Brittle-Ductile Deformation Behaviour in the Middle Crust, as exemplified by Mullions (Former “Boudins”) in the High-Ardenne Slate Belt Belgium

Nearly a century ago the terms ‘boudin’ and ‘boudinage’ have been introduced in the Ardenne-Eifel area to describe particular ‘sausage’-like structures. The term ‘boudin’ is now generally acknowledged in the geological terminology to describe structures that are the result of layer-parallel extension, i.e. ‘boudinage’. The structures in the Ardenne-Eifel area, however, turn out not to comply with the latter definition, which lead to a controversy on the true nature of these particular structures, i.e. the ‘boudinage question’. Eventually a renewed interest in the veins allowed to give a conclusive answer to this controversy. The key to the solution has to be found in a multidisciplinary approach, integrating structural, mineralogical, geochemical and microthermometrical analyses. What started of as a pure academic search for the solution to the ‘boudinage question’, ended up with a more general understanding of fundamental deformation processes occurring in the brittle-ductile transition zone in the middle crust, such as ‘hydraulic fracturing’ and ‘layer-parallel shortening. The Ardenne-Eifel area not only turns out to be an exposed fractured reservoir, it also bears the evidences of the viscous rheology of the middle crust.

GENERAL INTRODUCTION
Chapter 1: “Boudins” in the Ardenne-Eifel area. A historical review
1.1. The beginning
1.2. Introduction of the terms “Boudins” and “Boudinage” and discussion on the genesis of structures
1.3. Confusion
1.4. Conclusion

Chapter 2: Geodynamic setting
2.1. Overview
2.2. Stratigraphy of the Lower Devonian
2.3. Stratigraphy of the High-Ardenne slate belt
2.4. Structural architecture of the High-Ardenne slate belt
2.5. Metamorphism
2.6. Structural features
2.7. Synthesis

FROM “BOUDIN” TO MULLION
Chapter 3: Structural analysis
3.1. Studied outcrops in the High-Ardenne slate belt
3.2. Synthesis of meso- and macroscopic geometrical analysis
3.3. Fabric analysis
3.4. Interpretation
3.5. Evolutionary model
3.6. Discussion
3.7. Summary

Chapter 4: Fluid flow and formation of the veins
4.1. Fluid inclusions
4.2. Methodology
4.3. Fluids in the High-Ardenne slate belt
4.4. Mineralizing fluid
4.5. Spatial and chronological evolution of the fluids in the High-Ardenne slate belt
4.6. Determination of the P-T conditions at the time of veining
4.7. Discussion
4.8. Summary

Chapter 5: Kinematic model
5.1. Overview
5.2. Nomenclature
5.3. Kinematic model

MULLIONS AS A PALAEORHEOLOGICAL GAUGE
Chapter 6: Numerical mullion model and parameter sensitivity analysis
6.1. Mullion model
6.2. Parameter sensitivity analysis
6.3. Discussion
6.4. Summary

Chapter 7: Rheology of psammites deforming in the middle crust
7.1. Parameter estimation method
7.2. Case studies
7.3. Discussion

DISCUSSION AND CONCLUSIONS
Chapter 8: Discussion
8.1. Fractured reservoirs
8.2. Rheology of the middle crust

Conclusions

References
Appendix: content of CD-Rom
Acknowledgements

Ilse Kenis