PERTURBATIONS, BANDED, P, S,PS
Banded perturbations will be classified according to the nature and sense of the differential displacements described in the previous section.
In ideal P-bands, the only differential displacements are normal to the banding; in S-bands, they are parallel. A single P-band implies a local dilatation anomaly. In a p+ band, the dilatation is
positive, i.e. there is a volume gain; in a P- band, there is a volume loss. A single S-band implies a local shear anomaly.In an S+ band, the shear is sinistral; in an S+ band, dextral. Between the two ideal end members, there is a whole range of
bands of mixed type, PS-bands.
In general the differential displacements are of finite magnitude and it is appropriate to talk about finite P-bands or finite S-bands. However, over a small increment of deformation, the displacements may be small and it is then
convenient to talk of incremental bands. It is conceivable for a given band to be of incremental P-type at one stage of development, and of incremental S-type at a later stage: the finite end-product will then be a PS-band. Given the possible complexities in its evolution, no natural
band is likely to be of pure P- or S-type, but it may still be useful to talk loosely, for example, of a P-band when the differential displacements have been mostly of the extension (or shortening) variety.
The advantages of the proposed classification are several. First, the basis of the classification is purely geometric and nongenetic, permitting classification to proceed even if the mechanics
of formation are obscure. Second, there are few categories. Third, local deformations are distinguished
from the regional average. Fourth,
the names of categories are mnemonic and draw an analogy with seismic motions (P and S waves). Fifth, the classification includes the limits where deformation is discontinuous, as in faults. Sixth, there are no restrictions on the size of the bands. Finally, the classification by no means replaces existing names for geological
structures, but groups these according to
features that they share. One disadvantage of the classification is that
it is applicable ideally to deformation of continuous materials and therefore may break down, for example, on the scale of component grains of a rock. Second, the magnitude and occasionally the type of a perturbation may be difficult to on the presence of strain markers, symmetry
features, and other diagnostic evidence. A discussion of this problem is given in the following section.
Band-like Geological Structures in Rocks
In a number of different types of deformed
rocks, the deformation is sometimes band-like . Bands with a characteristic style of deformation have well-established names (e.g. kink bands, deformation bands, shear zones, pressure-solution seams, extension gashes, wrench faults, mylonite zones). These structures are easily identifiable because they contain
evidence of anomalously intense deformation (e.g. well developed mineral fabrics, highly deformed objects, anomalous layer orientations, anomalous chemical composition, relatively small grain size). The margins of the structures
are usually well defined. Their true form is often ellipsoidal or lenticular, but as a first approximation one may consider the structures to be bands with parallel planar margins of indefinite extent. Within the limits of this approximation, the theory developed in previous sections indicates
that the structures have developed by differential simple shear, differential simple extension, or a combination of both.
Diagnostic evidence from certain well studied examples will be reviewed below in an attempt to classify structures as P-bands or S-bands. Shear Belts or Shear Zones. Ramsay and Graham (1970) studied several examples of deformation anomalies in lowgrade and high-grade metamorphic rocks. They analyzed the shapes of deformed objects, the
displacement of cross-cutting planar features and the intensity and orientation of schistosity. They concluded that the rocks outside many deformation zones were themselves undeformed (or deformation was undetectable), whereas the rocks within had undergone a heterogeneous simple shear. In other examples, the rocks outside the zones were homogeneously deformed, whereas those within had undergone a more
complex deformation, with components of
homogeneous strain and heterogeneous, simple shear. Ramsay and Graham termed all these deformation anomalies "shear-belts" or "shear zones". Within the present classification, the structures would be finite S-bands.
Perturbation type P
'Pressure-solution' seams + Stylolites
Alternating parallel bands
Differentiated gneissic layering (?)
Extension gashes with
spaced cleavage (in
Alternating oblique bands
Conjugate 'normal' kinks
Kink bands are common in foliated rocks and have been produced by experimental deformation of rocks or foliated model materials (Paterson and Weiss 1966; Weiss 1968). The experiments show that as a kink band develops, the margins undergo differential shear displacements.
Deformation within the band is accomplished by a finite rotation of the layering or foliation. Individual layers undergo little internal strain,but slide past one another. At the margins of the kink band, the layers bend. The rotation,
sliding, and bending may be considered as
microcomponents of a heterogeneous simple shear, the shear direction being parallel to the margins. On a scale larger than that of the layer thicknesses, experimental kink bands are therefore
finite S-bands. Experimental kink bands resemble their natural counterparts in most respects and it is therefore highly probable that most natural kink bands are dominantly S-bands. The exceptions
to this generalization are normal kink bands, which apparently cannot develop without an internal volume reduction (Ramsay 1967). These structures are presumably PS-bands. 'Pressure-solution' Seams In many examples of seam-like structures, the apparent displacement of cross-cutting features and the orientation of internal cleavage indicate that the
seam margins have been displaced toward one another during deformation. Cleavage is not so intense outside the seams and also there is a difference in bulk chemical composition between a seam and its surroundings. Durney (1972) was
able to show that some constituents (e.g. quartz) are removed from a seam, leading to a relative enrichment in others (e.g. mica). This compositional differentiation is attributable to mass transport caused by stress ('pressure solution'). All the evidence points to a differential shortening
across the seams, which can therefore be
classified as P- -bands. Similar Folds (Including Chevrons) Many natural folds approximate to a 'similar' geometry (Ramsay 1967), with parallel .axial
planes. On a scale larger than that of individual layers, such folds can be formed only by a differential simple shear, a fact long recognized by certain geologists and demonstrated by Ramsay (1967) and, more conclusively, by Hobbs (1971). One way of generating the differential shear is by finite buckling of an anisotropic rock (Cobbold 1977b), although other mechanisms
seem feasible. The sense of differential shear reverses from limb to limb of similar folds and the structure is therefore composed of alternating S+ and S- bands. Associated with this perturbation is
an average strain. Other Structures
Using the arguments outlined in the previous section, a classification of many familiar bandlike structures or groups of them has been attempted. The list is almost certainly incomplete, and in some instances may be speculative or incorrect, but it serves the dual purpose of emphasizing basic geometric