediments and flysch rocks) show distinctively different metamorphic histories and also different climaxes (‘‘peaks’’) of Alpine metamorphism. This is shown by a wealth of Ku¨bler-Index, vitrinite and bituminite reflectance measurements, and K-white mica b cell dimension data. At least six main metamorphic events can be recognized in the area of Arosa–Davos–Klosters: (1) A pre-orogenic event, typical for the Upper Austroalpine and for instance found in the sediments at the base of the Silvretta nappe but also in some tectonic fragments of the Arosa zone (Arosa zone me´lange). (2) An epizonal oceanic metamorphism observed in the close vicinity of oceanic basement rocks units of the Arosa zone (South Penninic) is another pre-orogenic process. (3) A metamorphic overprint of the adjacent Lower Austroalpine nappes and structural fragments of the Lower Austroalpine in the Arosa Editorial handling: Fernando Nieto Garcı´a and Edwin Gnos. R. Ferreiro Ma¨hlmann (&) Technical and Low Temperature Petrology, Institut fu¨r Angewandte Geowissenschaften, Technische Universita¨t Darmstadt, Schnittspahnstrasse 9, 64287 Darmstadt, Germany e-mail: [email protected] R. Ferreiro Ma¨hlmann Lehrstuhl fu¨r Geologie, Geochemie und Lagersta¨tten der Kohle und des Erdo¨ls, RWTH Aachen University, Aachen, Germany
zone. This metamorphic overprint is attributed to the orogenic metamorphic processes during the Late Cretaceous. (4) A thermal climax observed in the South Penninic sediments of the Arosa zone can be bracketed by the Austroalpine Late Cretaceous event (3) and the middle Tertiary event (5) in the Middle Penninic units and predates Oligocene extension of the ‘‘Turba phase’’. (6) North of Klosters, in the northern part of our study area, the entire tectonic pile from the North Penninic flysches to the Upper Austroalpine is strongly influenced by a late Tertiary high-grade diagenetic to low-anchizone event. In the Arosa zone me´lange an individual orogenic metamorphic event is evidenced and gives a chance to resolve diagenetic–metamorphic relations versus deformation. Six heating episodes in sedimentary rocks and seven deformation cycles can be distinguished. This is well explained by the propagation of the Alpine deformation front onto the foreland units. Flysches at the hanging wall of the me´lange zone in the north of the study area (Walsertal zone) show data typical for low-grade diagenetic thermal conditions and are therefore sandwiched between higher metamorph
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