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 Ponding conditions of basalts erupted from fissure zones at São Miguel Island (Azores) were investigated through microthermometry of fluid inclusions, hosted in olivines and clinopyroxenes, and whole-rock and mineral chemistry. The Região dos Picos and Achada das Furnas fissure zones formed between central volcanoes and erupted geochemically similar magmas for the last 30 kyr. Hydrocarbonic fluid inclusions that survived to diffusion and re-equilibration recorded a maximum density value of 1000 kg m −3 (29.3 km) at both fissure zones, at the intersection with the feeding systems of older central volcanoes. The maximum density decreases to 875 kg m −3 (23.5 km) towards the central segment of Região dos Picos. Further trapping of low-density fluids is occasional and limited to a few samples. While the deepest event is interpreted to mark the vertical variation of the Moho at each location, all other events are only representative of short-time ponding with fluid re-equilibration. The deepening of the Moho towards the central volcanoes might be the effect of long-lasting underplating. However, the periodic stretching of the lithosphere, in response to the differential stress field acting at central volcanoes, would have been responsible for the thinning of the crust at the central segment of the fissure zone. Supplementary material: Whole-rock geochemistry with international standard used, and representative analyses of selected mineral phases from the two segments of the Região dos Picos fissure zone is available at www.geolsoc.org.uk/SUP18818

Analysis of regional joint orientations, vein mineral paragenesis, and fluid inclusion microthermometry provides valuable insights into the deformation history of the Central Appalachians. These data indicate that deformation occurred as a continuum from hinterland to foreland, and they reflect a progressive rotation in shortening direction. However, separate deformation milestones punctuate the history. Prior to North Mountain thrust emplacement, the rocks in the Valley and Ridge Province were fractured under a NNW-directed shortening. The emplacement of the North Mountain thrust sheet occurred under a more NW-directed shortening and resulted in rapid loading of the Paleozoic section in the footwall by 7–10 km of rock that extended 20–30 km toward the hinterland from the North Mountain thrust ramp. The loading of the Paleozoic section resulted in overpressuring of Cambrian Waynesboro décolle-ment and the development of the Valley and Ridge duplex, and the formation of the Adams Run–Cacapon Mountain anticlinorium in the northern part of the study area. Growth of this anticlinorium shed more sediments toward the foreland, thereby driving addition horse formation. Continued NW- and then WNW-directed shortening resulted in the formation of the Wills Mountain duplex under the load of the North Mountain thrust sheet and sediments derived from the Adams Run–Cacapon Mountain anticlinorium. The growth of the Wills Mountain duplex resulted in duplication of the Cambrian–Ordovician carbonate section with coeval syntectonic erosion, shedding 3–4 km of sediments into the plateau, and triggering the formation of several thrusts in the Cambrian–Ordovician carbonate section. A final E-W–directed shortening event resulted in extensive regional mesoscale fracturing.