Accretionary Wedge #19: Yes, we have a banana

It’s accretionary wedge time again an this month Dino Jim is asking us to ‘think outside the box’ when teaching geology. I seem to detect a food theme developing.

Here is a quick piece on using a banana as an analogue for rock deformation in general, and fault propagation folding in particular.

First take your banana and peel it.

Banana: Undeformed

Grasp an end in each hand leaving at least the central third free. Slowly move your hands towards each other.

Initially the banana will deform ductilely, and actually thicken. After the initial thickening, the banana will start to fold.

Banana: Thickening and Fold Initiation

As the fold develops into an anticline-syncline pair, note the extra compression on the inside of the folds generating buckling and extension on the outside of the folds generating tension cracking. If you look closely you can also see shearing starting to develop in the central limb between the two folds.

Banana: Folding developing

Deformation switches from ductile to brittle as shear failure through the central limb generates a thrust fault separating the hangingwall anticline from the footwall syncline.

Banana: fault propagation fold

And here is the real thing for comparison…

Broadhaven, Pembrokeshire fault propagation fold

You can see a gigapan and photosynth version of this structure in my previous blog post here.

Note: this isn’t my idea, I picked it up from Prof. Patrick James, Head of the School of Natural and Built Environments at the University of South Australia at a teaching and learning in geology conference.

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