Structural Geology lecture - Deformational fabrics

Lecture Index: Definitions for and description of fabrics. / Mechanisms of foliation formation. / Axial planar cleavages. / Linear structures and lineations. /


Key Terms:

Upper right: USGS photo from Appalachians of a well developed cleavage (the 'layering' running from lower left to upper right) as it intersects folded bedding, which is close to horizontal to the right, but bends to a subvertical position as one traces it to the left. This outcrop is part of an asymmetric synform. Image source: http://pubs.usgs.gov/of/2002/of02-437/gallery.htm .


Definitions for and description of fabrics

foliation: any planar, relatively penetrative (through going), fabric in a rock that is not primary. These are due to the interaction of deformational processes and metamorphic processes.

lineation: a relatively penetrative fabric in a rock that is not primary and which has a linear geometry.

Image to right of a schematic representation of a foliation versus a lineation. Note that foliations can also develop from simple shear, and not only by flattening. Image source: USGS site http://geomaps.wr.usgs.gov/parks/noca/nocaft4.html .

An easy initial association to make is that a foliation is the flattening plane of a pure shear deformation history driven by a cigar shaped stress ellipsoid, and that a lineation is the elongation direction of a pure shear deformation history driven by a pancake shaped stress ellipsoid. Not surprisingly, things are more complicated and these are just two possibilities of a much larger array of behavior.

Rocks with fabrics are anisotropic with respect to strength and seismic properties. This is a fundamental property.

How are foliations manifest in rocks?

spaced vs. penetrative fabrics: This distinction is a matter of scale, but one default perspective is hand specimen scale.

This image is from the Baraboo quartzite area in south-central Wisconsin. The lower pink material is quartzite, and a careful look brings to light preserved cross beds one of which is identified with a black arrow (with up-dip transport). The overall direction of bedding is traced by the blue line. Above the lense cap is a darker zone with some structural complexities. The white is vein quartz which is within a laminated, darker material which originally was finer grained more clay rich material. The surfaces/foliations of interest here are the spaced discontinuities within that darker material, identified with yellow arrows), that are counterclockwise and dipping more steeply than bedding here. They also occur in the overlying layer, and have a concentration of mica (pyrophyllite) along them. Note the apparent normal or hangingwall down offset on these surfaces (see area outlined by ellipse). This is opposite to the sense of shear one would expect from folding. However, solution along the spaced cleavage would produce the observed apparent offset. Another good possibility here is that the spaced foliation orginally formed as an axial planar cleavage (see below), and that was reactivated as a slip surface in a subsequent deformation phase.

View of migmatitic gneiss pavement. The gneissic layering here has been formed by either melt segregations or by intimate injection of igneous material. Deformation (flattening and shearing) then enhanced the layering. Note also the crenulations that deform the gneissic layering, and how the grain size and layer thickness is decreased in the short limbs of the crenulation. This can be considered a second, spaced fabric that developed. The deformation history of this rock is very complex.

 


Mechanisms of foliation formation

Foliations are polygenetic. Even in one specimen different mechanisms can contribute to the foliation character. Rock type or a dominant mineral often determine the type of mechanism.


Axial planar cleavages.

The above photo is of a slate in the Hecla Hoek basement rocks on Spitsbergen, Norway (specifically from Wedel Jarlsberg Land). The color bands are bedding (So), and here one can see the how the rock splits along cleavage planes that are in an axial planar position. Underneath the microscope it becomes clear that such cleavage planes are due to the preferred orientation of micas and other features that formed during the interplay between metamorphism and deformation.

This is an outcrop of the Precambrian Baraboo quartzite that can be found in south-central Wisconsin. The pink layers above and below are the quartzites, but the darker band inbetween is a phyllite (originally a muddier layer, and the mud metamorphosed to produce the micas, specifically pyrophyllite). Note the clear parting/fabric counter clockwise and dipping more steeply than the bedding. This is interpreted as an axial planer cleavage. In that case, given that the view is to the east, is a syncline or an anticline to the north (the left), and what can one say about the orientation of the limb on the othe side of the fold? A close look also shows the axial planar surface to bend as it disappears into the massive quartzite, at which time it also becomes more spaced and less developed. The bending is known as cleavage refraction.

A structural association and pattern between the fold and cleavage geometry exists that can be very useful in the field. We can develop the basic geometry for two different cases:

To the right is an image of Carboniferous sandstones and shales on Svalbard that are involved in an upright fold with a subvertical, semi penetrative cleavage that is preferentially developed in the shalier strata and can be seen as the planar vertical fractures in the fold hinge zone. If you carefully try to trace out layers small scale slip surfaces throughout this outcrop become apparent, some of them along the cleavage planes.

What information that can be garnered at one outcrop from bedding cleavage relationships?

You can practice on the image above. These are interbedded quartzites and phyllites, originally sandstones and mudstones, from the Hecla Hoek basement sequence in Svalbard (from western Wedel Jarlsberg Land). The exposure face is subvertical. Note the cleavage and the small parasitic fold. From what you see here what might you deduce about the larger fold pattern in this area?

What are deviations from a simple axial planar pattern?

This image is from the Baraboo Hills area of Wisconsin (same outcrop as image above, which is a close up of the upper right side), and shows a more phyllosilicate rich layer sandwiched between two more massive quartzite layers. The axial planar cleavage that is developed in these folded Precambrian metasediments is preferentially developed in the layer with phyllosilicate material because it has the appropriate composition that both concentrates the deformation and allows a good preferred orientation to be developed, defining the foliation. Note here how just above the camera lense the foliation parting bends in an S-shape. This geometry is known as sigmoidal cleavage, and one interpretation is that flexural slip related simple shear concentrated along the contact with the more massive and more competent quartzite has reoriented the cleavage. In this case, the direction of slip would be hangingwall up, a direction consistent with flexural slip during folding.

Looking at a recumbent tight fold in Triassic shales of Midterhuken, Spitsbergen. Note the hinge with a spaced cleavage just meters to the left and above the person for scale (OK - my brother).

Close up of fold hinge showing a fanning axial planar cleavage. Also note how the cleavage is not really planar, but anastomosing, even at this scale. The platey talus debris is not from breakage along bedding. but along the cleavage.

This is a photomicrograph of the cleaved, organic-rich, Triassic siltstone from Bellsund area, Miterhuken depicted above. The polarizers are in, as is the gypsum plate (which produces the pastel blues to purples). A lot can be learned from looking at foliations under the microscope. In this particular case the domainal character of the cleavage is evident, with a series of anastomosing dark seams running from the upper left to lower right, defining a folia. The folia is produced by very fine-grained dark mica (stilpnomelene likely), and concentrated opagues (and possibly organic material). Material on either side is composed of fine-grained quartz silt and carbonate material, and represents the microlithon, which preserves original textures. Note how the quartz grains within the folia are both smaller overall, and how they are also elongate in the plane of the folia, in contrast to the same grains in the microlithon. One explanation for this is that pressure solution along the individual seams has dissolved away the quartz on that side of the grain, both changing the grain shape and making them smaller. Consistent with that idea, the folia parallel length of the grains in the folia is similar to that outside the folia.

This is from the same specimen as above, but where the folia are more concentrated and defined.

The cleavage folia in this photomicrograph runs from lower left to upper right. The distinctive aggregate featured here represents a large opague grain with attached fibrous calcite. One way to make room for the calcite growth is to have cleavage parallel extension (consistent with the idea that the cleavage plane is a flattening place). The fibrous character of the calcite suggests it was growing while the strain was occurring. The fold of the calcite fibers seen symmetrically disposed one either side suggests some rotation during the fiber growth and cleavage formation episode.

This photomicrograph image is of the same cleaved Triassic strata as above, but without the polarizer in (plane light). Again, the well defined cleavage runs from lower left to upper right. Note the light colored fibrous calcite, indicating cleavage parallel elongation.

Axial planar cleavage and the history of fold development:

We will consider fabrics associated with ductile shears zones separately.


Linear structures and lineations

primary linear structures:

intersection lineations: the classic example is bedding cleavage intersection, but can be between any two surfaces.

mineral lineations: again, preferred growth position in a stress field. Easier to grow ends if in elongation direction. Common with amphiboles.

crinkle lineations: micro-folding, common in phyllites.

elongation lineations: due to a cigar shaped strain ellipse, nicely developed in metaconglomerates.

View of granitic dike in the Pelona schist. Note how the dike layer this and thickens, and how the schistosity bends into the area where the dike has thinned. This is known as a pinch and swell structure, and is due to layer-parallel extension, that is causing necking as the more competent granitic dike is stretched. Think of pulling taffy apart. If the deformation where to have continued, the dike segments would actually separate, producing boudins. Photo source: http://scamp.wr.usgs.gov/scamp/html/scg_sgm_vincent.html .

boudins:

This image is from the Baraboo Hills of Wisconsin (great place for a structure field trip). Just below the camera lense is a lense shaped body of pink quartzite, and below and to the left is another pod of the quartzite. Both are enclosed by pyrophyllitic phylllite, and are known as boudins. In 3-D the lenses of quartzite continue as a rod like form, with the axis parallel to the axis of local folding. Boudins form by layer parallel elongation. the quartz vein in the upper right boudin is part of the elongation strain. Note also how the internal bedding in the lower boudin is trunated. These boudins likely formed in part as slip parallel to the phyllitic surface, bent and cut through a layer of quartzite, separating the once continuous layer into asymmetric boudins, in which case these are associated with sinistral movement, which is opposite to that expected from simple flexural slip. This is part of the evidence that Baraboo Precambrian quartzites had a polyphase history.

mullions and quartz rods (viscosity contrasts in a stress field).


A few references for future follow-up:

Early thought on foliation development:

Some more recent thought:


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