Introduction |
|
|
It has been demonstrated experimentally (Means, 1968; Etheridge and Hobbs, 1974; Williams et al., 1977), that when deformed layersilicates are replaced by new layersilicate grains of the same or different composition or structure, the new grains tend to grow with one of two relationships to the host grain. Either the new grains tend to mimic the orientation of the host grain or they tend to grow along kinkband boundaries within the host with (001) parallel to the kinkband boundary. The first of these relationships may be epitaxial and the second may simply be a case of growth-selection, with grains oriented with their fast growth direction (parallel to (001)) parallel to the kinkband, growing preferentially along a zone of highly disordered lattice. A consequence of these relationships, in the development of axial plane foliations, is that if an old layersilicate foliation is transposed by folding into the new axial plane orientation and the layersilicates are recrystallised during or after deformation, the orientation of the transposed foliation will be preserved if growth is epitaxial (e.g. Williams, 1985) and enhanced if there is growth parallel to the axial planes of micro-kinks. Together these processes are mechanisms of mimetic growth which has been invoked as an important process in the development of axial plane foliations in layersilicate-rocks (Williams et al., 1977).
Support for this model comes from preservation of relic kinks and crenulations, intrafolial to the new foliation (Williams et al., 1977), and from the fact that such foliations are generally bimodal. If a layersilicate foliation develops from a previous foliation by transposition by folding, the new foliation is expected to have a bimodal (001) fabric, with the two maxima, corresponding to the alternate limbs of the micro-folds, overlapping and symmetrical about the new axial plane foliation. A similar bimodal fabric can develop without micro-folding if individual layersilicates rotate in a less ordered manner, in response to shortening parallel to the original foliation (Hobbs et al., 1976, p. 247). At large strains the two maxima will merge, but many rocks do not reach this stage and careful observation of natural layersilicate fabrics very commonly reveals a bimodal distribution. The micro-folding origin of crenulation cleavage is obvious, but it is less obvious that penetrative foliations commonly develop by micro-folding because the combined processes of transposition and recrystallisation tend to destroy the evidence. The rocks described here, from Woody Island in Notre Dame Bay, Newfoundland, are particularly good from the point of view of demonstrating that penetrative foliations can develop in this way. The rocks include shale clast conglomerates and microconglomerates and because of differences in strain history and because of unusual primary features they preserve much better evidence of their fabric development, than is generally true of foliated rocks. Of prime importance from this point of view, is the fact that internally bedded clasts make it possible to identify the orientation of the original shaly parting (parallel to internal bedding) and the original long dimension of the clasts (also parallel to internal bedding) in deformed clasts where, because of the intensity of transposition and recrystallisation, it would otherwise be impossible. These rocks are described here in detail, as an unusually instructive example of foliation development. An integrated qualitative analysis of clast shape and orientation, and associated microstructures, also makes it possible to show that fold development involved a large component of shortening perpendicular to the axial plane throughout the process and this has general implications. |
|