TECHNIQUES FOR VORTICITY ANALYSIS: A SELECTIVE REVIEW OF PROBLEMS ASSOCIATED WITH THEIR APPLICATION

Over the last fifteen to twenty years a considerable range of analytical techniques have been proposed for estimating flow vorticities in naturally deformed rocks. Microstructural input data for these techniques commonly include: a) orientation and shape factor of grains which are believed to have acted as rigid mineral phases rotating in a ductile deforming matrix; b) combined strain and quartz c-axis fabric data in which the orientation of the kinematic framework is inferred from the fabric data; c) data on the orientation of elongate dynamically recrystallized grains, combined with quartz c-axis fabric data, in which both data sets are used to infer kinematic framework. These microstructures/fabrics– based techniques have been applied in many tectonic settings including collisional, extensional and strike-slip dominated regimes. However, what is rarely included in these tectonics-scale studies are accounts of likely sources of error in individual vorticity analyses (e.g. degree of uncertainty in numerical values of input parameters) or discussion of how uncertainties in input parameter values may propagate through the analytical procedure. Allied problems include the “strain memory” of microstructures and fabrics during a progressive and potentially complex deformation history, and the assumption of plane strain deformation. In this presentation these likely sources of error are reviewed, taking examples from the Moine thrust zone mylonites of NW Scotland.