Fault propagation folds form important hydrocarbon traps. They form above the tips of reverse faults in compressional settings such as the Zagros Mountains and in the Caspian Sea. A nice seismic example is shown in Figure 1 - both the structure (the fold) and the causal fault can be clearly seen. Compare these to Inversion folds (see blog “Fault Reactivation: Inversion) which can be recognised by a thickened fault core.
As slip increases the fault tip advances and fold grows larger. The geometry of fault propagation folds is related to the underlying reverse fault - often having longer length to width ratios than pure buckle folds - as the length of the fault controls the fold (Sattarzadeh et al, 2000). Another thing is that they tend to have steeper forelimbs than back limbs - this is very relevant for well positioning as this is usually an area of high strain and fracturing (see blog “Natural Fracture Drivers”).
Figure 1. Fault propagation fold in deep water Niger Delta. The reverse fault is clear.
SOURCE: Virtual Seismic Atlas - https://www.seismicatlas.org/entity?id=aff4f079-131f-41bb-a8e1-8fe1cf4c149e. Author: David Iacopini
To show that fault propagation folds occur at different scales I’ve included a small outcrop example from dolomites in Marsden Bay, NE England (Figure 2). The causal reverse fault is much smaller (with smaller throws) than the one in Figure 1. Note the abundant open fractures which were present before the folding - you can see these bed perpendicular fractures on the unfolded layers above the fold. Consequently the folding appears quite brittle in nature as the fractured blocks became rotated into the shape of the fold. Also, there are no striations on the individual blocks.
Figure 2. Fault propagation fold in dolomite, Marsden Bay, NE England.
Reference
Sattarzadeh, Y, Cosgrove, J.W. Vita-Finzi, C. 2000. The interplay of faulting and folding during the evolution of the Zagros deformation belt. From Cosgrove, J.W & Ameen, M.S. (eds) Forced Folds and Fractures. Geological Society, London Special Publications, 169, 187 - 196.