OCCURRENCE AND DEVELOPMENT OF NORMAL FAULT-RELATED FOLDS WITHIN A HETEROGENEOUS SEDIMENTARY SEQUENCE: A CASE STUDY FROM INNER MORAY FIRTH, OFFSHORE SCOTLAND

We analyse the variability of folding along a normal fault system from Inner Moray Firth basin, offshore Scotland. Seismic data calibrated with wells show that folding is preferentially localized within mechanically incompetent stratigraphic intervals: the Lower-Middle Jurassic pre-rift shale-sandstone interbedded unit and the Upper Jurassic syn-rift shales. The propagation of the fault tip is, in first instance, inhibited by the weak lithologies, generating fault propagation folds, with amplitudes of ~50 m. Folds are also generated, or amplified by translation of the hangingwall over curved, convex-upward fault planes. These fault bends are a consequence of vertical fault segmentation and linkage in contractional steps within mechanically incompetent layers. In areas where opposite-dipping, conjugate normal faults intersect, the maximum displacements do not occur within the competent, pre-rift sandstone layers, where faults presumably nucleated, but are skewed upwards towards the base of syn-rift sequence (i.e. the free surface at the time the fault initiated) without developing significant folding ahead of the propagating tip. Mechanical models of conjugate faults predict restricted downward fault propagation due to high compressive stresses in the vicinity of the intersection region and an asymmetric displacement distribution, skewed toward the upper tip, with higher 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 and development of folding and controls parameters such as propagation/slip ratios and displacement rates, lending empirical support to the results of previous kinematic and analogue models.