Lineations

Generally, a lineation is a term used to describe linear elements in a rock, this can occur in deformed or undeformed rocks (Fossen 2016). Lineations are geometrical arrangement of component features in the rock (fabric element) in which one dimension is considerably longer than the other two dimensions, as seen in the diagram below (Fossen 2016). Structural geologists focus on lineations that occur due to deformation.

• Lineations isolated to a single surface (such as a fault surface). These occur in the brittle regime.

Striations or slickenlines are a direct result of frictional sliding. Slickenlines are found on shear fractures and form by physical abrasion (Fossen 2016). There are two types of slickenlines: (1) groove lineations and (2) fibrous lineations.

• Type 1: Groove Lineations

Groove lineations are linear abrasions that form due to the movement of a fault. They show the movement of a fault in a shear zone, especially in hard brittle rock (asperties). Groove lineations are also known as fault grooves (Fossen 2016).

• Type 2: Fibrous Lineations

Minerals can grow in a preferred direction on fractures. These lineations form when there is an extensional component during deformation and are defined by the fibrous growth on that deformational surface. These lineations require some sort of opening during deformation (Fossen 2016).

Groove and fibrous lineations can be found together along a fault depending on the fault movement throughout time.

• Build up of a linear fabric throughout the rock (not isolated to a single surface).

• Line of intersection between two planar fabrics (Wilkerson 2019). Intersecting planar fabrics could be cleavage intersecting with bedding or two sets of cleavage intersecting.

• Stretching lineations are defined by the shape of deformed objects within the rock. Stretching lineations occur due to plastic deformation in rocks.

Commonly deformed objects in rocks include but are not limited to...

• Ooids (shown in image 9)
• Pebbles
• Fossils
• Various minerals

Boudins & Boudin Lineation

• "Boudins are competent rock layers that have been stretched into segments" (Fossen 2016). Boudins, like lineations, are longer in one dimension than in the other two dimensions, but typically are seen from the shorter orientations.

• Mineral lineations in the brittle regime result in fibrous lineations (as preciously discussed), but when discussing mineral lineations in the plastic regime we should think about minerals that are precipitating in a certain orientation due to stress.

Strain/Pressure Shadows

"Sectors on each side of a rigid object in a shear zone where minerals may crystallize into tails" (Fossen 2016). If these appear to be asymmetric, they can provide a shear direction and/or transport direction.

Orientation of the lineations depict the direction of maximum elongation.

Lineations, because of their relationship to transport/shearing/folding direction, provide important information on the deformational history of the sample or rock. So structural geologists in particular need to pay close attention to what lineations are telling us about the objects in question.

Image 1: Stute, M., 2019, Photo of Duawnt Lake, Canada showing glacial lineations that carved the landscape, Google Earth Pro

Image 2: Eby, Nelson, 2014, Lineations:  http://faculty.uml.edu/Nelson_Eby/89.520/Instructor%20pdfs/Chapter%2014.%20Lineations.pdf  (Accessed November 2019).

Image 3: Qasim, M., 2015, Lineations in the Brittle Regime:  http://geologylearn.blogspot.com/2015/08/lineations-in-brittle-regime.html  (Accessed November 2019)

Image 4: Fossen, H., 2016, Structural Geology, University Press, Cambridge

Image 5: Department of Geology, 2017, Kafrelsheikh University, Lineation and Linear Structure: https://www.slideshare.net/SaifAboKhashaba1/lineation-and-linear-structural

Image 6: Burg, Jean-Pierre, 2009, Swiss Federal Institute of Technology, Lineation Structure: http://www.diogenes.ethz.ch/diogenes_details.asp?colum1_res=last+10+added&colum2_res=&hiddenvalue=140

Image 7: Gotthard, Roland, 2004, Large-Scale Crenulation Forming Kink-folds

Image 8: Department of Geology, 2017, Kafrelsheikh University, Lineation and Linear Structure: https://www.slideshare.net/SaifAboKhashaba1/lineation-and-linear-structural

Image 9: Zawacky, E., 2016, Ooids and Sedimentary Spheres: https://itssedimentary.com/2016/07/16/ooids-sedimentary-spheres-while-these-dark/

Image 10-12: Fossen, H., 2016, Structural Geology, University Press, Cambridge

Image 13: Zulauf et al., 2011, Chocolate Tablet Structures, Journal of Structural Geology: https://www.sciencedirect.com/science/article/pii/S0191814114002028#fig12

Image 14: Wilkerson, M.S., 2019, Mineral Lineation in Quartzite, https://ic.uscs.edu/"casey/eart150/Lectures/Foliations&Lineations/Foliations&lineations.htm

Fossen, H., 2016, Structural Geology, University Printing House, Cambridge, pp. 301-311.

Wilkerson, M.S., 2019, GEOS 350: Structural Geology and Tectonics, DePauw University.