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Atlantic-type continental margins have long been considered "passive" tectonic settings throughout the entire postrift phase. Recent studies question the long-term stability of these margins and have shown that postrift uplift and reactivation of preexisting structures may be a common feature of a continental margin's evolution. The Namaqualand sector of the western continental margin of South Africa is characterized by a ubiquitously faulted basement but lacks preservation of younger geological strata to constrain postrift tectonic fault activity. Here we present the first systematic study using joint apatite fission track and apatite (U-Th-Sm)/He thermochronology to achieve a better understanding on the chronology and tectonic style of landscape evolution across this region. Apatite fission track ages range from 58.3 ± 2.6 to 132.2 ± 3.6 Ma, with mean track lengths between 10.9 ± 0.19 and 14.35 ± 0.22 μm, and mean (U-Th-Sm)/He sample ages range from 55.8 ± 31.3 to 120.6 ± 31.4 Ma. Joint inverse modeling of these data reveals two distinct episodes of cooling at approximately 150-130 Ma and 110-90 Ma with limited cooling during the Cenozoic. Estimates of denudation based on these thermal histories predict approximately 1-3 km of denudation coinciding with two major tectonic events. The first event, during the Early Cretaceous, was driven by continental rifting and the development and removal of synrift topography. The second event, during the Late Cretaceous, includes localized reactivation of basement structures as well as regional mantle-driven uplift. Relative tectonic stability prevailed during the Cenozoic, and regional denudation over this time is constrained to be less than 1 km.

International audienceIntracontinental deformation accommodated along major lithospheric scale shear zone systems and within associated extensional basins has been well documented within West, Central and East Africa during the Late Cretaceous. The nature of this deformation has been established by studies of the tectonic architecture of sedimentary basins preserved in this part of Africa. In southern Africa, where the post break-up history has been dominated by major erosion, little evidence for post-break-up tectonics has been preserved in the onshore geology. Here we present the results of 38 new apatite fission track analyses from the Damara region of northern Namibia and integrate these new data with our previous results that were focused on specific regions or sections only to comprehensively document the thermo-tectonic history of this region since continental break-up in the Early Cretaceous. The apatite fission track ages range from 449 ± 20 Ma to 59 ± 3 Ma, with mean confined track lengths between 14.61 ± 0.1 μm (SD 0.95 μm) to 10.83 ± 0.33 μm (SD 2.84 μm). The youngest ages (c. 80–60 Ma) yield the longest mean track lengths, and combined with their spatial distribution, indicate major cooling during the latest Cretaceous. A simple numerical thermal model is used to demonstrate that this cooling is consistent with the combined effects of heating caused by magmatic underplating, related to the Paraná-Etendeka continental flood volcanism associated with rifting and the opening of the South Atlantic, and enhanced erosion caused by major reactivation of major lithospheric structures within southern Africa during a key period of plate kinematic change that occurred in the South Atlantic and SW Indian ocean basins between 87 and 56 Ma. This phase of intraplate tectonism in northern Namibia, focused in discrete structurally defined zones, is coeval with similar phases elsewhere in Africa and suggests some form of trans-continental linkage between these lithospheric zones

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The timing and mechanisms involved in creating the elevated, low‐relief topography of the South African plateau remain unresolved. Here we constrain the thermal history of the Southwest African plateau since 300 Ma by using apatite fission track (AFT) and (U‐Th‐Sm)/He (AHe) thermochronology. Archean rocks from the center of the Kaapvaal Craton yield AFT ages of 331.0 ± 11.0 and 379.0 ± 23.0 Ma and mean track lengths (MTLs) of 11.9 ± 0.2 and 12.5 ± 0.3 µm. Toward the southwest margin of the craton and in the adjacent Paleozoic mobile belt, AFT ages are significantly younger and range from 58.9 ± 5.9 to 128.7 ± 6.3 Ma and have longer MTLs (>13 µm). The range of sample AHe ages complements the AFT ages, and single‐grain AHe ages for most samples are highly dispersed. Results from joint inverse modeling of these data reveal that the center of the craton has resided at near‐surface temperatures (<60°C) since 300 Ma, whereas the margins of the craton and the off‐craton mobile belt experienced two discrete episodes of cooling during the Cretaceous. An Early Cretaceous cooling episode is ascribed to regional denudation following continental breakup. Late Cretaceous cooling occurs regionally but is locally variable and may be a result of a complex interaction between mantle‐driven uplift and the tectonic setting of the craton margin. Across the entire plateau, samples are predicted to have remained at near‐surface temperatures throughout the Cenozoic, suggesting minimal denudation (<1 km) and relative tectonic stability of the plateau.

Extension of the continental lithosphere can lead to the formation of rifted margins with contrasting tectonic and geomorphologic characteristics. Many of these characteristics depend on the manner extension spatially distributed. Here we investigate the feedback between tectonics and the transfer of material at the surface resulting from erosion, transport, and sedimentation and discuss how they influence the rifting process. We use large-scale (1, 200 × 600 km), high-resolution (1 km) numerical experiments coupling a 2-D upper-mantle-scale thermo-mechanical model with a planform 2-D surface processes model. We test the sensitivity of the coupled models to varying crust-lithospheric rheology and erosional efficiency. We confirm that the development and long-term support of topography is dependent on the strength of the coupling between the crust and the mantle lithosphere. Strong coupling promotes high topography as the integrated strength of the lithosphere is sufficient to support the additional stress. Weak coupling results in the stress being relaxed via viscous flow in the middle/lower crust and leads to more subdued topography. Erosion and transport of sediment modulates this behavior but has only minor effect on the overall structure of the rift. High erosion efficiency counters the development of high topography and creates complex landscape morphologies while low erosion efficiency allows for longer standing high topography and results in more simple landscape morphologies. The transfer of mass between the continent and the basin alters the stress field at the onshore-offshore transition and facilitates the development of faults, increasing their offsets and keeping them active over a longer period.Our understanding of the coupling between surface processes and tectonics has developed primarily for convergent orogens (e.g.

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