lineation

lineation ‘Lineation’ is a term used in structural geology for a repeated or penetrative linear structure in a rock mass. A lineation may be present on discrete planes or developed throughout the body of the rock; in the latter case the rock is called an L tectonite. Most lineations are associated with the processes of deformation and metamorphism; they are less common in igneous and sedimentary rocks. Any discussion of lineations requires reference axes or planes, as illustrated in Fig. 1a. The tectonic axis a is the direction of tectonic transport ‘up’ the cleavage; b is parallel to the fold axis; and c is perpendicular to the ab plane, which in cleaved rocks is the plane of slaty cleavage. Deformation may be multi-phase (that is, it may take place in more than one episode), in which case the successive phases are designated D₁, D₂, etc., yielding folds F₁, F₂, etc., planar structures S₁, S₂, etc., and linear structures L₁, L₂, etc. S₀ is used for the original surface, which is usually a bedding surface. In the investigation of deformed rocks it is important to relate the linear and planar features to the appropriate phases of deformation in order to be able to elucidate the geological history.

Lineations take a variety of forms, as shown in Fig. 1b: the intersection of two planar surfaces, elongation of minerals or mineral aggregates, elongation or rodding of pebbles, deformed ooids and reduction spots, oriented fold axes, mullions and boudins, grooves or mineral growths on fault surfaces, preferred orientation of larger crystals (phenocrysts) in igneous intrusions; variety of sole marks, and primary current lineations in sedimentary rocks.

During deformation at low temperatures rock layers may become flexed and the upper ones may slip over the lower and ride up towards the hinge zone of the flexure in the same way that playing cards will move over each other when a pack of cards is flexed. This bedding-plane slip produces slickensides, which may be in the form of grooves or elongation of minerals or mineral aggregates or growth of new minerals such as quartz or calcite. This elongation lineation is perpendicular to the fold axis in the direction of slip.

With low to moderate temperatures and increasing deformation, a planar closely spaced cleavage develops parallel to the axial plane (ab) of the fold, although in some instances the cleavage fans and is not strictly parallel to the axial surface. The intersection of the cleavage on the bedding and of the bedding on the cleavage (see Fig. 1) gives an intersection lineation which is parallel to the fold axis (b). In slates a mineral stretching lineation commonly develops ‘up’ the cleavage plane and at right-angles to the fold axis; if there are porphyroblasts of, say, pyrite in the slate, then new grains of quartz may grow in their ‘shadow’ also ‘up’ the cleavage. In some slates, especially in purple ones, there may be green spots which were originally spherical reduction spots but are now triaxial ellipsoids whose eccentricity is a measure of the strain the rocks have undergone. The maximum elongation which gives a type of lineation is in a direction perpendicular to the fold axis and parallel to (a), the transport direction; the maximum flattening is at right-angles to the cleavage. Ooids in a limestone with a composition similar to the matrix are deformed in a similar manner to the reduction spots and give similar lineations.

Pebbles and even boulders deformed under even higher temperature and pressure, when both fragments and matrix become more ductile, undergo considerable elongation with the long axes of the pebbles most often parallel to the fold axis, a ‘b’ lineation. There is some debate about how there can be such a high degree of stretching parallel to the fold axis. In these high-temperature conditions new inequant minerals such as hornblende, tourmaline, and sillimanite grow with their long axes along the fold axis; in a later deformation event some of these may be fractured across their length and pulled apart along their length. Further evidence of multiphase folding is given by crenulation or microfolding with hinge zones a centimetre or so apart which crinkle an earlier cleavage or schistosity. These small-scale folds are parallel to a later phase of folding.

On the limbs of folds affecting alternations of competent and incompetent beds where there has been considerable layer-parallel extension, segmentation and extension of the competent beds results, with the incompetent beds squeezing into the spaces between and new minerals growing in the gaps with an elongation parallel to the direction of stretching. The result is a set of long, parallel sausage-shaped bodies called boudins, elongated in the direction of the fold axis (Fig. 1c); mullions have a similar orientation and shape with deformed bounding surfaces which have not fractured.

Slickensides as defined above but associated with faults are usually not penetrative but are restricted to specific planes. If shearing along thrust planes was intense, penetrative lineations may extend through a considerable thickness of rock. The direction of these lineations indicates the relative movements of the rocks involved.

R. Bradshaw

Bibliography

Hobbs, B. E.,, Means, W. D.,, and and Williams, P. F. (1976) An outline of structural geology. John Wiley and Sons, New York.