Lineation

A lineation is any linear feature or element in a rock , and can occur as the product of tectonic, mineralogical, sedimentary, or geomorphic processes. Lineations are the one-dimensional counterparts of foliations, and both are part of the fabric (geometric organization of features) of a rock. Lineations and foliations are said to possess preferred orientations, meaning that the spatial orientation of the features comprising the lineation or foliation is similar throughout the rock mass.

The spatial orientation of a lineation is described by two angles known as bearing and plunge. The plunge angle is the inclination of the lineation relative to an imaginary horizontal plane (ranging from 0 to 90 degrees), whereas the bearing angle is the compass direction of the lineation in the direction of the plunge (ranging from 0 to 360 degrees).

Structural lineations are those that are formed by tectonic activity such as folding, faulting, or metamorphism . Structural lineations can be either discrete or constructed. Discrete lineations are formed by the deformation and alignment of objects such as fossils or initially spherical pebbles. When a rock containing discrete objects (such as fossils or nearly spherical pebbles) is subjected to stress, the objects can be deformed into ellipsoids that share a preferred orientation throughout the rock. Constructed lineations are those that are formed during deformation and therefore do not involve preexisting objects. Constructed lineations include those formed by the intersections of two planes (e.g., the intersections some combination of foliations, fractures, or bedding planes) and slickenlines (also referred to as slickensides) along fault surfaces.

Mineral lineations are formed by the preferred orientation of either individual mineral grains or clusters of mineral grains. Mineral lineations can be formed by the nearly parallel alignment of mineral grains or clusters that have a needle like habit (e.g., amphiboles), by elongation of mineral grains or clusters during deformation, or by the preferentially oriented growth of minerals in response to the ambient state of stress during metamorphism. A mineral lineation formed by elongation of grains is similar to a discrete structural lineation.

Sedimentary lineations include pebbles aligned in the direction of stream flow and the crests of ripple marks. They are typically, although not exclusively, found on bedding planes in sedimentary rocks .

The slip surfaces of landslides can contain slickenlines similar to those found on fault surfaces, although they are generally considered to be of geomorphic rather than tectonic origin. Likewise, preferentially aligned clasts in sheared glacial till are similar to discrete structural lineations in tectonically deformed rocks, but most geologists do not consider them to be tectonic features.

See also Bedforms (ripples and dunes); Metamorphic rock; Shear zones

lineation
1. Any linear feature that appears on the surface of a rock. Lineation may be formed during deformation by the parallel alignment of minerals, fossils, or pebbles; by parallel crenulation cleavages; or by striations and grooves resulting from the movement of a rock over a plane, e.g. a fault surface (see SLICKENSIDE), or flexural slip during folding. An intersection lineation is caused by the crossing of any two planes, e.g. cleavage and bedding.

2. Lineations are a series of parallel lines on a rock surface, formed by tectonic processes, by the transportation and deposition of sand under upper-flow-regime plane-bed conditions, or by the movement of glacial ice over the rock surface.

lin·e·a·tion / ˌlinēˈāshən/ • n. the action or process of drawing lines or marking with lines. ∎  a line or linear marking; an arrangement or group of lines: magnetic lineations. ∎  a contour or outline. ∎  the division of text into lines: the punctuation and lineation are reproduced accurately.