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Squished clasts show how rocks flow at the base of faults

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Photo 1. Squished (strained) clasts that have changed shape during ductile deformation within the Funzie Conglomerate (pronounced: finny), with rocky cliffs surrounding Funzie Bay in the background.
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Snir Attia
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Figure 2. Maps successively closing in on Fetlar within the Shetland Islands, Scotland, United Kingdom. Fetlar is also known as the “Garden of Shetland” for its green fields and fertile soil.
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Photo 3. Alexander Lusk (right) and Snir Attia (left) in front of dramatic sea cliffs common to the study area.
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Snir Attia

— August 5, 2022

Close to the earth’s surface, rocks undergo brittle deformation by breaking and sliding along faults when subjected to external forces like when continents collide or break apart. However, just ~15 kilometers beneath the surface, higher temperature and pressure cause rocks to flow in response to these same forces and undergo ductile deformation. At these depths, the mineral grains and clasts that make up rocks accumulate penetrative strain and change shape, allowing the entire rock to also change its shape! The depth where the change in deformation style takes place is called the brittle-ductile transition.

When thinking about deformation, geologists often talk about strain, the change in shape, size, orientation, and position that deformed rocks have experienced. Lateral offset of a railroad track across an active fault is an example of measurable brittle strain. Ductile strain can be measured by looking at objects within rocks that underwent ductile flow such as clasts that have changed shape and orientation within a deformed rock.

This spring, Bureau geologist Snir Attia joined UW Madison postdoctoral researcher Alexander Lusk for three weeks of fieldwork in the Shetland Islands, Scotland to measure clasts within a conglomerate (a rock consisting rounded fragments of other rocks) that experienced ductile deformation over 400 million years ago. Although quite far away from New Mexico (three days of travel by plane, motorhome, and ferry ride!), the remote island of Fetlar is home to some of the most spectacular known exposures of ductiley deformed conglomerate! This collaboration aims to combine cutting edge field measurements with new statistical methods to uncover just how rocks respond to ductile strain, so we can better understand how active fault zones behave. Tune in Friday for another postcard about this work!

Snir Attia, Field Geologist, NMBGMR