8 March 2013

Friday fold: Fault propagation fold from Cristo Rey

Posted by Callan Bentley

The Friday fold is an overturned antiform that’s associated with a fault. It occurs in limestone and shale on the flanks of the mountain called Cristo Rey, near the spot where Texas meets New Mexico and Chihuahua, Mexico. Here’s Joshua Villalobos of El Paso Community College, standing next to the fold & fault:

This deformation is associated with the intrusion of a laccolith of andesitic magma 49 million years ago, and the sloughing off of the (formerly overlying) sedimentary layers in all directions. As the magma rose to the south, strata went tumbling down to the north, and pushed these Cretaceous layers out of their way.

Zooming in on the fold and fault:

Let me rotate that a bit, so the fault is parallel with the bottom of the photo, and back out a bit, and you’ll take note of something else that caught my eye:

There’s a neat cleavage fan splaying out from the fold hinge area. This cleavage appears to be better developed in the weaker, more friable shale layer, and then to grade out into the more massive limestone which makes up the upper portion of that bed:

I like how the hammer imitates the fault motion arrow.

Happy Friday!