Fault Bend Fold
Not every fault on the planet reaches the surface. In fact, most of them don’t. Many faults form at depths of several kilometers below the Earth’s surface and as they move, the rocks around them can be bent and folded without breaking.
This photograph shows a lovely example of a classic “Fault Bend Fold”. The lower layers in the shot where the hammer sits are soft and deformable enough that you can’t see the fault itself, but the folded layers are stronger and tell you exactly how it worked.
A small thrust fault formed, allowing the softer rocks to squeeze upwards on a ramp. As that happened, the stronger layers bent around it. They rode upwards on the ramp on the left side, flattened out at the top, then came back down to their more normal level on the right hand side. The lower layers may even be soft enough to have deformed without actually breaking, flowing into the center of the fold and forming the same pattern.
We find these kinds of folds at all scales, from smaller than seen here to across entire regions. They form the same pattern – rocks bending upwards then bending back downwards – and can be recognized even in large scale by pairs of folds called anticlines and synclines.
-JBB
Image credit: https://flic.kr/p/PZD8k
Miguel Vera León
