Abstract The South Atlantic and Gulf of Mexico conjugate-margin salt basins display similar relationships between crustal architecture and presalt and salt sequences. 3D and 2D depth-migrated seismic data reveal that: (1) the base salt is mostly smooth but drops into an outer marginal trough just landward of and deeper than oceanic crust; (2) the Moho climbs basinward to shallow depths and is largely absent below the trough; (3) faults are low-angle and asymmetrical beneath the smooth base salt but steeper and symmetrical in the trough; (4) the trough contains triangular highs between faulted lows; and (5) the smooth base salt is underlain by sag sequences that often dip and thicken basinward. The observations suggest that these salt basins shared a common evolution. Crustal faulting gradually shifted basinward. Consequent thermal/loading subsidence plus lower-crustal thinning generated basinward-shifting accommodation for sag sequences, but slow sedimentation relative to subsidence resulted in deep depressions. A switch to symmetrical boudinage of thinned crust created the troughs and possible reactive mantle diapirs. Evaporites formed during this stage, with deposition near sea-level in proximal positions but 2–3 km deep in basin centres. Oceanic spreading separated the salt into conjugate basins, with allochthonous flow out over oceanic crust.

Abstract We applied a computational method to aid in clustering 41 alluvial fans along the southern coast of the Gulf of Corinth, Greece. The morphology of the fans and their catchments was quantitatively expressed through 12 morphometric parameters estimated using geographical information system techniques and the relationships among the geomorphometric features of the fans and their catchments were examined. Self-organizing maps were used to investigate the clustering tendency of fans based on morphometric variables describing both the fans and their corresponding catchments. The results of unsupervised classification through the self-organizing maps method revealed correlations among the morphometric parameters and five groups of alluvial fans were identified. These groups had a clear physical explanation, showed a preferred geographical distribution and reflected the processes related to the development of the fans. The geographical distribution of the fan catchment groups was partially controlled by variations in the relative tectonic uplift rate, which was the main control on the accommodation space for the development and accretion of the fans. The smaller fans were located in the central part of the study area, where the uplift rates were higher, whereas larger fluvial-dominated fan deltas formed to the east and west of the central group, where the uplift rates were lower.

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: The northern North Sea region has experienced repeated phases of post-Caledonian extension, starting with extensional reactivation of the low-angle basal Caledonian thrust zone, then the formation of Devonian extensional shear zones with 10–100 km-scale displacements, followed by brittle reactivation and the creation of a plethora of extensional faults. The North Sea Rift-related approximately east–west extension created a new set of rift-parallel faults that cut across less favourably orientated pre-rift structures. Nevertheless, fault rock dating shows that onshore faults and shear zones of different orientations were active throughout the history of rifting. Several of the reactivated major Devonian extensional structures can be extrapolated offshore into the rift, where they appear as bands of dipping reflectors. They coincide with large-scale boundaries separating 50–100 km-wide rift domains of internally uniform fault patterns. Major north–south-trending rift faults, such as the Øygarden Fault System, bend or terminate against these boundaries, clearly influenced by their presence during rifting. Hence, the North Sea is one of several examples where pre-rift basement structures oblique to the rift extension direction can significantly influence rift architecture, even if most of the rift faults are newly-formed structures.

Abstract: Salt is mechanically weaker than other sedimentary rocks in rift basins. It commonly acts as a strain localizer, and decouples supra- and sub-salt deformation. In the rift basins discussed in this paper, sub-salt faults commonly form wide and deep ramp synclines controlled by the thickness and strength of the overlying salt section, as well as by the shapes of the extensional faults, and the magnitudes and slip rates along the faults. Upon inversion of these rift basins, the inherited extensional architectures, and particularly the continuity of the salt section, significantly controls the later contractional deformation. This paper utilizes scaled sandbox models to analyse the interplay between sub-salt structures and supra-salt units during both extension and inversion. Series 1 experiments involved baseline models run using isotropic sand packs for simple and ramp-flat listric faults, as well as for simple planar and kinked planar faults. Series 2 experiments involved the same fault geometries but also included a pre-extension polymer layer to simulate salt in the stratigraphy. In these experiments, the polymer layer decoupled the extensional and contractional strains, and inhibited the upwards propagation of sub-polymer faults. In all Series 2 experiments, the extension produced a synclinal hanging-wall basin above the polymer layer as a result of polymer migration during the deformation. During inversion, the supra-polymer synclinal basin was uplifted, folded and detached above the polymer layer. Changes in thickness of the polymer layer during the inversion produced primary welds and these permitted the sub-polymer deformation to propagate upwards into the supra-salt layers. The experimental results are compared with examples from the Parentis Basin (Bay of Biscay), the Broad Fourteens Basin (southern North Sea), the Feda Graben (central North Sea) and the Cameros Basin (Iberian Range, Spain).

Abstract Transform-margin development around the Arctic Ocean is a predictable geometric outcome of multi-stage spreading of a small, confined ocean under radically changing plate vectors. Recognition of several transform-margin stages in the development of the Arctic Ocean enables predictions to be made regarding tectonic styles and petroleum systems. The De Geer margin, connecting the Eurasia Basin (the younger Arctic Ocean) and the NE Atlantic during the Cenozoic, is the best known example. It is dextral, multi-component, features transtension and transpression, is implicated in microcontinent release, and thus bears close comparison with the Equatorial Shear Zone. In the older Arctic Ocean, the Amerasia Basin, Early Cretaceous counterclockwise rotation around a pole in the Canadian Mackenzie Delta was accommodated by a terminal transform. We argue on geometric grounds that this dislocation may have occurred at the Canada Basin margin rather than along the more distal Lomonosov Ridge, and review evidence that elements of the old transform margin were detached by the Makarov–Podvodnikov opening and accommodated within the Alpha–Mendeleev Ridge. More controversial is the proposal of transform along the Laptev–East Siberian margin. We regard an element of transform motion as the best solution to accommodating Eurasia and Makarov–Podvodnikov Basin opening, and have incorporated it into a three-stage plate kinematic model for Cretaceous–Cenozoic Arctic Ocean opening, involving the Canada Basin rotational opening at 125–80 Ma, the Makarov–Povodnikov Basin opening at 80–60 Ma normal to the previous motion and a Eurasia Basin stage from 55 Ma to present. We suggest that all three opening phases were accompanied by transform motion, with the right-lateral sense being dominant. The limited data along the Laptev–East Siberian margin are consistent with transform-margin geometry and kinematic indicators, and these ideas will be tested as more data become available over less explored parts of the Arctic, such as the Laptev–East Siberia–Chukchi margin.

Abstract The study focuses on the offshore Guyana–Suriname–French Guiana region. It draws from seismic, well, gravimetric and magnetic data. They indicate that the continental break-up along the western margin of the Demerara Plateau took place during the Callovian–Oxfordian, associated with the Central Atlantic opening, and accommodated by normal faults. The continental break-up in the SE offshore Guyana accommodated by strike-slip faults was coeval. The continental break-up along the NE and eastern margins of the Demerara Plateau took place during the late Aptian–Albian, associated with the opening of the Equatorial Atlantic, and accommodated by dextral strike-slip and normal faults, respectively. Different spreading vectors of the Central and Equatorial Atlantic required development of the Accommodation Block during the late Aptian/Albian–Paleocene in their contact region, and in the region between the Central Atlantic and its southernmost portion represented by the Offshore Guyana Block, which were separated from each other by the opening Equatorial Atlantic. Its role was to accommodate for about 20° mismatch between the Central and Equatorial Atlantic spreading vectors, which has decreased from the late Aptian/Albian to Paleocene down to 0°. Differential movements between the Central and Equatorial Atlantic oceans were also accommodated by strike-slip faults of the Guyana continental margin, some active until the Paleocene.

Abstract We use the Bay of Biscay and Western Pyrenees as a natural laboratory to develop and apply an approach to characterize and identify distinctive rifted margin domains in offshore and onshore settings. The Bay of Biscay and Western Pyrenees offer access to seismically imaged, drilled and exposed parts of one and the same hyperextended rift system. Offshore, we use gravity inversion and flexural backstripping techniques combined with seismic interpretation to provide estimates of accommodation space, crustal thickness and lithosphere thinning. Onshore, we focus on key outcrops of the former rift domain to describe the nature of sediment and basement rocks, and of their interface. This qualitative and quantitative characterization provides diagnostic elements for the identification of five distinct structural domains at magma-poor rifted margins and their fossil analogues (proximal, necking, hyperthinned, exhumed mantle and oceanic domains). This new approach can be used to reconcile offshore and onshore observations, and to aid interpretation when only local observations are available. Onshore remnants can be placed in an offshore rifted-margin context, enabling the prediction of first-order crustal architecture. For the interpretation of offshore seismic reflection sections, geological insights into rift structures and basement nature can be suggested based on onshore analogies. Supplementary material: Sensitivity of backstripping results to flexural rigidity is available at http://www.geolsoc.org.uk/SUP18778 .