Die Mobilisierung der austroalpinen Dent Blanche Decke auf Fluid-induzierten Scherzonen während alpiner Hochdruckmetamorphose
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Abstract
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.
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Metadata
Contributors
Supervisor:
Dates
Created: 2003Issued: 2004-06-18Updated: 2005-03-16
Faculty
Fachbereich Geowissenschaften
Publisher
Philipps-Universität Marburg
Language
ger
Data types
DoctoralThesis
Keywords
ScherzonenShear zonesDent Blanche nappeStructural geologyDent Blanche Deckemetamorphe Fluide ,Metamorphic fluidsHigh-pressure metamorphism
DFG-subjects
StrukturgeologieHochdruckmetamorphoseWestalpen
DDC-Numbers
550
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Hellwig, Dirk (109493842): Die Mobilisierung der austroalpinen Dent Blanche Decke auf Fluid-induzierten Scherzonen während alpiner Hochdruckmetamorphose. : Philipps-Universität Marburg 2004-06-18. DOI: https://doi.org/10.17192/z2004.0139.
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This item has been published with the following license: In Copyright