Abstract: The boundaries between pairs of adjacent fault segments within normal fault arrays define a spectrum of structures, from relay ramps where the length of overlap between the fault segments is much larger than the separation, through low aspect ratio (overlap/separation) relay ramps and ultimately to underlapping fault segments. Where fault segments underlap, transfer of displacement between them is accommodated by a connecting monocline. When displacement increases and a through-going fault forms, relay ramps are preserved as fault-bounded zones of elevated bed dip and monoclines are preserved as areas of normal drag. Therefore, the orientation and magnitude of bed dips within and adjacent to a fault zone, and the numbers of segments seen on a cross-section through it, depend largely on the aspect ratios of relay ramps in the initial fault array. The aspect ratio of relay ramps varies between different fault systems. An analysis of the geometry of 512 relay ramps from 13 different fault systems suggests that the main controls on aspect ratio are the strength of the sequence at the time of faulting and the underlying structure.

Abstract: 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.

Abstract The 50 km (31 mi) long Hat Creek fault, located along the western margin of the Modoc Plateau in northern California, is a geometrically complex segmented normal fault that offsets Pleistocene lavas by at least 570 m (1870 ft) of cumulative throw. Three subparallel, ∼NNW-trending sets of scarps (Rim, Intermediate, and Recent) reflect a progressive westward migration of surface rupture locations that offset progressively younger Pleistocene volcanic deposits during a ∼1 Myr fault history. The 50 km (31 mi) long Rim scarp comprises predominantly right-stepping segments with a maximum throw of ∼370 m (1214 ft) in ∼925 ka lavas. The 17.5 km (10.9 mi) long Intermediate scarp occurs 0.4 to 3.5 km (0.2–2.2 mi) west of the Rim, comprising left-stepping segments with a maximum throw of ∼177 m (581 ft). The 30.5 km (19 mi) long Recent scarp occurs several tens of meters west of the bases of older scarps, and is composed of left-stepping segments with a maximum throw of 56 m (184 ft). The northernmost segment of the Recent scarp offsets 53.5 ± 2 ka basaltic lavas, whereas the remaining segments offset 24 ± 6 ka basalt flows that erupted into Hat Creek Valley, indicating a youthful scarp system. Vertical propagation of the fault through young lavas produced fault-trace monoclines with amplitudes of up to 30 m (98 ft). The monoclines are commonly breached along their upper hinges by a vertical, dilational fault scarp. Shaking associated with repeated earthquakes progressively broke down these monoclines, causing disaggregation or partial to complete