Lineation (geology)
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Lineations in structural geology are linear structural features within rocks. There are two types of lineations, intersection lineations and stretching lineations.
[edit] Intersection lineations
Intersection lineations are imaginary linear structures formed by the intersection of two planes in a three dimensional space. Intersection lineations are measured as lines with a plunge and a plunge azimuth.
Intersection lineations are measured in relation to the two structures which intersect to form them. For instance, according to the measurement conventions of structural geology, original bedding, S0 intersected by a fold's axial plane foliation, forms an intersection lineation L0-1, with an azimuth and plunge defined by the fold. This is the typical cleavage-bedding intersection angle and is diagnostic of the plunge of the fold on all parts of the fold.
Intersection lineations can be formed by the intersection of any two surfaces. However, the usefulness and importance of the intersection lineation to the observer is heavily dependent on recognising and correctly identifying the lineation and understanding what it means.
[edit] Stretching lineations
Stretching lineations are a special form of lineation formed by shearing of rocks during assymmetric deformation of a rock mass. Stretch lineations record primarily the vector of greatest stretch, which is perpendicular to the principle plane of shortening.
It is best to visualise the stretching lineation as a ball of treacle which when gripped and pulled forms a cigar-shaped rod parallel to the direction in which the treacle is pulled. This is parallel with the direction at which a shear zone or shearing force is stretching the rock. Shortening occurs at the same time perpendicular to this stretched rod, equally in all directions.
With reference to the image at right (top), the conglomerate pebbles most likely were deposited as sub-spherical pebbles and boulders. During deformation the rock was flattened and then stretched by movement along a ductile shear zone within which this outcrop resides. The spherical conglomerate pebbles stretched along the direction of movement of this shear zone, attaining their current somewhat flattened cigar-shaped form. The pebbles thus record important information on the orientation of the shear zone (subvertical) and the direction of movement of the shear zone.
Stretching lineations may also manifest as linear features upon pre-existing surfaces such as foliations within shear zones (see image at right, below). In such a case the lineation may not be as obvious in plan and may require measurement as a rake upon a planar surface. In this case, the two lineations are formed in the same deformation event but manifest differently based upon the rheology of the rocks upon which they are inflicted.
Finally, the key difference between a stretching lineation and an intersection lineation is that stretching lineations carry no information on the orientation of other planar fabrics within a rock mass. In the case of the illustrated lineations within the sandstone, they do not record an earlier deformation event's foliation and cannot be used to infer orientation information for folds or original bedding.
