Die Mobilisierung der austroalpinen Dent Blanche Decke auf Fluid-induzierten Scherzonen während alpiner Hochdruckmetamorphose

Die Dent Blanche Decke ist ein Element des Austroalpins der Westalpen, welches das oberste strukturelle Stockwerk im alpinen Deckenstapel bildet. Als größte austroalpine tektonische Klippe stellt die Dent Blanche Decke die nordwestliche Fortsetzung der Sesia Zone dar. Zwei tektonische Elemente ba...

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Bibliographic Details
Main Author: Hellwig, Dirk
Contributors: Vogler, Stefan (Professor) (Thesis advisor)
Format: Doctoral Thesis
Published: Philipps-Universität Marburg 2003
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The Dent Blanche nappe belongs to the Austroalpine domain of the western Alps which represents the highest structural level in the Alpine nappe pile. Between the Rhone-Valley and Aosta-Valley, the Dent Blanche forms a huge klippe which consists of two subnappe units, the lower Arolla series (granites and country rocks) and the upper Valpelline series (pre-alpine basement rocks). Different parts of the Dent Blanche nappe memorized different stages of the Alpine tectonometamorphic evolution. Previous work in the southern Dent Blanche nappe (Roisan Zone, Höpfer & Vogler 1994, Höpfer 1995) revealed an alpine PTd-path including high pressure-low temperature (HP-LT) metamorphism during subduction to eclogite-facies conditions and concomittant major thrusting with nappe-formation during the first and second deformation phase (D1 and D2). The work documented in this dissertation focused on the northern Dent Blanche nappe. There, most of the Dent Blanche outcrop area consists of Arolla series metagranitoids and some metasediments whose metamorphic imprint was for decades interpreted as greenschist-facies, due to their "typical greenschist assemblages". For the first time, PT-data from these "greenschist-assemblages" is presented and give evidence that the northern Dent Blanche nappe experienced HP-LT metamorphism at 11,5-13 kbar and 360-440°C (blueschist to eclogite facies) during D1 and D2. Geobarometry was carried out utilizing the white-mica barometer of Massonne & Schreyer (1987) while geothermometry was performed on chlorite (Chl-thermometer, Cathelineau 1988) and on amphibole-plagioclase (Amph-Pl-thermometer, Blundy & Holland 1990). Alpine deformation continued in the northern Dent Blanche through the major folding-phase D3* at greenschistfacies conditions (5-6 kbar, ~300°C) and following brittle deformations D4 and D5. Compiled from the PT-data of this work, the first PTd-path for the northern Dent Blanche nappe differs from the classic "Western Alps type" (Ernst 1988) due to a significant amount of cooling during the first phase of decompression. The northern Dent Blanche nappe differs strongly from the southern Dent Blanche and other units of the western Alps due to its style of shear zone deformation. During D1 and D2, the Arolla rocks were heterogeneously deformed by shear zones of variable width, leaving large parts of the rock only barely deformed and creating a deformation gradient with highest strains acting in the center of the shear zones. Along these shear zones, the northern Dent Blanche nappe was segmented into a SE-dipping imbricate structure. Within the shear zones, the rocks affected by hydrolytic weakening processes, were mylonitised and phyllonitised. The most important deformation mechanisms active in the shear zones therefore were hydration, solution and precipitation. Hydration led to the important grain size reduction that facilitated diffusion along grain boundaries. Solution and precipitation enabled the mass transfer in the shear zones, well documented by the growth of quartz, white mica and actinolite on extensional fractures. Though affected by dynamic recrystallisation, solution and brittle deformation, quartz lasted longest within the strain-gradient. The strong fluidactivity which is documented by these mineral reactions, combined with a focused deformation lead to a progressive fluid-permeability in the shear zones. Shear zone rocks and country rocks experienced a profound geochemical equilibration which indicates a diffusive fluid-transport, along the shear zones and through the country rocks as well. The shear zone processes, summarised as "phyllonitisation", were essential for thrusting within and for the transport and emplacement of the Dent Blanche nappe as a whole. For the first time, this work correlates shear zone phyllonitisation with alpine tectonic nappe transport and stresses its importance for the tectono-metamorphic evolution of the Alps. Kinematic analyses (microstructural indicators, texture analysis, paleostressanalysis)throughout the northern Dent Blanche shear zones reveal a constant stress-field with a rotational component top-NW being active throughout all stages of the tectonic alpine evolution. There is no evidence of any SE-directed nappe movements which could be attributed to alpine exhumation.