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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Understanding the evolution of submarine channel-lobe systems on salt-influenced slopes is challenging as systems react to seemingly subtle changes in sea-floor topography. The impact of large blocking structures on individual deep-water systems is well documented, but understanding of the spatio-temporal evolution of regionally extensive channel-lobe systems in areas containing modest salt movement is relatively poor. We use 3-D seismic reflection data to map Late Miocene deep-water systems offshore Angola within a c. 450 ms TWTT thick interval. Advanced seismic attribute mapping tied to standard seismic stratigraphic, seismic facies analysis and time-thickness variations, reveals a wide variety and scale of alterations to sediment routing and geomorphology. Five seismic units (SU1-SU5) record a striking change in sediment pathways and structural relief within eight evolving minibasins. Observations within these units include gradual channel diversion through lateral migration during times of relatively high structural growth, opposed to abrupt channel movement via avulsion nodes during times of relatively high sediment accumulation. The results of the study led to the development of conceptual models for influences on deep-water systems during characteristic structural development in the contractional salt domain, these stages being initiation, maturity, and decay. The initiation stage contains small-segmented folds with mostly system bypass, while the maturity stage contains linked high-relief structures and prominent minibasins leading to ponding, large-scale diversion and localized slump deposits derived from nearby highs (SU1-SU3). The less studied final stage of topographic decay contains decreased length and relief of structures leading to a more complicated array of channel-lobe bypass, diversion, ponding and subtle control on avulsion nodes (SU4-SU5). These observations contribute to the understanding of channel-lobe stacking, routing and control over transition zones in tectonically active areas, ultimately improving our general understanding of the effects of significant through to subtle sea-floor topography, and can be a guide in other salt-influenced basins.