Cleavage Refraction

[Martta]

CLEAVAGEREFRACTION Cleavage refraction occurs in layered rocks, when there is appreciable rheologic difference in response to stress. The smaller cleavage-hedding angle is always characteristic of the less competent layers, the greater angle is found in the more competent layers. The numerical

difference in value of the angle is a function of competence contrast and of the location in the fold. If the geometry of the fold is associated with higher strains on the overturned limb than on the normal limh (as is frequent), the cleavage refraction is more marked on the normal limb than on the overturned limb. Generally the axial surjace of the fold lies with an orientation between the two directions of the cleavage developed in competent and incompetent layers.

The geometric relationships described above hold true irrespective whether the fold is an upward or downward facing structure whether this fold is an antiformal anticline or antiformal syncline etc. definitions . If the polarity of the layers in a fold can be determined (e.g. with sedimentation structures such as cross bedding or lithological grading) then these observations can be combined with cleavage bedding relationships to determine the jacing direction of a old. Once this facing direction has been established then the cleavage -bedding relationships at individual outcrops can be used to determine the stratigraphic polarity of the beds.

Clockwise from top: Refraction in Martinburg formation; refraction on the western limb of Rhoscolyn anticline; refraction in turbidite sequence at Barmhan (Mahakoshal group) and interpretation of cleavage refraction as regards the finite strain state.

In metamorphic rocks forming at depth in tectonic regions, cleavage forms by the planar alignment of minerals. In layered metamorphic rocks, this cleavage will develop in different orientations based on the strength of the various rock types comprising the layers, causing cleavage refraction across layer boundaries. We can use this cleavage refraction to estimate the strength differences (in terms of effective viscosity ratios) between layers, a critical parameter that is poorly constrained in naturally-deformed rocks. Effective viscosity ratios can be used to evaluate whether rocks deformed by a simple linear-viscous model or whether a more complex rheologic model is needed. Layered quartzites and phyllites exposed near Baraboo, Wisconsin provide an ideal natural laboratory for study. The rocks contain varying proportions of quartz (strong phase) and pyrophyllite (weak phase) with only minor amounts of hematite and other minerals. Previously collected cleavage orientation and mineralogy data are used from those layers to estimate effective viscosity ratios and relate them to mineralogy between not only adjacent layers, as has been previously reported, but nonadjacent layers as well. Within a particular outcrop, the qualitative relationship between quartz content and cleavage orientation is clear; the angle between cleavage and bedding is smaller in layers with lesser amounts of quartz (and conversely more pyrophyllite). Graphs comparing the effective viscosity ratios and volume of strong phase (quartz) show an exponential relationship that fits within theoretical end-member curves of two phase mixtures with strong and weak phases. However, the data plot closer to the curve for strong inclusions in a weak matrix rather than the curve for weak inclusions in a strong matrix, which is contrary to what we expect from the mineralogy and microstructural evidence. Nevertheless, the data demonstrate that we can link mineralogy to effective viscosity ratios in naturally deformed rocks.

Cleavage is the product of shortening and to some extent a reduction in rock volume by dissolution during folding and metamorphism. Cleavage planes are defined by an alignment of micas (hence the ability to cleave) that grow as other more labile minerals (e.g. clays, particularly illite) are recrystallized. The growth of mica crystals and progressive development of cleavage go hand in hand.

There is no void space between cleavage planes and although they are mechanically weak, the rock is continuous. Hence, cleavage differs fundamentally from joints. Joints also are regularly spaced, commonly pervasive planar structures, but they form by extension and brittle failure of a rock mass. This process results in void space, or fracture porosity that during burial will become the locus for precipitation of minerals such as calcite and quartz.

Cleavage has a geometric relationship with folds. Arrays of cleavage are commonly arranged as fans about axial surfaces and in tight folds cleavage closely parallels axial surfaces. Both cases are referred to as axial planar cleavage. Fans diverge upwards in anticlines and converge upwards in synclines. This relationship furnishes us with yet another tool for deciphering fold type and fold orientation.

Cleavage is not always uniformly planar. In layered successions cleavage planes are commonly refracted or bent. The change in orientation reflects how different lithologies respond to stress; common examples of cleavage refraction occur in successions of alternating sandstone (mechanically strong) and shale (mechanically weaker). A couple of examples are shown below.

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.)

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.

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.

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).

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.

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.

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.

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.

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