Influence of Strain Symmetry and Vorticity of Flow on Thrust Sheet Evolution at Mid-Crustal Levels in the Caledonides of NW Scotland: Implications for Transport-Parallel Extrusion

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Geometric analyses of thrust systems in mountain belts commonly make the general assumption that strain is essentially two dimensional, with no along-strike extension or contraction. Most orogenic belts, however, display a wide spectrum of grain shape fabrics ranging from pure S through L-S to L tectonites, suggesting that plane strain deformation may be more the exception than the rule. Formation of S and L tectonites presents major apparent space problems. For example, do S tectonites indicate flattening strains associated with along-strike extension, while L tectonites indicate constriction associated with along strike shortening? If these 3D strains are real then, in order to maintain strain compatibility, how is along-strike extension and contraction accommodated? Alternatively, do these tectonites merely reflect 'apparent' strains caused by strain superposition or volume change, with no real along-strike extension or contraction? Even if strain is two dimensional potential space problems may remain if microstructural or petrofabric data indicate a significant component of pure shear (coaxial) deformation. This is because pure shear deformation in turn implies a significant component of sub-horizontal transport-parallel extension. Some authors regard this space problem as a compelling reason for assuming simple shear deformation, while others have argued that components of pure shear may be of critical importance in extruding mid-crustal rocks towards the topographic surface. Numerical modeling of strains associated with thrust sheet emplacement has almost exclusively taken a 2D approach by assuming that observed 3D strains are 'apparent' strains produced by various combinations of plane strain (pure and simple shear) deformation oriented parallel to the transport direction. Arguably a stage has now been reached where strain modeling is more advanced than the field data needed to constrain it. Future meaningful progress can only be made by determining the actual strain paths (including strain magnitudes, symmetries and flow vorticities) associated with evolution of structurally well-constrained natural systems, thereby "ground-truthing" the assumptions made in these sophisticated models. This project involves generating such natural strain path data by integrated field mapping, strain and vorticity analysis in representative transport-parallel and orogenic strike-parallel traverses across the Moine Nappe and underlying mylonites of the Moine thrust zone. This part of the Caledonian Orogen is an outstanding natural laboratory for investigating domainal variation in 3D strain and vorticity of flow at mid-crustal levels because: a) our reconnaissance work indicates a wide range of strain, petrofabric and vorticity analysis methods are applicable to the plastically deformed rocks of the Moine Nappe and underlying Moine thrust, b) previous mapping over the last 120 years by generations of geologists from the British Geological Survey and academia has resulted in an exceptionally well-constrained structural framework for our strain/crystal fabric and vorticity data. In collaboration with co-workers, the natural data generated by this study are being used to develop more realistic 3D numerical models for flow associated with mountain building. The research is providing support for a Ph.D. student, will facilitate collaboration between U.S. universities and the British Geological Survey, and results from the project will form the nucleus for symposia and fieldtrips associated with a conference on 'Continental Tectonics and Mountain Building' that will be based in Scotland in May 2007.