Conclusions |
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(1) In the Woody Island rocks, rotation is the single most important process in the development of preferred orientation of clasts and mica grains to define an axial plane foliation. The mica grains are of metamorphic origin and probably mimetic after sedimentary grains. The rotation may occur at a clast or granular-scale and may be random or it may be more spatially organised and a product of micro-folding (crenulation) of the initial foliation. At large strains crenulations may tighten until they completely transpose the original foliation into a new foliation. Rotation tends to produce bimodal fabrics and the ubiquity of such fabrics elsewhere, suggests that this mechanism is a very common one in the development of axial plane foliations in general.
(2) Folding on Woody Island involves dynamic and kinematic components and the kinematic component appears to be large. The fact that the foliations and folds are typical of those seen elsewhere in similar sedimentary sequences, including for example turbidites, suggests that this may generally be true in sedimentary sequences. (3) Whatever the mass transfer mechanism for differentiation in rocks, there is a correlation between strain path partitioning and compositional domains. (4) Axial plane foliations developed by the mechanism proposed here will not track strain axes and will not ultimately lie precisely parallel to a principal plane of strain. They are likely to be close to a principal plane however, and how close will depend on the relative importance of dynamic and kinematic components during folding. The larger the kinematic component the closer the orientation. (5) Rocks that contain more than the usual amount of evidence of their history are by definition unusual and therefore less common than most. It is important to recognise the value of such rocks. Particularly important in the Woody Island rocks are the shale clasts, especially those that contain recognisable bedding. The bedding makes it possible to recognise Si as opposed to S2i, and to know whether a clast has maximised rotation or shortening, because its original shape can reasonably be assumed once bedding is recognised (the long dimension is generally parallel to bedding in shale clasts). By the same argument, irrespective of its origin, since it is observed to be parallel to bedding, Si must also have been parallel to the original long dimension of the clast. Since Si is now a metamorphic foliation (albeit mimetic after bedding), in the absence of compositional bedding, its orientation with respect to clast shape would be unknown.
Acknowledgements The writer acknowledges many interesting discussions with Win Means over the years on this and other topics. It is a pleasure to be associated with this production. Dazhi Jiang is gratefully acknowledged for stimulating discussion and for critical reading of this contribution. The work was supported by a research grant from NSERC. |
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