The late Variscan rapid extrusion of ultra-high pressure metamorphic rocks into a preexisting nappe stack is the striking feature of the Erzgebirge, N-Bohemian Massif. Complex deformation increments, the large scatter of orientation and geometry of the finite strain ellipsoid as well as partly inverted metamorphic and age profiles are controversially discussed.
Structural analysis and geothermobarometry show that deeply buried continental crust emplaced under transpression with horizontal σ1 (NNW-SSE) and σ3 stress axes. Thereby, west-directed lateral escape of isothermally exhumed high-pressure units led to the formation of an exhumation channel. The pervasive fabric of quartz-feldspar rocks formed between 400–650 °C. Based on Ar-Ar geochronology, the deformation in the exhumation channel is framed between 340 and 335 Ma.
This preliminary results allow a modern texture analysis of natural shear zones, i.e.
electron back scattering and neutron diffraction of quartz from shear zones of the
exhumation channel. Because of an extensive and complex dataset, the crystallographic orientation of quartz is statistically analysed. I applied multidimensional scaling of the error between orientation density functions to visualize quartz textures together with additional microstructural features. I show that the temporal coexistence of two crystallographic orientation endmembers is the exclusive result of varying strain rates and differential stress. This thesis combines for the first time crystallographic textures of the Erzgebirge with modern plate tectonic concepts of the European Variscan orogeny.:Table of contents
PREFACE
Channel exhumation models in collisional Orogens
Texture evolution of quartz in orogenic shear zones
The structure of the thesis
PART 1: THE EXHUMATION CHANNEL OF THE ERZGEBIRGE: GEOLOGICAL
CONSTRAINS
1 Introduction
2 Geological Setting
2.1 The Variscan orogeny
2.2 The Saxo-Thuringian Zone as part of the European Variscides
2.3 Tectonics – constraints for an exhumation channel (<340 Ma)
3 Methods and Data processing
3.1 Field work, sample collection and selection
3.2 Geochemistry
3.3 MLA
3.4 EMP analyses and pressure-temperature estimations
3.5 Ar-Ar dating
3.6 Ar-Ar data handling and statistical treatment
4 Results
4.1 Geochemistry and Mineral Content of the Channel Rocks
4.2 Tectonics of the exhumation channel
4.2.1 Mica schists – roof of the channel
4.2.2 Paragneisses and Orthogneiss (type 1)
4.2.3 Orthogneiss (mgn)
4.2.4 Orthogneiss (type 2) – footwall of the channel
4.3 Petrology and Mineral chemistry
4.3.1 Garnet
4.3.2 Plagioclase
4.3.3 White mica
4.3.4 Biotite
4.4 Geothermobarometry
4.5 40Ar/39Ar – geochronology
4.5.1 Step heating
4.5.2 Single grain fusion
4.5.3 Ar-Ar and mineral chemistry
4.5.4 Ar-Ar and structural geology
5 The tectonometamorphic evolution of the exhumation channel
5.1 Local change in finite strain ellipsoid orientation
5.2 Evidence for advective heat transfer during exhumation
5.3 Position of the gneiss complex Reitzenhain-Catherine
5.4 Do Ar-Ar ages of the Erzgebirge represent cooling or recrystallization?
6 The channel model
6.1 Pre-channel stage – subduction
6.2 Channel stage – lateral extrusion
6.3 Post-channel stage – extensional doming
7 The Constrains for Texture analyses in a channel-type exhumation shear zone
PART 2: QUARTZ TEXTURE AND MICROSTRUCTURAL EVOLUTION IN A
CHANNEL-TYPE EXHUMATION SHEAR ZONE
1 Introduction
2 State of the Art
2.1 Dynamic recrystallization mechanism in quartz.
2.2 Texture evolution from natural and experimental deformed quartz
2.3 Quartz c-axis and textures in the Erzgebirge
3 Sample description
3.1 Mineral content
3.2 Quartz microstructures
3.2.1 Type 1 – Predominance of GBM
3.2.2 Type 2 – GBM overprints SGR
3.2.3 Type 3 – Equal ratio of GBM and SGR
3.2.4 Type 4 – Predominance of SGR
4 Methods
4.1 Time of Flight data processing and analysis
4.2 EBSD data processing and analysis
4.3 Multidimensional scaling
5 Results
5.1 Pole figure geometry
5.2 Multidimensional scaling
5.3 Texture properties and recrystallization
5.4 Grain and sample properties
5.5 Intragranular misorientation
5.6 Subgrain misorientation axes, slip systems and Schmid factor
6 Discussion
6.1 The dependence of quartz content and distribution and the particular CPO
6.2 The context between grain sizes, shape preferred orientation (SPO) and crystal
preferred orientation (CPO)
6.3 Active slip systems during ductile quartz deformation
6.4 Recrystallization mechanism and texture
7 Conclusions
GENERAL CONCLUSIONS
REFERENCES
APPENDIX
A Isochemistry during metamorphism
B Confidentiality of the PT estimations
C Discrepancy of WPA and WMA
D Appendix Figures
E Appendix Tables
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:71605 |
| Date | 28 August 2020 |
| Creators | Hallas, Peter |
| Contributors | Kroner, Uwe, Schaeben, Helmut, Pfänder, Jörg, Froitzheim, Nikolaus, TU Bergakademie Freiberg |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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