Occurrence and development of folding related to normal faulting within a mechanically heterogeneous sedimentary sequence : a case study from Inner Moray Firth, UK

Folds associated with normal faults are potential hydrocarbon traps and may impact the connectivity of faulted reservoirs. Well-calibrated seismic reflection data that image a normal fault system from the Inner Moray Firth basin, offshore Scotland, show that folding was preferentially localized within the mechanically incompetent Lower–Middle Jurassic pre-rift interval, comprising interbedded shales and sandstones, and within Upper Jurassic syn-rift shales. Upward propagation of fault tips was initially inhibited by these weak lithologies, generating fault propagation folds with amplitudes of c. 50 m. Folds were also generated, or amplified, by translation of the hanging wall over curved, convex-upward fault planes. These fault bends resulted from vertical fault segmentation and linkage within mechanically incompetent layers. The relative contributions of fault propagation and fault-bend folding to the final fold amplitude may vary significantly along the strike of a single fault array. In areas where opposite-dipping, conjugate normal faults intersect, the displacement maxima are skewed upwards towards the base of the syn-rift sequence (i.e. the free surface at the time of fault initiation) and significant fault propagation folding did not occur. These observations can be explained by high compressive stresses generated in the vicinity of conjugate fault intersections, which result in asymmetric displacement distributions, skewed towards the upper tip, with high throw gradients enhancing upward fault propagation. Our observations suggest that mechanical interaction between faults, in addition to mechanical stratigraphy, is a key influence on the occurrence of normal fault-related folding, and controls kinematic parameters such as fault propagation/slip ratios and displacement rates.