Plastic deformation lineations
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| Strain data can be represented in (a) the Flinn diagram (linear or logarithmic axes) or (b) the Hsu ¨ diagram. The same data are plotted in the two diagrams for comparison. |
Mineral lineations
A penetrative linear fabric is typically made up of aligned prismatic minerals such as amphibole needles in an amphibolite, or elongated minerals and mineral aggregates such as quartz–feldspar aggregates in gneiss. Mineral lineations can form by several processes:
Minerals and mineral aggregates can form a linear fabric by means of recrystallization, dissolution/ precipitation or rigid rotation.
Physical rotation of rigid prismatic minerals in a soft matrix can in some cases occur during deformation. An example is amphibole or epidote crystals in micaschist, where statically grown amphiboles become aligned in zones of localized deformation. In most cases the competence contrast between elongate-shaped minerals and their matrix is not high enough for rotation to be important. Instead, synkinematic recrystallization by means of plastic deformation mechanisms or a dissolution/precipitation process reshapes minerals and mineral aggregates. In addition, precipitation of quartz in pressure shadows or strain shadows is a common way to facilitate growth of minerals or mineral aggregates in a preferred direction. Even crushing or cataclasis of brittle minerals and mineral aggregates enclosed in a ductile matrix can reshape mineral aggregates to linear fabric elements.
Cataclasis, pressure solution and recrystallization all contribute to change the shape of minerals and mineral aggregates during deformation.
In a homogeneously strained rock, if a mineral aggregate had a spherical shape at the onset of deformation, its shape after deformation would represent the strain ellipsoid. In most cases the original shape is unknown so that the final shape only gives us a qualitative impression of the shape of the strain ellipsoid. Nevertheless, deformed mineral aggregates in gneisses have been used for strain analysis, although the difference in viscosity between the aggregates and their surroundings may add uncertainty to the results. In cases where the initial shape is known and the competence contrast is small, such analyses are particularly useful. Deformed conglomerates or oolites are examples of rocks where linear shapes can quantitatively be related to strain. These and other lineations defined by the shape of deformed objects are named stretching lineations and the related fabric is called a shape fabric. Stretching lineations and shape fabrics are extremely common in plastically deformed rocks such as gneisses.
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| Stretching lineation in quartzite conglomerate. The long axes of the pebbles are plunging to the right. The Bergsdalen Nappes, West Norway Caledonides. |
Stretching of minerals and mineral aggregates into a penetrative stretching lineation forms the most common type of lineation in deformed metamorphic rocks.
Quartz, calcite and some other common minerals can grow well-aligned fibrous crystals that define linear elements. Such mineral lineations are referred to as mineral fiber lineations. Fibers may grow in the instantaneous stretching direction (ISA1), but may also grow perpendicular to the face of the opening walls. Besides, once formed they may rotate away from this direction as a result of progressive deformation. In addition to their occurrence in veins in retrograde metamorphic rocks and un-metamorphosed sedimentary rocks such as over-pressured mudstones, they are commonly found in strain shadows of porphyroclasts in metamorphic rocks during low-grade metamorphism. Fibers do not form if pressure and temperature get too high, and seldom above middle greenschist-facies conditions. This rather restricted occurrence makes fiber lineations less common and also less penetrative than many other lineation types.
Rodding describes elongated mineral aggregates that are easily distinguished from the rest of the rock. Quartz rods are common in micaschists and gneisses where striped quartz objects occur as rods or cigars in the host rock. Rods are often considered as stretching lineations, but are commonly influenced by other structure-forming processes. They may represent isolated fold hinges, or be related to boudinage or mullion structures, or to deformed veins with an originally elongated geometry.
Intersection lineations
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| Intersection lineations appearing on bedding or foliation surfaces that are intersected by a later foliation. |
Many deformed rocks host more than one set of planar structures. A combination of bedding and cleavage is a common example. In most cases such planar structures intersect, and the line of intersection is regarded as an intersection lineation. Where the first tectonic cleavage (S1) cuts the primary layering or bedding (S0), the resulting intersection lineation (L1) appears on the bedding planes. Intersection lineations formed by the intersection of two tectonic foliations are also common. In most cases intersection lineations are related to folding, with the lineation running parallel to the axial trace and the hinge line. Note that for transected folds there will be an angle between the intersection lineation and the axial trace.
In some deformed rocks an intersection lineation appears only locally. In most cases, however, their frequency and distribution are large enough that the lineation can be considered penetrative. Like other lineations, intersection lineations may be folded about later folds, showing their use in tracing the deformation history of a rock.
Fold axes and crenulation lineations
Fold axes are generally regarded as linear structures, despite being theoretical lines related to the geometrical shape of the folded surface. Some rocks have a high enough density of parallel fold axes that they constitute a fabric. This is often the case with phyllosilicate-rich metamorphic rocks, where small-scale folds or crenulations constitute a crenulation lineation. Crenulation lineations are thus composed of numerous millimetre to centimetre-scale fold hinges of low-amplitude folds. They are commonly seen in multiply deformed phyllites, schists and in micaceous layers in quartz-schists, mylonites and gneisses. Crenulation lineations are closely associated with intersection lineations but are different in that they are comprised of fold hinges identifiable to the naked eye. During folding of layered rocks crenulation cleavages and crenulation lineations form at an early stage, while larger folds form later on during the same process. It is therefore of interest to compare the orientation of early crenulation lineations with related but slightly younger fold axes. If there is a difference in orientation this could be related to how layers rotated during deformation. We are already familiar with the concept of transected folds, where the lineation makes an angle to the hinge line.
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| Cleavage transecting the axial surface of a transected fold. |
Boudinage
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| Cylindrical pinch-and-swell structures (above) and boudins (below) represent linear elements in many deformed rocks. |
Boudins are competent rock layers that have been stretched into segments. Individual boudins are commonly much longer in one dimension than the other two and thus define a lineation. Such linear boudins form where the X-axis of the strain ellipsoid is significantly larger than Y. Chocolate-tablet boudins can form when X Y. When occurring in folded layers, boudins typically appear on the limbs of the fold with their long axes oriented in the direction of the fold axis.
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| Common connection between folding and boudinage. The fold hinges are thickened while the limbs are extended and boudinaged. The strain ellipse is indicated. |
In general, boudinage structures are most easily recognized in sections perpendicular to the long axes of the boudins. Because of this fact they may be difficult to recognize as linear features in deformed rocks. It is also true that boudins are restricted to competent layers and therefore more restricted in occurrence than most other lineations.
Mullions
Mullion is the name that structural geologists use for linear deformation structures that are restricted to the interface between a competent and an incompetent rock. The term mullion has been used in several different ways in the literature, ranging from striations on fault surfaces (fault mullions) to layer-interface structures formed during layer-parallel extension as well as contraction. We will relate the term to layer-interface structures where the viscosity contrast is significant. In such cases the cusp shapes of mullions always point into the more competent rock, i.e. the one with the higher viscosity at the time of deformation.
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| Mullion structures form lineations at the interface between rocks of significantly different competence (viscosity). |
Such mullions are closely related to buckle folds in the sense that their formation is predicted by a contrast in viscosity, they form by layer parallel shortening, and their characteristic wavelength is related to the viscosity contrast. But they differ from buckle folds in having shorter wavelengths and they are restricted to a single layer interface. A common place to find mullion structures in metamorphic rocks is at the boundary between quartzite and phyllite or micaschist. Mullions also occur on the surface of quartz pods in micaschists.
Pencil structures
The formation of pencil structures occurs as a result of discrete interference between compaction cleavage and a subsequent tectonic cleavage, or between two equally developed tectonic cleavages. Pencil structures have a preferred orientation and form a lineation in un-metamorphosed and very low-grade metamorphic rocks.
Credits: Haakon Fossen (Structural Geology)