Planation surfaces as a record of mantle dynamics: The case example of Africa

Abstract

International audienceThere are two types of emerged relief on the Earth: high elevation areas (mountain belts and rift shoulders) in active tectonic settings and low elevation domains (anorogenic plateaus and plains) characteristic of the interior of the continents i.e. 70% of the Earth emerged relief. Both plateaus and plains are characterized by large erosional surfaces, called planation surfaces that display undulations with middle (several tens of kilometres) to very long (several thousands of kilometres) wavelengths, i.e. characteristic of lithospheric and mantle deformations respectively.Our objective is here (1) to present a new method of characterization of the very long and long wavelength deformations using planation surfaces with an application to Central Africa and (2) to reconstruct the growth of the very long wavelength relief since 40 Ma, as a record of past mantle dynamics below Central Africa.(i) The African relief results from two major types of planation surfaces, etchplains (weathering surfaces by laterites) and pediplains/pediments. These planation surfaces are stepped along plateaus with different elevations. This stepping of landforms records a local base level fall due to a local tectonic uplift.(ii) Central Africa is an extensive etchplain-type weathering surface – called the African Surface – from the uppermost Cretaceous (70 Ma) to the Middle Eocene (45 Ma) with a paroxysm around the Early Eocene Climatic Optimum. Restoration of this surface in Central Africa suggests very low-elevation planation surfaces adjusted to the Atlantic Ocean and Indian Ocean with a divide located around the present-day eastern branch of the East African Rift.(iii) The present-day topography of Central Africa is younger than 40–30 Ma and records very long wavelength deformations (1000–2000 km) with (1) the growth of the Cameroon Dome and East African Dome since 34 Ma, (2) the Angola Mountains since 15–12 Ma increasing up to Pleistocene times and (3) the uplift of the low-elevation (300 m) Congo Basin since 10–3 Ma. Some long wavelength deformations (several 100 km) also occurred with (1) the low-elevation Central African Rise since 34 Ma and (2) the Atlantic Bulge since 20–16 Ma. These very long wavelength deformations record mantle dynamics, with a sharp increase of mantle upwelling around 34 Ma and an increase of the wavelength of the deformation and then of mantle convection around 10–3 Ma

Cradles and museums of generic plant diversity across tropical Africa

“…), possibly related to the topographic and edaphic complexity of East Africa. Indeed, East Africa has a greater elevation range than West or Central Africa (Guillocheau et al , ) and the tectonic activity that occurred in East Africa generated places that differ edaphically from surrounding areas. Edaphic complexity plays a great role in generating endemism (Bruchmann & Hobohm, ; Rahbek et al , ), and the habitat heterogeneity and fragmentation of montane regions are often associated with range‐restricted taxa (Moritz et al , ; Hughes & Eastwood, ; Kier et al , ; Fjeldså et al , ).…”

Ice‐margin fluctuation sequences and grounding zone wedges: The record of the Late Palaeozoic Ice Age in the eastern Karoo Basin (Dwyka Group, South Africa)

“…Synthetic depositional model for the Dwyka Group in the study area, depicting the three fluctuations of the ice‐margin and their associated deposits. The last cartoon shows the modern‐day situation where Cenozoic (Guillocheau et al , ) and sub‐modern erosion exhumed glacial landscapes carved by the Dwyka ice sheets.…”

“…The path of the modern Tugela and Buffalo rivers (Figures and C) closely follows the network of pre‐Dwyka fluvial and/or glacial valleys that had been completely filled up with glaciogenic sediments during the glacial period and subsequently re‐excavated during Cenozoic and Recent times. Hence, in KwaZulu‐Natal Province, in areas where glaciogenic deposits have been entirely eroded away, the modern topography exhibits Dwyka‐aged polished and striated surfaces as well as roches moutonnées carved into the ancient basement and hence corresponds to the exhumed Permo‐Carboniferous glacial landscape that prevailed at the onset of the deposition of the Dwyka sediments (von Brunn, , ; see also Guillocheau et al , ). Such an exhumed LPIA glacial landscape echoes those of the same age found in Namibia (Andrews et al , ), Chad (Le Heron, ) and in South America (Assine et al , ).…”

“…It should be noted, however, that although amplitudes of ice-margin advance-retreat fluctuations are not constrained here, the virtual absence of prominent deposits lacking glacial features, like the interglacial black shales in the western Karoo Basin (Visser, 1997;Isbell et al, 2008) suggests that F I G U R E 1 1 Synthetic depositional model for the Dwyka Group in the study area, depicting the three fluctuations of the ice-margin and their associated deposits. The last cartoon shows the modern-day situation where Cenozoic (Guillocheau et al, 2018) and sub-modern erosion exhumed glacial landscapes carved by the Dwyka ice sheets. retreat phases of the three fluctuations were probably of restricted amplitude and of short term.…”

The adjoint equations for thermochemical compressible mantle convection: derivation and verification by twin experiments

“…Through convective stresses, it also maintains a portion of the observed topography known as dynamic topography [1,2]. Recently, there has been much effort to quantify dynamic topography and its evolution as a link to mantle flow processes [3,4], for instance, from studies of continental scale stratigraphy, planation surfaces, elevated passive margins, thermochronology and basin analysis [5][6][7][8][9], although considerable uncertainty remains.…”