Turbidity currents descending the slopes of deep‐water extensional basins or passive continental margins commonly encounter normal‐fault escarpments, but such large‐magnitude phenomena are hydraulically difficult to replicate at small scale in the laboratory. This study uses advanced computational fluid dynamics numerical simulations to monitor the response of large, natural‐scale unconfined turbidity currents (100 m thick and 2000 m wide at the inlet gate) to normal‐fault topography with a maximum relief of nearly 300 m. For comparative purposes, the turbidity current is first released on a non‐faulted pristine slope of 1·5° (simulation model 1). The expanding and waxing flow bypasses the slope without recognizable deposition within the visibility limit of 8 vol.% sand grain packing. Similar flow is then released towards the tip (model 2) and towards the centre (model 3) of a normal‐fault escarpment. In both of these latter models, the sand carried by flow tends to be entrapped in four distinct depozones: an upslope near‐gate zone of flow abrupt expansion and self‐regulation; a flow‐transverse zone at the fault footwall edge; a flow‐transverse zone at the immediate hangingwall; and a similar transverse zone near the crest of the hangingwall counter‐slope, where some of the deposited sand also tends to be reshuffled to the previous zone by a secondary reverse underflow. The near‐bottom reverse flow appears to be generated on a counter‐slope of 1·1°, increased to 2·0° by deposition. The Kelvin–Helmholtz interface instability plays an important role by causing three‐dimensional fluctuations in the flow velocity magnitude and sediment concentration. The thick deposits of large single‐surge flows may thus show hydraulic fluctuations resembling those widely ascribed to hyperpycnal flows. The study indicates further that the turbiditic slope fans formed on such fault topographies are likely to be patchy and hence may differ considerably from the existing slope‐fan conceptual models when it comes to the spatial prediction of main sand depozones.

This natural-scale experimental study combines structural modelling of soft-linked normal-fault relays with a CFD (computational fluid dynamics) numerical simulation of a range of unconfined turbidity currents overrunning the relay-system topography. The flow, released from an upslope inlet gate 2000-m wide and 50-m to 100-m high, rapidly expands and adjusts its thickness, velocity and sediment load to the substrate slope of 1.5°. A lower initial sediment concentration or smaller thickness renders the quasi-steady flow slower and its sediment-transport capacity lower. A 3D pattern of large interfering Kelvin-Helmholtz waves causes fluctuations of the local flow velocity magnitude and sediment concentration. Four zones of preferential sediment deposition are recognized: a near-gate zone of abrupt flow expansion and self-regulation; a flow-transverse zone on the counterslope of fault footwall edges; a flow-transverse zone at the fault-scarp toes and a similar transverse zone near the crest of the hanging wall counter-slopes. The sand deposited on the counter-slope tends to be re-entrained and fed back to the current by a secondary reverse underflow. The spatial extent and sediment accumulation capacity of depozones depend upon the released current volume. The impact of relay system on an overrunning current depends upon the fault separation distance and stage of tectonic evolution. An early-stage relay system, with small vertical displacement and little overlap of faults, is bypassed by the current with minimum flow disturbance and no pronounced deposition. An advanced-stage system, with greater fault displacement and overlap, gives a similar hydraulic effect as a single fault segment if the fault separation is small. If the separation is relatively large, the flow tends to be internally redirected sideways from the ramp into the hanging wall synclinal depressions. Since normal-fault relays are common features in extensional basins, the study bears important implications for turbiditic slope-fan models and for the spatial sand prediction in subsurface exploration of faulted submarine slopes.

Sea‐floor topography of deep‐water folds is widely considered to have a major impact on turbidity currents and their depositional systems, but understanding the flow response to such features was limited mainly to conceptual notions inspired by small‐scale laboratory experiments. High‐resolution three‐dimensional numerical experiments can compensate for the lack of natural‐scale flow observations. The present study combines numerical modelling of thrusts with fault‐propagation folds by Trishear3D software with computational fluid dynamics simulations of a natural‐scale unconfined turbidity current by MassFlow‐3D™ software. The study reveals the hydraulic and depositional responses of a turbidity current (ca 50 m thick) to typical topographic features that it might encounter in an orthogonal incidence on a sea‐floor deep‐water fold and thrust belt. The supercritical current (ca 10 m sec−1) decelerated and thickened due to the hydraulic jump on the fold backlimb counter‐slope, where a reverse overflow formed through current self‐reflection and a reverse underflow was issued by backward squeezing of a dense near‐bed sediment load. The reverse flows were re‐feeding sediment to the parental current, reducing its waning rate and extending its runout. The low‐efficiency current, carrying sand and silt, outran a downslope distance of >17 km with only modest deposition (<0·2 m) beyond the fold. Most of the flow volume diverted sideways along the backlimb to surround the fold and spread further downslope, with some overspill across the fold and another hydraulic jump at the forelimb toe. In the case of a segmented fold, a large part of the flow went downslope through the segment boundary. Preferential deposition (0·2 to 1·8 m) occurred on the fold backlimb and directly upslope, and on the forelimb slope in the case of a smaller fold. The spatial patterns of sand entrapment revealed by the study may serve as guidelines for assessing the influence of substrate folds on turbiditic sedimentation in a basin.

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Studies of salt‐influenced rift basins have focused on individual or basin‐scale fault system and/or salt‐related structure. In contrast, the large‐scale rift structure, namely rift segments and rift accommodation zones and the role of pre‐rift tectonics in controlling structural style and syn‐rift basin evolution have received less attention. The Norwegian Central Graben, comprises a complex network of sub‐salt normal faults and pre‐rift salt‐related structures that together influenced the structural style and evolution of the Late Jurassic rift. Beneath the halite‐rich, Permian Zechstein Supergroup, the rift can be divided into two major rift segments, each comprising rift margin and rift axis domains, separated by a rift‐wide accommodation zone – the Steinbit Accommodation Zone. Sub‐salt normal faults in the rift segments are generally larger, in terms of fault throw, length and spacing, than those in the accommodation zone. The pre‐rift structure varies laterally from sheet‐like units, with limited salt tectonics, through domains characterised by isolated salt diapirs, to a network of elongate salt walls with intervening minibasins. Analysis of the interactions between the sub‐salt normal fault network and the pre‐rift salt‐related structures reveals six types of syn‐rift depocentres. Increasing the throw and spacing of sub‐salt normal faults from rift segment to rift accommodation zone generally leads to simpler half‐graben geometries and an increase in the size and thickness of syn‐rift depocentres. In contrast, more complex pre‐rift salt tectonics increases the mechanical heterogeneity of the pre‐rift, leading to increased complexity of structural style. Along the rift margin, syn‐rift depocentres occur as interpods above salt walls and are generally unrelated to the relatively minor sub‐salt normal faults in this structural domain. Along the rift axis, deformation associated with large sub‐salt normal faults created coupled and decoupled supra‐salt faults. Tilting of the hanging wall associated with growth of the large normal faults along the rift axis also promoted a thin‐skinned, gravity‐driven deformation leading to a range of extensional and compressional structures affecting the syn‐rift interval. The Steinbit Accommodation Zone contains rift‐related structural styles that encompass elements seen along both the rift margin and axis. The wide variability in structural style and evolution of syn‐rift depocentres recognised in this study has implications for the geomorphological evolution of rifts, sediment routing systems and stratigraphic evolution in rifts that contain pre‐rift salt units.

Abstract. Current models of gravitational tectonics on the structural styles of salt-influenced passive margins typically depict domains of upslope extension and corresponding downslope contraction separated by a mid-slope domain of translation that is rather undeformed. However, an undeformed translational domain is rarely observed in natural systems as extensional and contractional structures tend to interfere in the mid-slope area. In this study, we use sandbox analogue modelling analysed by digital image correlation (DIC) to investigate some of the factors that control the structural evolution of translational domains. As in nature, experimental deformation is driven by slowly increasing gravitational forces associated with continuous basal tilting. The results show that a translational domain persists throughout the basin evolution when the pre-kinematic layer is evenly distributed. However, a thin (1 mm in the experiment, 100 m in nature) pre-kinematic layer can render the translational domain relatively narrow compared to settings with a thicker (5 mm) pre-kinematic layer. In contrast, early differential sedimentary loading in the mid-slope area creates minibasins separated by salt diapirs overprinting the translational domain. Similarly, very low sedimentation rate (1 mm per day in the experiment, < 17 m Ma−1 in nature) in the early stage of the experiment results in a translational domain quickly overprinted by downslope migration of the extensional domain and upslope migration of the contractional domain. Our study suggests that the architecture of passive margin salt basins is closely linked to the pre- and syn-kinematic cover thickness. The translational domain, as an undeformed region in the supra-salt cover, is a transient feature and overprinted in passive margins with either low sedimentation rate or a heterogeneous sedimentation pattern.

Salt tectonics is an important part of the geological evolution of many continental margins, yet the four-dimensional evolution of the minibasins, the fundamental building block of these and many other salt basins, remains poorly understood. Using high-quality 3D seismic data from the Lower Congo Basin, offshore Angola we document the long-term (&gt;70 Myr) dynamics of minibasin subsidence. We show that, during the Albian, a broadly tabular layer of carbonate was deposited prior to substantial salt flow, diapirism, and minibasin formation. We identify four subsequent stages of salt-tectonics and related minibasin evolution: (i) thin-skinned extension (Cenomanian to Coniacian) driven by basinward tilting of the salt layer, resulting in the formation of low-displacement normal faults and related salt rollers. During this stage, local salt welding led to the along-strike migration of fault-bound depocentres; (ii) salt welding below the eastern part of the minibasin (Santonian to Paleocene), causing a westward shift in depocentre location; (iii) welding below the minibasin centre (Eocene to Oligocene), resulting in the formation of a turtle and an abrupt shift of depocentres towards the flanks of the bounding salt walls; and (iv) an eastward shift in depocentre location due to regional tilting, contraction, and diapir squeezing (Miocene to Holocene). Our study shows that salt welding and subsequent contraction are key controls on minibasin geometry, subsidence and stratigraphic patterns. In particular, we show how salt welding is a protracted process, spanning &gt;70 Myr of the salt-tectonic history of this, and likely other salt-rich basins. The progressive migration of minibasin depocentres, and the associated stratigraphic architecture, record weld dynamics. Our study has implications for the tectono-stratigraphic evolution of minibasins.