Regolith and landscape evolution in Peninsular India and West Africa: Morphoclimatic evolution of the two continents over the Cenozoic

International audience; Shields’ surfaces of the tropical belt have been continuously shaped over the Cenozoic under thecombined or alternating effects of chemical weathering and mechanical erosion that left steppedrelict lateritic paleolandsurfaces exposing different generations and type of regolith in today’ssceneries. These lateritic paleolandscape remnants are well preserved in West Africa and in highlandPeninsular India, particularly on Deccan Traps. The stepped character of such paleolandscaperemnants allows to describing a common geomorphic sequence of three successive sub-continentalscale lateritic paleolandsurfaces on the two sub regions. The first surface is defined by the oldestremnants, which are generally topped by Al-Fe (mostly bauxitic) lateritic duricrusts upon distant kmscalemesas or as larger provinces on high relictual topographic massifs (e.g., Fouta Djalon in WestAfrica or Nilgiri hills in South India). The relict bauxitic landforms generally dominate from less thanca. 300 m the relicts of a second geomorphic level (so-called “intermediate” surface), which ismantled by ferruginous lateritic duricrusts. The third and last paleolandsurface remnants lie less thanca. 400 m below the bauxitic landforms, and consist in a weathered lateritic pediment that is locallycapped by a ferricrete.The ages of these continental-scale lateritic paleolandsurfaces may be bracketed using 40Ar/39Ardating of K-Mn oxides (cryptomelane) formed in their underlying weathering profiles in the Africanand Indian contexts [1,2,3]. The first surface is Eocene and correlates with the Eocene climaticoptimum (ca. 50 Ma) that is recorded throughout the tropical belt by the production of bauxite. InSouth India, the Intermediate surface has evolved by dominant chemical weathering since the LateEocene (ca. 37 Ma) and records peak weathering activity in the Late Oligocene. In West Africa, thatpaleolandsurface seems to have only record the late Oligocene interval (ca. 29-24 Ma) of intenseweathering. Abandonment of the Intermediate landscape as a result of its dissection by the rivernetwork took place in the Latest Oligocene on both continents. By contrast, the later pedimentseems to have been shaped quite rapidly (ca. 32-29 Ma) and was weathered around the Oligocene-Miocene boundary (ca. 29-24 Ma) in India, whereas it took longer to form (ca. 24-18 Ma) and wasweathered mostly during the Mid-Miocene (ca. 18-11 Ma) in West Africa. The contrasts in themorphoclimatic record of the two sub regions are linked to the spatial diversification of climaticregimes after the Eocene climatic optimum. However, the combination of the ages with the elevationdifferences between each lateritic paleolandsurface documents denudation rates with comparableand very low amplitudes (5-15 m/m.y.) in these two continents over the last 50 Ma.[1] Beauvais A and Chardon D (2013) Geochem Geophys Geosyst 14:1590-1608, doi:10.1002/ggge.20093.[2] Bonnet NJ et al. (2014) Earth Planet Sci Lett 386:126-137, doi:10.1016/j.epsl.2013.11.002[3] Bonnet NJ et al. (2016) Chem Geol, in press.