Cleavage development

Cleavage Development

There are many types of cleavages and a rich terminology is available. To efficiently deal with cleavages and foliations it is useful to keep an eye on crustal depth and lithology. Crustal depth is related to temperature (and pressure), and with increasing temperature we first obtain increasing mobility of minerals and at yet higher temperatures the possibility that minerals will recrystallise. Around 350–375 C we leave the realm of cleavage and enter that of schistosity and mylonitic foliations. Lithology and mineralogy are important because different minerals react differently to stress and temperature. Phyllosilicates are particularly important in cleavage development. In general, if there are no phyllosilicates in the rock there will not be a very strong cleavage or schistosity. Cleavage formation in calcareous rocks is controlled by the mobility of carbonate and the easy formation of stylolites.

Cleavage is the low-temperature version of foliation and is best developed in rocks with abundant platy minerals.

We will here consider the most common types of foliation that develop due to deformation during prograde metamorphism, i.e. as a rock is being buried to progressively greater depths.

Compaction cleavage 

The first secondary foliation forming in sedimentary rocks is related to their compaction history. Reorientation of mineral grains and collapse of pore space result in accentuation and reworking of the primary foliation (bedding). For a clay or claystone, the result is a shale with a marked compaction cleavage. 

Theoretical cleavage development in a mudstone.

In this process there is also dissolution going on, and in some quartzites we can find pressure solution seams. Such structures are much more common in limestone, in which dissolution of carbonate produces sub-horizontal and irregular seams where quartz or carbonate has been dissolved and where clay and other residual minerals are concentrated. The seams are stylolites or pressure solution seams, and the foliation can be called a stylolitic cleavage, which is a type of (pressure) solution cleavage. The spacing of the seams in calcareous rocks is usually several centimetres, and the cleavage is therefore a widely spaced cleavage. In fact, the stylolitic surfaces may be too far apart to define a cleavage. In contrast, the compaction cleavage in shale is recognizable under the microscope and therefore a continuous cleavage. These non-tectonic cleavages are usually regarded as S0 foliations.

Early tectonic development and disjunctive cleavage 

A tectonic foliation commonly results when a sedimentary rock is exposed to tectonic stress that leads to progressive horizontal shortening of sedimentary beds - a condition typical of the foreland regions of orogenic belts. In limestone and some sandstones, the first tectonic foliation to form is a pressure solution cleavage that typically is stylolitic (toothlike, showing a zigzag suture in cross section). If s1 is horizontal, a vertical pressure solution cleavage forms that will make a high angle to S0 and previously formed compaction-related stylolites. 

Pressure solution is also important when shales are exposed to tectonic stress. In this case extensive dissolution of quartz causes concentration and reorientation of clay minerals. At some point the secondary cleavage will be as pronounced as the primary one, and clay minerals will be equally well oriented along S1 and S0. The shale will now fracture along both S1 and S0 into pencil-shaped fragments, which explains why the cleavage is known as pencil cleavage. Pencil cleavage also occurs where two tectonic cleavages develop in the same rock due to local or regional changes in the stress field. Such purely tectonic pencil cleavage is associated with some thrust ramps, formed close in time during the same phase of deformation. 

Pencil cleavage in shale in the Caledonian foreland fold-and-thrust belt near Oslo.

If the tectonic shortening persists, it will eventually dominate over the compactional cleavage. More and more clay grains become reoriented into a vertical orientation as quartz grains are being dissolved and removed, somewhat similar to a collapsing house of cards. Micro-folding of the clay grains may also occur. The result is a continuous cleavage that totally dominates the structure and texture of the rock. The rock is now a slate and its foliation is known as slaty cleavage. 

The formation of slaty cleavage occurs while the metamorphic grade is very low, so that recrystallization of clay minerals into new mica grains appears to have just started. A close look at a well-developed slaty cleavage reveals that a change has taken place in terms of mineral distribution. There are now domains dominated by quartz and feldspar, known as QF-domains, that separate M-domains rich in phyllosilicate minerals. The letters Q, F and M relate to quartz, feldspar and mica, and we need a microscope to discern the individual domains, which are considerably thinner than 1 mm. The QF-domains are typically lozenge- or lens-shaped while the M-domains form narrower, enveloping zones. As shown below, many types of cleavages and foliations show such domainal structures, and they can all be referred to as domainal cleavages. 

Disjunctive cleavage types. Stylolitic (limestones) and anastomosing (sandstones) cleavages are usually spaced, while continuous cleavages in more fine-grained rocks are separated into rough and smooth variants, where the rough cleavage can develop into the smooth version. All disjunctive cleavages are domainal, and the cleavage domains (C) are separated by undeformed rock called microlithons (M).

The term disjunctive cleavage is commonly used about early tectonic domainal cleavage in previously unfoliated rocks such as mudstones, sandstones and limestones. This term implies that the cleavage cuts across, rather than crenulating (folding), pre-existing foliations.

It was once thought that slaty cleavage formed by physical grain rotation. We now know that so called wet diffusion or pressure solution is what chiefly produces the domainal structure that characterizes slaty cleavage. Grains of quartz and feldspar are dissolved perpendicular to the orientation of the cleavage and achieve lensoid shapes (disk shapes in three dimensions). Where this happens, phyllosilicates are concentrated and M-domains form. The importance of dissolution or pressure solution allows us to use the term (pressure) solution cleavage also for slaty cleavage.

Cleavage forms through grain rotation, growth of minerals with a preferred direction and, most importantly, wet diffusion (pressure solution) of the most solvent minerals in the rock.

Wet diffusion implies that dissolved minerals diffuse away through a very thin film of fluid located along grain boundaries. The material is precipitated in so-called pressure shadows of larger and more rigid grains in the QF-domains or becomes transported out of the rock. In fact, very significant amounts of matter appear to have left most slate belts, which represent interesting aspects in terms of thermodynamics and fluid flow through the upper crust.

Greenschist facies: from cleavage to schistosity 

New phyllosilicate minerals grow at the expense of clay minerals in shales and slates when they enter the field of green schist facies metamorphism. A phyllite forms and the cleavage changes into a phyllitic cleavage. The new mica minerals grow with their basal plane more or less perpendicular to the Z-axis of the strain ellipsoid, and more or less perpendicular to s1. The newly formed mica grains are thus parallel and a phyllitic cleavage is established. The cleavage is still a continuous one, and the development of QF- and M-domains is more pronounced than for slaty cleavage. The domainal cleavage becomes better developed because dissolution (wet diffusion) becomes more efficient as green schist facies temperatures are reached. 

Phyllitic cleavage bears similarities to crenulation cleavage when viewed under the microscope. The difference is that phyllitic crenulations are microscopic and thus invisible to the naked eye. In this case a phyllitic cleavage dies out towards a folded competent lamina.

When original claystone reaches upper green schist facies and perhaps lower amphibolite facies, the mica grains grow larger and become easily visible in a hand sample. At the same time, the foliation becomes less planar, wrapping around quartz–feldspar aggregates and strong metamorphic minerals such as garnet, kyanite and amphibole. The foliation is no longer called a cleavage but a schistosity, and the rock is a schist. 

(a) Phyllitic cleavage in lower greenschist facies phyllite. (b) This cleavage formed higher into the greenschist facies, showing very well-developed QF- (middle) and M-domains and coarser grain size.

Schistosity is also found in quartz-rich rocks such as quartz schists and sheared granites. Here the M- and QF-domains are on the millimetre or even centimetre scale and they appear more regular and planar than for mica schists. This is why quartz schists and sheared granites split so easily into slabs that can be used for various building purposes. In summary, while wet diffusion (solution) and grain reorientation dominate the formation of slaty cleavage, recrystallization is more important during the formation of schistosity.

Secondary tectonic cleavage (crenulation cleavage) 

An already established tectonic foliation can be affected by a later cleavage (S2 or higher) if the orientation of the ISA changes locally or regionally at some point during the deformation, or if a later cleavage-forming deformation phase occurs. Because cleavages tend to form perpendicular to the maximum shortening direction (X), a new cleavage will form that overprints the preexisting one. In many cases this occurs by folding the previous foliation into a series of microfolds, in which case the cleavage is called a crenulation cleavage. Hence, a crenulation cleavage is a series of micro-folds at the centimetre scale or less with parallel axial surfaces. Depending on the angle between the existing foliation and the secondary stress field, the crenulation cleavage will be symmetric or asymmetric. 

Asymmetric crenulation cleavage affecting a mylonitic foliation. The cleavage is discrete in the middle, micaceous layer, while it is zonal and less well developed in the more quartz-rich adjacent layers.

A symmetric crenulation cleavage has limbs of equal length, while an asymmetric crenulation cleavage is composed of small, asymmetric folds with S- or Z-geometry. 

Crenulation cleavage through which the earlier foliation can be traced continuously is known as zonal crenulation cleavage. In the opposite case, where there is a sharp discontinuity between QF- and M-domains, the cleavage is called a discrete crenulation cleavage. The M-domains here are thinner than the QF-domains and mimic micro-faults. Discrete and zonal crenulation cleavages can grade into each other within a single outcrop. 

Crenulation cleavage is restricted to lithologies with a pre-existing well-developed foliation that at least partly is defined by phyllosilicate minerals. It is commonly seen in micaceous layers while absent in neighbouring mica-poor layers. The domainal thickness of the affected foliation is connected with the wavelength of the new crenulation cleavage: thicker domains produce longer crenulation wavelengths. This is the same relationship between layer thickness and wavelength, where the viscosity contrast was shown to be important. A close connection is also seen between crenulation cleavage and folding. 

Crenulation cleavage affecting the phyllitic cleavage. The crenulation cleavage is seen to be axial planar to decimetre-scale folds.

We can find any stage of crenulation cleavage development, from faint crenulation of foliations to intense cleavage development where recrystallization and pressure solution have resulted in a pronounced domainal QF-M-structure.In the latter case the original foliation can be almost obliterated in a hand sample although usually observable under the microscope. Progressive evolution of crenulation cleavage is accompanied by progressive shortening across the cleavage, and eventually a crenulation cleavage can transform into a phyllitic foliation.

Courtesy © Text and Photos from Structural Geology (book) by Haakon Fossen 

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