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Mylonite

Down in the hot middle to lower crust where temperatures are higher, rocks behave differently. At geologic timescales they can flow, shear, and deform plastically without fracturing under shear stresses that produce strain, and generally do not produce earthquakes. This ductile environment is where the deep fault shear zones are. These zones are generally wider than faults and can accommodate displacements from centimeters to tens of kilometers. Nearby rocks may show little, or no strain at all.

Movement of the shear zones form foliated, deformed, lineated rocks called mylonite, and lab experiments show the fine-grained textures are formed by recrystallization, grain-size reduction, and metamorphic reactions. These textures can provide clues as to the displacement, or the relative movement of the rocks on either side of the shear zones, called shear-sense indicators. In the example shown below, the sense of movement is “dextral” or to the right (to the left is called “sinistral”). Dextral movement has caused “drag” lineations to form on either side of the rock. If you turn the rock upside down in your imagination, the sense of movement is still dextral facing the shear zone.

The shear-sense indicators in rocks are important tectonic clues for structural geologists to determine how mountains formed and plates collided.

More information from EGU Blog authors Samuele Papeschi, Sandra McLaren and Hannah Davies are at the link: https://blogs.egu.eu/divisions/ts/2021/01/25/features-from-the-field-shear-zones-and-mylonites/

More on mylonites here: https://en.m.wikipedia.org/wiki/Mylonite

Photo of metagranite mylonite and interpretation and annotation is by Samuele Papeschi.

#mylonite #ShearZones #ShearSenseIndicators #rocks #geology #ScienceMastodon