Deep beneath our feet, deformation of rocks at high temperature produces impressive structures such as shear zones, that localize the movement of two volumes of rock with respect to one another. Shear zones are strongly deformed bands with strongly foliated structures (i.e., with rocks that look like a pile of leaves) and kinematic indicators, such as S-C fabrics, that tell us geologists which way rocks moved during deformation.
But… how do we know in which direction rocks moved? If only there were one easily recognizable structure in the rock volume that we could use to learn the direction of tectonic stretching… Ah, wait, we can use lineations!
What is a lineation?
We have already shown in this series that when rocks deform, some platy minerals and layers get flattened and produce foliations. The same mechanisms can stretch objects in a specific orientation, producing linear fabrics that we can see and measure in rocks: stretching lineations.
Stretching lineations (also called object lineations) are linear fabrics outlined by deformed grains and objects in rocks. They commonly consist of mineral grains and aggregates that stretch, rotate, or grow along the direction of tectonic transport during deformation. The minerals that most commonly define lineations are those with prismatic or needle-like habit, like amphibole, pyroxene, epidote, or tourmaline. Stretching lineations can also be defined by rods or aggregates of minerals or deformed objects such as clasts and fossils.
Lineations are useful
When geologists map an area, they commonly measure foliations, fold axes, and lineations. Lineations tell us the direction where the rocks ‘stretched’ or moved when they deformed. Therefore, if we are looking for kinematic indicators, we need to find a section parallel to stretching lineations and perpendicular to the foliation. If correctly interpreted, stretching lineations help us understand the sense of motion/tectonic transport in deformed rock volumes and can be linked to the kinematics of shear zones, vergence of folding and, at larger scale, the direction in which two continents collided, just to list a few examples.
References and Further Reading
Introduction to Geology by Chris Johnson, Matthew D. Affolter, Paul Inkenbrandt, and Cam Mosher, Salt Lake Community College. Open Geology Project.
Burg, J. P., Bale, P., Brun, J. P., & Girardeau, J. (1987). Stretching lineation and transport direction in the Ibero-Armorican arc during the Siluro-Devonian collision. Geodinamica Acta, 1(1), 71-87.
Cloos, E. (1946). Lineation: a critical review and annotated bibliography. Geological Society of America.
Fossen H. (2010). Structural Geology. Cambridge University Press.
Lin, S., & Williams, P. F. (1992). The geometrical relationship between the stretching lineation and the movement direction of shear zones. Journal of Structural Geology, 14(4), 491-497.
Piazolo, S., & Passchier, C. W. (2002). Controls on lineation development in low to medium grade shear zones: a study from the Cap de Creus peninsula, NE Spain. Journal of Structural Geology, 24(1), 25-44.
Tikoff, B., & Greene, D. (1997). Stretching lineations in transpressional shear zones: an example from the Sierra Nevada Batholith, California. Journal of Structural Geology, 19(1), 29-39.
Turner, F. J. & Weiss L. E. (1963). Structural analysis of metamorphic tectonites, Francis J. Turner, Lionel E. Weiss. McGraw-Hill. New York. US.