Belts dominated by slate and other low-grade metamophosed sedimentary rocks are found in many orogens. They seem to come from a variety of depositional settings, e.g.:
Most characteristic rock types
We will look at cleavage and folds in turn.
Cleavage is the fabric that defines slate.
Originally, slaty cleavage was defined as a planar fabric that is totally penetrative. In other words, the fabric is continuously present at any magnification, on scales ranging down to the individual grains. (In other words, however much you magnify the rock, it's not possible to find a spot that is 'between' two cleavage planes, unless you go to the scale of individual mineral grains.)
[Photomicrograph of near-penetrative cleavage in slate]
Under high magnification, most cleavages are domainal at some scale; in other words its possible to see between the cleavage planes. Under these circumstances we distinguish between
In bedded rocks, slaty cleavage is almost always associated with a bedding-cleavage intersection lineation.
Sometimes there is also a faint mineral lineation on the cleavage surface, which is known as a down-dip lineation, from its most common orientation.
Most slate belts have a proportion of sandstone, varying from zero to 90%. Typically the sandstone layers are obviously folded, and less obviously cleaved than the mudrocks. Primary layering in slate intervals are sometimes less tightly folded.
Sandstone layers typically show class 1 geometry, with tighter curvature on the inside of the folds, known as buckle folds.
Where slate is more abundant than sandstones, the folds in the sandstone layers are typically rounded, with class 1C or 1B geometry. Wavelength is dependent on layer thickness, producing disharmonic folds if the sandstone layers are of variable thickness.
Where sandstone layers are closely spaced, the style tends to be more angular. Kink folds have typically 120° interlimb angles; Chevron folds have typically 60° inter-limb angles.
The typical fold-cleavage relationship in slate belts is 'axial-planar' with an intersection lineation parallel to the fold hinge.
[Ramsay and Humber 10-21, 21-32]
Commonly cleavage is not everywhere exactly parallel to the axial surface. the most common departure is cleavage refraction. Cleavage planes bend so that they are more nearly perpendicular to layering in more competent layers (e.g. sandstone).
In some slate belts the cleavage is rotated consistently either clockwise or counterclockwise of the fold axial surfaces. The folds are said to be transected by the cleavage.
Some slate belts display conspicuous joints and veins.
There is a complex terminology of veins, related to a now-obsolete system of fold "axes" a, b & c.
Joints parallel to the profile plane of a fold are known as "ac joints"
Joints parallel to folded layers, thickening in the hinge region, are known as "saddle reefs", famously mineralized in a number of goldfields.
The origin of cleavage has been the subject of some controversy. In particular, in some cases cleavage planes have been regarded as planes of shear, whereas other examples have been interpreted as planes of flattening.
To answer this question we need to look at strain markers in cleaved rocks. The overwhelming conclusion is that cleavage planes are usually very close to the S1S2 (or XY) plane, and therefore perpendicular to S3 (or Z) the direction of maximum shortening.
"Down-dip" lineations, where present, are probably stretching lineations, parallel to S1 (X).
A number of phenomena of deformed rocks are easily explained by cleavage perpendicular to shortening.
For example, folded layers typically have cleavage at a high angle to bedding, whereas the fine-grained slates have cleavage at lower angles to bedding. This phenomenon is commonly known as cleavage refraction.It indicates that the orientation of the strain ellipsoid varies between layers with different properties, and that the shear strain on bedding planes is higher in the slates.
Cleavage refraction
To understand the strain significance of cleavage refraction it is helpful to look at two idealized kinematic models of folding: Both are constructed for ideal parallel folds (class 1B).
Cleavage refraction can be thought of as the result of varying amounts of pure shear and simple shear relative to bedding planes.
Example of cleavage refraction at fold hinge
Notice incidentally that the kink construction (which we met in balancing cross-sections) implies that layers deform by simple shear on bedding surfaces only - ie flexural slip.
We therefore regard cleavage mainly as a product of shortening, and we have to ask how does shortening produce cleavage? In most examples, more than one process contributes:
Many types of rock have inhomogeneities that are more or less equant in an undeformed state. Examples include clasts, burrows, etc. All of these can be regarded as domains of one composition surrounded by another. On deformation, these domains become distorted, and a fabric is defined by the distorted domains.
Any strain results in the rotation of lines and planes toward the long axis of the strain ellipsoid and away from the short axis. These rotations lead to predictable development of fabrics as shown in the diagram.
Diagram of fabrics generated by randomly oriented lines and planes
Transposition describes the process by which a fabric (usually a planar layering) is separated into portions which are independently rotated into a new orientation.
Transposition process
Photo of transposed slate
Pressure solution: - occurs by stress concentration at grain contacts, leading to distortion of crystal lattices and eventually to preferential solution. Because grains are preferentially dissolved at contacts perpendicular to maximum compressive stress, then a fabric is generated. In some rocks it has been suggested that up to 50% volume loss occurred by pressure solution.
Pressure solution may also be concentrated in domains producing a layered fabric.
Stylolytic cleavage in limestone
Pressure solution spaced cleavage in sandstone
Where does the dissolved material go? In some rocks it seems to leave the system (maybe ending up in hydrothermal veins or cements higher in the crust.)
In others, signs of new mineral growth are found in 'pressure shadows' more accurately termed 'strain shadows' on the ends of grains.
Some features of mineral growth:
Mineral beards
Mineral beards
Strain shadows around pyrite
These types of features, if developed throughout a rock, may add new mineral grains with a strong preferred orientation, contributing to a fabric
Fabric of a typical slate
Single layers embedded in weak material undergo buckling when subjected to differential stress with the maximum compressive stress nearly parallel to the layers. A deflection nucleates a first wave. This then propagates along the layer.
Some theoretical analyses have predicted the wavelength of folds, but for materials of idealized properties (e.g. viscous, ideal plastic etc.). For newtonian (viscous) material these analyses show that wavelenth of a single buckled layer is controlled primarily by layer thickness.
Some experiments have been done to see what happens if multiple competent layers are present.