Inversion fold, Kilve Pill, Somerset, UK

A fold related to a fault ?

Large inversion fold on the cliffs at Kilve Pill, Somerset coast.

The structure on the right hand side of the fault (red line) below is clearly a fold as package has been pushed up along the fault, bending the beds of limestone (pinkish grey) and shale (dark greys). This is a cliff exposure on the Somerset coast at Kilve Pill.

The folded package is on the hanging wall of the fault (part that has moved) and the footwall (left hand side of the fault) has unfolded beds. Note that the hanging wall package is much thicker than the footwall package - this tells us that the fault was once a normal fault and that the sediment was deposited whilst the fault was moving (step a in cartoon). This thicker hanging wall package of limestones and shales is called the growth package or the synrift.

Much later on in geological time, the fault becomes reactivated as a reverse fault (steps b + c), which pushes the hanging wall package up into a fold. This reversal in structural history is often called inversion and the fold that we see on this cliff is an inversion fold or an inversion structure. Inversion at Kilve Pill has taken place during the Tertiary period.

Ductile or brittle deformation ?

The fold-forming fault is interpreted in solid red and potentially extends higher up through the grassy ledge (dashed red line). It is a very ductile looking deformation as a result of the volume of shale involved and there are other faults that can be interpreted in the hanging wall. Note also that the limestone beds (light pink) are heavily fractured whereas the dark grey shales are relatively unfractured.

Summary of geological history

The yellow lines in (a) are growth lines within the growth bedding. The black arrows represent pulling apart or extension early in the structural history whereas the later black arrows which face each other represent the compressive inversion stage. This pushes the growth strata up into a fold with a steep forelimb and more gentle backlimb. The thickening of fault core (step c) is a key feature of this type of fault propagation fold, as growth strata (yellow) are involved. 

Various steps in creation of inversion structures - modified from some work by…

Bailey, C.M. Giorgis, S. & Coiner, L. 2002: Tectonic inversion and basement buttressing: an example from the Appalachian Blue Ridge province. J. Struct. Geol. 24, 925-936.