The modification of chevron folds by simple shear: examples from north Cornwall and Devon

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Abstract
This contribution considers the effect of a horizontal simple shear deformation on an initially upright chevron fold pair using geometrical models. The models utilize an initial fold geometry based on structures observed in the coastal sections of north Cornwall and Devon, and consider various mechanisms by which the simple shear could be accommodated. Due to their different initial orientations the limbs of the chevron folds rotate at different rates during simple shear, and this is accommodated by the modification of the fold structure. Possible modification structures include: normal faulting in the opposite direction to the sense of shear on either the axial planes or the limbs of the folds; and folding on the limb which dips in the same general direction as the sense of shear. The results of the modelling can be used to interpret the structures observed in the studied sections. It is shown that a southerly-directed simple shear deformation acted in discrete zones resulting in the local modification of certain chevron folds. This deformation is in the opposite direction to the main Variscan movements in SW England. Nevertheless, it is argued that the simple shear is an integral component of the main deformation and represents an analogous process to the mechanism of backthrusting present in most thrust belts.