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Book Chapter

Structures: shear zones and folds

Published:
January 01, 2014

Abstract

New and existing methods for the analysis of finite strains in shear zones involving volume changes are reviewed. By assuming that the wall rocks are undeformed, the position gradients tensor can be determined from data derived from deformed passive markers with or without knowledge of the orientation of cleavage within the shear zone. The new methods are both algebraic and graphical and include those based on off-axis Mohr circles. Application of these methods to deformed sandstones at Marloes Sands reveals important volume changes that would invalidate an approach that assumes simple shear.

Abstract

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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Geological Society, London, Special Publications

Deformation Structures and Processes within the Continental Crust

S. Llana-Fúnez
S. Llana-Fúnez
Universidad de Oviedo, Spain
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;
A. Marcos
A. Marcos
Universidad de Oviedo, Spain
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;
F. Bastida
F. Bastida
Universidad de Oviedo, Spain
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Geological Society of London
Volume
394
ISBN electronic:
9781862396708
Publication date:
January 01, 2014

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