Strain and foliation refraction patterns around buckle folds
Marcel Frehner, Ulrike Exner, 2014. "Strain and foliation refraction patterns around buckle folds", Deformation Structures and Processes within the Continental Crust, S. Llana-Fúnez, A. Marcos, F. Bastida
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Axial plane foliation associated with geological folds may exhibit a divergent or convergent fan. Commonly, the foliation is assumed to reflect the major principal finite strain orientation. Here, the strain orientation around numerically simulated single-layer buckle folds is analysed in detail. Four different strain measures are considered: (1) finite strain, (2) infinitesimal strain, (3) incremental strain (recording the strain history from a certain shortening value until the end), and (4) initially layer-perpendicular passive marker lines. In the matrix at the outer arc of the fold, all strain measures result in similar divergent fan patterns. Therefore, divergent foliation fans around natural folds cannot readily be associated with the finite strain orientation as they may reflect other strain measures. In the simulated folds, the convergent fans in the stronger layer show differences between the different strain measures, which are associated with a 90°-switch of the major principal strain from a layer-perpendicular to a layer-parallel orientation at the outer arc. A similar observation is made in one of three studied natural folds (near Ribadeo and Luarca, NW Spain). It is suggested that the convergent foliation fan pattern inside a fold is better suited for strain estimates than the divergent fan around a fold.
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Deformation Structures and Processes within the Continental Crust
This volume presents a selection of papers showing the current focus of studies of deformation structures and processes within the continental crust. The selected contributions use a large range of analytical techniques suited to the full range of structure sizes and fine-tuned to the physical process that controls the deformation, from the grain boundary at the micro-scale, the lithological contact at the meso-scale to the plate boundary at the global scale.
The papers in the volume are grouped into three sections relating to specific lines of research within the analysis of rock deformation structures and processes, in particular in respect to the continental crust: structures within shear zones and faults; magmatic structures, and microstructures and rheology. These sections include papers describing field studies, experimental rock deformation and numerical modelling of deformation processes.