Underneath a faultHere’s the kind of question you might not have thought about if you’re not a geolo

Underneath a faultHere’s the kind of question you might not have thought about if you’re not a geologist; what does the bottom of a fault look like?Small faults can actually just end, but huge, continent-scale faults like the San Andreas Fault in California or the Anatolian Fault in Turkey have hundreds of kilometers of movement between the rocks on either side. What does the bottom of a faultlike that look like?Strike-slip faults tend to only go down about 10 or 15 kilometers deep. Megathrust faults at subduction zones can go deeper, but for now let’s focus on a strike-slip fault. In that type of fault, the rocks are sheared, sliding past each other. Shallow, close to the surface, the rocks are actually able to break leading to earthquakes, but at depths of over 10 kilometers, it gets so hot that the rocks don’t break, they flow.Rocks at the bottom of a fault are still sheared, but they are ductile. The minerals are able to change shape through one of several mechanisms depending on the temperature. The stresses of the fault reorient the minerals into new patterns, creating a rock like this called a mylonite with long strings of minerals created by the shear.This rock specifically is an S-C mylonite. See how there are light and dark mineral bands with an angle between them? Those bands form as the shear stress causes the minerals to twist and regrow. Minerals with planar shapes like micas form bands in one direction in-between the white minerals that are rolled or otherwise twisted into the other bands. The angle between these two directions is defined by how the minerals respond to the stress and how much deformation has occurred after the bands formed. As strain continues to increase, the minerals rotate even more and new bands form, creating a process of shear band growth and twisting that can be used to reconstruct which direction a fault was moving.This rock formed more than 10 kilometers deep in Earth’s crust, buried deep beneath a fault. Ever wanted to know what the bottom of a fault looks like? This is the bottom of an ancient fault, now exposed at the surface.-JBBImage credit: https://flic.kr/p/9WqmtSRead more:http://65.54.113.26/Publication/40487400www.geo.arizona.edu/geo3xx/geo304/Foliations_Lineations.ppt -- source link

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