Abstract The thermal evolution of continental rifted margins is key to understanding margin subsidence and hydrocarbon prospectivity. Observed heat-flow values, however, do not always comply with classic rifting models. Here, we use 2D numerical models to investigate the relationship between rifting, sedimentation and thermal history of margins. We find that during the synrift, the basement heat-flow and temperature are not only controlled by extension factor, but also by synrift sediment thickness and the evolution of deformation. As this progressively focuses oceanward, the proximal sectors thermally relax, while the distal sectors experience peak temperatures. In the post-rift, the lithosphere under the hyperextended margins does not return to its original state, at least for c. 100 Myr after break-up. Instead, it mimics that of the adjacent oceanic plate, which is thinner than the original continental plate. This results in heat-flow increasing oceanward at post-rift stages, when classic rifting theory predicts complete thermal relaxation. Our models also predict slightly increased heat-flows in the adjacent oceanic crust, potentially extending hydrocarbon plays into distal margins and oceanic crust, previously discarded as immature. Finally, our models indicate that commonly used temperature approximations to calculate heat-flow during rifting may strongly differ from those occurring in nature.

Abstract This paper presents the time and space evolution of crustal deformation and their respective sedimentary infill of the 600 km wide, asymmetric conjugate rifted margin of the Santos–Benguela basins. Based on a geoseismic transect obtained with interpretation of long-offset seismic reflection and tied by wells, we interpret six main synrift unconformities, corresponding to different deformation phases processed from the Valanganian to Early Albian. Confined by these unconformities, sedimentary growths with progressively young relative ages towards the boundary with the oceanic crust are interpreted as evidence of oceanward rift migration. The combination of this information with crustal structure derived from long-offset seismic reflection illuminating the deep crust of the Santos–Benguela conjugate margins, resulted in a complete view of sedimentary infill, internal compartments, and crustal structure. These data were used to guide a dynamic model of rifting resulting in a simulated lithospheric section. We show that the margin architecture can be explained by the combination of an early, protracted phase of distributed deformation, followed by basinward rift migration. Distributed deformation lasted from the Valanginian to Early Aptian (135–117 Ma), initiating with isolated lakes that later coalesced into a wide basin-scale lake (>450 km). From the Mid Aptian to Early Albian (117–110 Ma), rift migration formed the main structural compartments and unconformities, as well as the distal hinge zones we observe today in the seismic lines. During this time, the inner proximal margins were left behind to thermally subside, whereas outer proximal and distal margins were tectonically active. Coexistence of these two processes explains the enigmatic simultaneous formation of proximal sag-like geometries, with late synrift accumulation of a salt layer up to 3 km thick, with tectonically active faults in the distal margin, promoting crestal block uplift that could explain the deposition of Late Aptian, shallow water, pre-salt carbonate rocks.

Abstract Hyperextended, magma-poor margins are characterized by a wide continent–ocean transition and anomalously small fractions of magmatism during mantle exhumation prior to oceanic spreading. Here, I bring together several aspects of their rift to drift transition and give a coherent picture of their evolution from platform to deep-sea environments. I focus mainly on the West Iberia Margin (WIM)–Newfoundland (NF) conjugates in the North Atlantic Ocean. The architectural evolution of these margins is characterized by upper-crustal faulting and lower-crustal deformation that are tightly coupled, resulting in fault displacement that is accompanied by underlying, equal and coeval crustal thinning. Lower crust deforms first ductilely, but then progressively switches to brittle due to enhanced conductive cooling at very slow extension velocities (< c. 6 mm a −1 half-rate). The switch from ductile to progressively brittle lower crust is accompanied by the emergence of a dominant basinwards fault dip and oceanwards younging of fault activity. It is shown that these processes, acting in concert: (1) reconcile the horizontal extension on faults with crustal thinning without the need of lower-crustal flow; (2) explain, within one common Andersonian framework (faults active at 65°–30°), the change in fault geometry from planar to listric to detachment-like with increasing extension; and (3) generate the tectonic asymmetry observed between conjugate pairs. This work also discusses a high-resolution seismic section of the WIM showing that the ‘detachment-like’ fault S is truncated prior to the peridotite ridge where mantle exhumation first takes place. This suggests that serpentinized mantle rises due to its own buoyancy, separating and pulling the thinned crustal blocks apart. Once the crust has been separated, further mantle exhumation takes place by magma-poor extension of the underlying mantle. I show with numerical models that either a small reduction of mantle potential temperature ( c. 25–50 °C), a mantle depletion of more than 10% or very slow half-extension velocities ( c. 6 mm a −1 ) are required to reproduce the small amount of magmatism inferred. Available observations support either a very slow extension velocity or a smaller than normal mantle temperature; however, estimation errors may be large. Ultimately, unravelling which of these factors most contribute to magma-poor mantle exhumation will provide an improved understanding of the mantle lateral homogeneity and the three-dimensional nature of the rifting to drifting process.