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Simple shear modification of chevron folds: implications for facing interpretation, strain analysis and deformation history

  • Chapter
Evolution of Geological Structures in Micro- to Macro-scales

Abstract

The fold facing concept has been widely utilized in the interpretation of multiply deformed terranes. The facing direction is unambiguous when the orientation of the axial plane and the younging direction of the folded layer can both be determined. Difficulties with the application of fold facing arise when either or both of these properties are ambiguous. Furthermore, the validity of the cleavage facing concept is critically dependent on the cleavage forming and remaining parallel to the axial planes of the associated folds. A problem arises when folds are subsequently modified and cleavages do not maintain an axial planar orientation (e.g. in shear zone environments). In this contribution we are concerned with establishing some geometrical properties of fold modifications which can affect the determination of fold facing and hence its applicability to the interpretation of complexly deformed terranes. Examples are taken from the Variscan fold and thrust belt of SW England, where initially upright chevron folds have been modified by folds and faults due to a subsequent southerly directed simple shear deformation. Finally, we use our observations to suggest a possible explanation for the so-called Variscan ‘facing confrontation’ said to exist in part of SW England.

Acknowledgements: Original field-work (GEL, JSW) was supported by UK NERC research grant GR3/4461. Subsequent field work expenses were provided by (GEL) Department of Earth Sciences, University of Leeds; and (JSW) Department of Geology, University of Portsmouth. The conscientious collection of field observations by Leeds University and (the then) Portsmouth Polytechnic undergraduate students (Nick Chinnery, Mark Enfield, Ben Follows, Marian Gadsby, Roy Gillchrist, Ruth Herman, Sandra Jackson, Mike Knowles, Phil Merrin, Simon Palmer, Sara Spencer and Samantha Willis) during their final year mapping projects contributed significantly to our interpretations. JSW gratefully acknowledges discussions with Prof. Robert Shackleton during field-work in the Padstow area.

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Authors and Affiliations

  1. Department of Earth Sciences, University of Leeds, UK

    Geoffrey E. Lloyd

  2. Department of Geology, Portsmouth University, UK

    John S. Whalley

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  1. Geoffrey E. Lloyd
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  2. John S. Whalley
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  1. Department of Geological Sciences, Jadavpur University, Calcutta, India

    S. Sengupta

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Lloyd, G.E., Whalley, J.S. (1997). Simple shear modification of chevron folds: implications for facing interpretation, strain analysis and deformation history. In: Sengupta, S. (eds) Evolution of Geological Structures in Micro- to Macro-scales. Springer, Dordrecht. https://doi.org/10.1007/978-94-011-5870-1_21

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  • DOI: https://doi.org/10.1007/978-94-011-5870-1_21

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