Sander's theory of schistosity is based upon the results of petrofabric analyses—especially statistical optical investigation of grain-orientation—of schistose rocks (Sander, 1930). There is an extensive German literature on the subject, much of which is not available in New Zealand. A summary in English of the principles upon which the method is based and the results achieved in the field of metamorphic petrology has recently been given by Mrs E. B. Knopf (1933), while instructive illustrations of the theory have appeared in papers by Gilluly (1934), Sander (1934), Knopf (1935) and Fairbairn (1935, 1935a).*
In addition a useful and clearly presented introduction (in English) to the whole subject of petrofabric analyses has recently been published by H. W. Fair-bairn (1935b), of Queen's University, Canada.
An outline of Sander's conclusions as to the origin and significance of schistosity is given below, the petrofabric evidence upon which his theory is based being as far as possible omitted:—
The first permanent deformation in a rock mass is accomplished by differential movement along parallel slip-surfaces. This movement involves rotation of mineral grains and gliding of individual grains along crystallographiec gliding-planes, and in this way for each mineral a regular crystallo graphic (space-lattice) orientation of the component grains is achieved. The study of the type of orientation-patterns and the degrees of orientation thus produced in the various minerals is an important part of petrofabric analysis, and yields valuable evidence bearing upon problems of schistosity.
The schistosity thus initiated typically is parallel to the slip-surfaces (cf. Becker). In some instances, however, schistosity is determined by elongation of mineral grains
[Footnote] *See also E. Ingerson, Am. Jour. Sci., vol. xxxi, pp. 161–187, 1936.
(e.g., quartz or calcite) in a plane (“plaiting-surface ”) bisecting the acute angle between two intersecting slip-surfaces, and hence is parallel to the AB plane of the strain ellipsoid (cf. Leith). Fairbairn (1935, 1935b) has suggested that schistosity parallel to plaiting-surfaces may prove to be more widely developed than has hitherto been recognised.
The slip-planes may be mechanically-induced surfaces arising subparallel to the circular sections of the strain ellipsoid. This condition usually holds in the case of relatively isotropie rocks such as shales, so that the resultant slaty cleavage of the deformed rocks has no relation to the original bedding.
In markedly anisotropic rocks the slip-planes typically are pre-existing surfaces of weak cohesion such as planes of bedding, flow, foliation or previously developed schistosity, all of which are included by Sander in a single category as s-planes.- During deformation of a mass consisting of alternating beds of variable competency, slipping along the intervening s-planes is often accompanied by development of flexures in the competent beds. The incompetent beds simultaneously yield by slipping, giving rise to minor drag-folds which are usually essentially slip-folds* (Knopf, 1935). When deformation is complete the original s-planes have been transposed into a new direction (Knopf, 1931, pp. 16–18) but still retain their identity. The schistosity so developed is parallel to the transposed s-planes and has been termed “transposition cleavage,” in contrast with “slaty cleavage ” which cuts across the original s-planes. This “transposition eleavage ” may come to lie in a direction normal to that of the external causal force (cf. Leith's view). Strain-slip cleavage may be regarded as an intermediate stage in the development of. “transposition cleavage ” and is therefore a feature of the early stages of deformation of anisotropic bedded rocks. It must be noted, however, that when an initially isotropic shale has acquired schistosity of the “slaty cleavage ” type, it has become markedly anisotropic; further deformation may result in isoelinal folding of the slaty cleavage, giving first strain-slip cleavage and perhaps ultimately a new schistosity of the “type. In slaty rocks of initially relatively isotropie character the development of strain-slip cleavage, therefore, is characteristic of the late stages of deformation, or may indicate repeated metamorphism (Knopf, 1935)
Schistose rocks which owe their structure to differential movement of the constituent grains during deformation belong to the major group of rocks which Sander terms
[Footnote] * As explained clearly by Mrs Knopf (Knopf, 1933, p. 464) slip-folds are structures resulting from pure non-homogeneous shearing along planes parallel to the axial plane of the slip-fold. Their presence therefore does not imply the existence of a compressive force acting normal to the fold axis.
tectonites. Two main divisions of the class are recognised on the basis of the type of motion involved in the deformation, as indicated by petrofabric analysis. In S-tectonites the motion is dominantly one of crystal gliding in the shear-planes (cf. the lateral displacement of a pack of cards); in B-tectonites the motion is essentially an external rotation of the component particles about the B axis of the strain ellipsoid, comparable with the movement of balls in a ball-bearing. The two classes of tectonite are not sharply separable: some degree of gliding of grains in the slip-planes in one or more directions perpendicular to B is always indicated in B-tectonites, while orientation-diagrams of S-tectonites invariably show traces of minor effects of external rotation. In many tectonites, especially B-tectonites, a linear element in the schistosity is developed at right angles to the direction of translatory movement and parallel to the axis of rotation (i.e., to B of the strain ellipsoid.) This linear direction has thus the same tectonic significance as the direction of fold axes. It is usually clearly marked in the hand-specimen, and may be due to (a) intersection of two slip-planes, or (b) minute corrugations and folds, or (c) elongation of mineral grains (e.g., quartz, calcite, amphibole, etc.) parallel to the B axis of the strain ellipsoid.*
In many schists, especially those of coarse grain, the structure is the result of crystallisation after cessation of tectonic movement. The schistosity is here due to growth of crystals with their long axes in the direction of greatest ease of growth, i.e., parallel to the best-developed s-planes. Crystallisation of this type, whereby a pre-existing anis-totropy is preserved and sometimes even accentuated in the recrystallised rock, is termed by Sander (1930, p. 172) Abbildungskristallisation (“mimetic crystallisation” of F. E. Suess and E. B. Knopf, “portrait crystallisation ” of H. W. Fairbairn). Two cases are to be distinguished:—
In porphyroblastic schists large crystals of albite, mica, hornblende, etc., with purely dimensional orientation may be enclosed in a tectonite matrix having space-lattice orientation. “Crystallisation-schistosity ” resulting from mimetic crystallisation has hitherto often been explained as due to the operation of Riecke's principle governing recrystallisation. Petrofabric analysis shows this explanation to be incorrect.
[Footnote] *For discussion on this latter point see Fairbairn, 1935b, pp. 46–48.
Any theory of schistosity should account for the fact that there is a single predominant direction of cleavage in most schistose rocks. Leith's hypothesis that cleavage is developed parallel to the AB plane of the strain ellipsoid readily explains this phenomenon; but if schistosity-planes are interpreted as shear-planes, as in the hypotheses advanced by Becker, Schmidt and Sander, the problem is more difficult to solve. Sander (1930, especially pp. 101–103) has discussed fully this question of Einscharigkeit. Petrofabric analyses, while not yet proving the truth or otherwise of Becker's ingenious explanation of the development of single schistosity by rotational strain (see under 5, p. 11), has thrown much light upon the question as a whole. Sander believes that the grain-fabric in most tectonites indicates the existence of at least two sets of s-planes. When one of these is more strongly marked than the others it usually defines the schistosity. When there are two equally well-developed sets of s-planes the result may be either a single macroscopic schistosity (plaiting-surface) bisecting the acute angle of intersection, or alternatively a symmetrically developed double schistosity. In cases where the original rock was markedly anisotropic, e.g., in a well-bedded sediment, shearing along only one set of s-planes (the original s in the undeformed rock) is to be expected.
From the above discussion it is clear that Sander's conception of schistosity is much broader than-previous hypotheses. Several distinct types of schistosity are recognised :—-
Schistosity parallel to shear-planes (s-planes), whether developed mechanically for the first time or moulded upon s-surfaces of a pre-existing anisotropy. Note that though the greatest importance is attached by Schmidt and Sander to Becker's hypothesis, it is clearly recognised that the latter is not a universal theory of s-planes nor does it explain all those structures to which the term schistosity is applied (Sander, 1930, p. 99).
Schistosity parallel to plaiting-surfaces, believed to be parallel to the AB plane of the strain ellipsoid (cf. Leith).
Schistosity developed parallel to pre-existing s-planes by mimetic crystallisation (cf. Leith). This includes post-tectonic crystallisation in the late stages of metamorphism, or non-tectonic erystallisation governed-by the s-planes of an undeformed anisotropie rock.
Tension joints perpendieular to B typically are well developed in tectonites. Sometimes these may be so closely spaced that they impart to the rock a pronounced fissility or schistosity which is thus parallel to the AC plane of the strain ellipsoid (Sander, 1930, p. 219).
It is also clear that the direction of the external deforming force can seldom be deduced from the schistosity of a rock. On the other hand, the “tectonic axis” parallel to B of the strain ellipsoid, and the orientation of shear-planes can usually be determined with considerable precision.