What is Flexural Slip ?

It is a kinematic type of folding commonly found in fold and thrust belts such as the Zagros Mountains (Figs 1 - 2). Nicely explained by Fossen (2016) - Try folding a double sandwich with jelly. The sandwich maintains its thickness even though slip occurs between pieces of bread, until the fold becomes too tight. This will also preserve the layer length.

Another mechansim flexural shear has the shear plane along the bedding - it also preserves length and thickness and therefore also area.

How is it used ?

Commonly to unfold anticlines such as those created through fault propagation (Fig. 3) The mechanism usually needs a strong mechanical anisotropy so is ideal in folded alternating sand-shale layers.

Also, an increase in dip can result in increased flexural slip - therefore dip maps may be used to drive the occurrence of shear fractures on anticlines (work by Gary Couples).

restoration Example from middle east

As flexural slip maintains layer length and thickness, we can carry out a simple restoration. The horizons in Fig. 3 can be successfully restored, although there is some misfit on the blue horizon in the more complex, right hand section. This type of restoration needs a pin line (red arrow) in Fig. 3) to provide control to the restoration.

lets Try a Restoration

This example is from Fig. 21.5 of Fossen (2016),

• Using a ruler or thread, measure the length of each marker in the deformed section.

• Then move each marker to their original location i.e., (where they were cutoff at the ramp) and straightened

• Bed thickness and length should be maintained. If not, another restoration method may be required.

Can it be used in extensional environments ?

Usually not, as layer parallel shear is not common in extension as slip likely initiates at 20 - 30 degrees to sigma 1. Shear across layering is therefore taken care of by simple shear approaches.

Reference

Fossen. 2016. Structural Geology. Cambridge Univ. Press.