Late Neogene passive margin denudation history—cosmogenic isotope measurements from the central Namib desert

In this paper, we review ideas on the geomorphological history of the southwest African passive margin, focusing on the central Namib sector and presenting new evidence on the late Neogene landscape evolution of this region. The hyperarid central Namib Desert occupies the 100–150-km-wide pediment at the foot of the Great Escarpment and forms part of the southwest African passive margin, which formed after breakup in the South Atlantic at around 118 Ma. Previous apatite fission track (AFT) and cosmogenic isotope studies and numerical models of coupled tectonic-surface processes in the same area suggest that long-term denudation rates of this passive margin (after a period of significant post-rift denudation) have been very low, 5 m/my. Aridity of the Namib Desert is generally assumed to have started with the onset of Benguela upwelling in the SE Atlantic at 10–15 Ma and to have prevailed ever since. It has been implied that during this period, the landscape has undergone only marginal change. Here, we present new evidence from in situ cosmogenic 21Ne dating of pediment surfaces, river terraces and river cut bedrock benches. Correlation of terrace sediment bodies defining ancient river courses is supported by a sediment provenance study using heavy mineral analyses. Our results generally confirm previous work, adding new insights in the late Neogene landscape evolution of the central Namib. They indicate that denudation rates since at least 5 Ma have been even lower (of the order of 0.5–1 m/my) than those calculated by previous studies of inselbergs. In the central Namib, inselbergs appear to be located in or near areas of Plio/Pleistocene incision and badland erosion. These inselbergs therefore are not the best sites to obtain representative passive margin denudation rates. We obtained a 21Ne age of 5.18±0.18 Ma from a resistant quartz vein projecting above pediment unaffected by recent incision, the oldest exposure age from the Namib so far. The pediment surface stretching from the base of the escarpment to the ocean, on average, probably has undergone less than 10 m of surface lowering since the mid Miocene. Our results also indicate that a late Neogene episode of accelerated denudation in the central Namib Desert and incision of deep canyons started around 2.81±0.11 Ma ago. We interpret this as a signal of increasingly wet conditions in this region, in response to Plio/Pleistocene global cooling. As aggradation of the river systems is followed by degradation, their drainage patterns change from braided to meandering. Evidence of, yet undated, earlier incision episodes suggests that the denudation history of this passive margin is one of slow long-term denudation—as other workers concluded on the basis of AFT and cosmogenic nuclide analyses—interrupted by pulses of rapid denudation.