Basin extension often gives way to Structural Inversion. Not to be confused with Seismic Inversion, this is when a normal fault becomes reactivated with a reverse sense. The Southern North Sea (offshore UK) is a good example of a basin that has experienced inversion (see earlier blog "Some Benefits of Fracture Cements").
The various steps are shown in Figure 1 which is modified from some work by Bailey et al (2002) from the Appalachians. Step (a) shows a rollover anticline overlain by growth strata (yellow shading with growth lines) that has formed during rifting (black arrows pointing away from each other). These growth strata are often called the synrift. In (b) the structure is subjected to compression (black arrows pointing towards each other) and the listric fault reactivates as a reverse fault. 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.
Figure 1. stages in Structural Inversion (a) Creation of rollover anticline (synrift in yellow), (B) reactivation of fault in reverse sense to create a fold, (c) Full Inversion with steep forelimb and thickened core. Modified from Bailey et al. (2002).
An outcrop example of steps b - c (Figure 1) is given in Figure 2. The fold-forming fault is interpreted in solid red (Figure 2b) 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 how the hanging wall (synrift) beds are much thicker than those in the foot wall and that the limestone beds (light pink) are heavily jointed.
Figure 2. Inversion Structure on the cliffs at Kilve Pill (Somerset, UK) formed during the Alpine Orogeny (a) uninterpreted, (b) interpreted: fold-forming fault (red line) potentially extends higher up (dashed red line). other faults can be interpreted in the hanging wall. Jointed limestones (light pink) alternate with grey shales (Lower Jurassic age).
Reference
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.