Discussion: Foliation development |
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The different clasts in the material described here, are representative of different deformational environments in folds. The clasts oriented such that Ssi and Sse are parallel and perpendicular to S2e respectively (Fig. 4f), represent fold hinges and as in that environment the foliation in each layer or clast is oriented parallel to the general orientation of S2. Clasts oriented with Ssi perpendicular to Sse and therefore parallel to S2 (Fig. 4d.) represent fold limbs that have rotated without first crenulating so that no axial plane cleavage has developed. The folded foliation has simply rotated towards parallelism with the axial plane of the fold. The clasts inclined to S2 with asymmetrical crenulation cleavage (Fig. 4e), represent the limbs of folds where a crenulation developed prior to, or in the early stages of folding, and became increasingly asymmetrical as folding continued. Such axial plane foliations are generally inclined to the axial plane of the folds and in competent layers form a convergent fan such that the limb and cleavage are rotated in opposite directions relative to the axial plane. This is the same relationship as recorded within the shale clasts. The transposition model invoked here for the development of an axial plane foliation has previously been proposed on the basis of observations of folded layers in pelite (e.g. Hobbs et al., 1976, p.242). The Woody Island rocks allow us to successfully test the model. It provides different constraints to those implicit in the original data set but does not require any modifications to the model. The whole interpretation is internally consistent with all of the observations including variation in clast shape and orientation, cleavage morphology and the orientation of bedding within the clasts. Crenulation cleavage is a feature of S2i in the shale clasts and of S2e in some of the most mica-rich layers. Where not a crenulation cleavage, S2 is mostly a bimodal fabric. It may be defined for example by anastomosing mica-films or by the orientation of the long dimension of the clasts. It is suggested here that the processes operating in the development of the crenulation cleavage and of the various bimodal fabrics are essentially the same, except that deformation in the non-crenulated rocks is less ordered. Depending on their initial orientation with respect to the bedding-parallel bulk shortening direction, shale clasts and mica grains may rotate in opposite directions towards the same S2 orientation (Hobbs et al., 1976, p. 247; Means et al., 1984). There is no order to the sense of rotation of the individual grains and the result is a bimodal fabric unless the magnitude of strain is sufficient for the two maxima to coalesce. In the case of crenulations, rotation of the individual mica grains is subordinate to crenulation of the foliation so that there is a gradual variation in orientation from grain to grain. Neither crenulations nor bimodal fabrics are likely to form in situations where large-scale folding has a predominant dynamic component from the start, i.e. where folding is largely a buckling process with little deformation achieved by homogeneous bulk shortening parallel to the initial bedding orientation (cf. Fletcher, 1974). This is because the spin of the limbs will rapidly rotate all grains and clasts to an orientation clockwise of the shortening direction on a clockwise rotating limb (Fig. 8b) and anticlockwise on an anticlockwise rotating limb. All rotation relative to the axial plane, will then be in the same direction on a given limb and neither microfabric will be able to develop. Either fabric however, will still be able to develop in the fold hinge which explains why folds in some areas have crenulations and/or cleavage in the hinge but not on the limbs. Crenulation cleavage is typically developed in layersilicate-rich rocks and it is very noticeable in rhythmic sediments such as turbidites that the foliation can change in appearance from crenulation cleavage to, for example, an anastomosing foliation, in going from layersilicate-rich horizons to horizons rich also in quartz and lithic clasts. This suggests that the rotation mechanism, i.e. by folding or by rotation of individual grains, is controlled by the perfection of the initial anisotropy and/or the homogeneity of the initial grain-scale fabric. The presence in a sedimentary rock of quartz and lithic grains, which are less elongate than the layersilicates, generally tends to reduce the degree of preferred orientation of the latter, thus reducing the anisotropy. In addition, the mere presence of scattered strong grains must have an affect that is independent of its influence on the mica fabric. Intuitively, increasing the number of strong grains that lack internal planar anisotropy, will cause deflections in the stress axes and will render the anisotropy less penetrative even if the grains are as elongate as the layersilicates, and have a similar preferred shape orientation. The situation will be further exacerbated if the strong grains are less elongate and/or lack preferred shape orientation. Grain-scale heterogeneity in the orientation of the stress axes and reduction in the planarity of the foliation will cause grains or clasts to rotate independently, rather than following a pattern dictated by the folding of the initial foliation. Bimodal fabrics are common in layersilicate rocks, most slates and schists are bimodal and crenulation cleavage can be considered a special case of a bimodal fabric. It is suggested therefore that rotation of layersilicate grains either by crenulation of earlier foliation or by rotation of individual grains in a less organised manner is a major mechanism for the development of axial plane foliations. |
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