Discussion: Implications for folding |
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If folding were purely dynamic (Fletcher, 1974) with no homogeneous pure shear component, it would have to be accommodated by some form of flexural slip (can include tangential longitudinal strain in isolated layers). In layers accommodating the flexural slip component on a clockwise rotating limb, all clasts would rotate anticlockwise (Fig. 8c), relative to bedding. For a spherical clast the maximum rotation possible (assuming homogeneous flexural flow) is given by f=
a/2, where a is the dip of the limb (Williams and Jiang, in press). Thus on the clockwise rotating limb of an isoclinal fold a spherical object may rotate 45° anticlockwise relative to bedding. It would thus rotate 45° clockwise with respect to the axial plane of the fold. It is possible to increase the amount of rotation by localising the simple shear component of the flexural slip (cf. N-value of Jiang, 1994). However, large N-values are required to prevent the clasts rotating clockwise with respect to the axial plane and the problem is exacerbated by the fact that for non spherical objects, which do not rotate at a constant rate as a function of shear, the rotation is minimised when the clasts are parallel to the shear-plane. Thus clasts initially oriented parallel to bedding could never be rotated to define an axial plane cleavage unless it was in a very narrow layer with a very high N-value. There would then be no mechanism for developing an axial plane foliation in the fold hinge or the limb.
If folding were purely kinematic (Fletcher, 1974) and a product of homogenous pure shear, folds could only form by amplification of pre-existing perturbations and assuming that the shortening was parallel to the initial orientation of bedding, clasts with an initial variation as in Figure 8a, would rotate both ways on the same limb and both synthetic and antithetic cleavages would exist in equal numbers. The data presented here indicate that both dynamic and kinematic components of folding operated synchronously during deformation. It is probable that the bulk pure shear component was particularly large in relative terms at the beginning and end of the process, as is commonly believed (e.g. Ghosh, 1993, p.273). However, this component must have been large enough throughout the deformation, assisted by a weak simple shear component, to prevent at least a large percentage of the clasts, dipping in the opposite direction to the host-limb, from rotating so that they dipped in the same direction as the host-limb. A more thorough analysis of this material might place quantitative constrains on the relative importance of the dynamic and kinematic components, but is outside the scope of this paper. |
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