Résumé
Most shields of the tropical belt show extended lateritic landform relicts, which result fromconjugated chemical weathering and mechanical erosion processes over the Cenozoic. After DeccanTraps extrusion ca. 65 m.y. ago, Peninsular India has been shaped by successive periods of lateriticweathering and erosion on both sides of the Western Ghats escarpment (WGE), which separates acoastal lowland from a highland plateau. The weathering periods are documented from 40Ar/39Ardating of K-Mn oxides (cryptomelane, Kx Mn8-xIV MnxIII O16, nH2O) formed in Mn ore deposits and Fe-Mn lateritic weathering profiles, which are developed from Archean supracrustal rocks and exposedon stepped remnants of paleolandsurfaces on either side of the escarpment. The 40Ar/39Ar agesobtained document three major weathering periods [1], ca. 53-45 Ma both in the highland and thelowland, ca. 37-24 Ma only in the highland [2], and ca. 24-19 Ma only in the lowland [3]. These ages’series document evolving geomorphic and climatic patterns on either side of the WGE over theCenozoic.The intense lateritic weathering at 53-45 Ma and 37-24 Ma reflect the Early Eocene optimum and theonset of Asian monsoon regimes, when the peninsula drifted across the equatorial belt. Synchronousearly Eocene weathering on both sides of the WGE and the preservation of laterites as old as 47 Maon its piedmont attests to the stabilization of this marginal escarpment before that time, and furtherdocuments a slow denudation of the lowland since then [3]. The ages series also indicate that thehighland landsurfaces were mostly dissected after ca. 24 Ma, while little incision affected thelowland after ca. 19 Ma. Finally, the 40Ar/39Ar ages document a divergent erosion and weatheringhistory on either side of the escarpment suggesting a dual climatic regime across this topographicbarrier after the Eocene climatic optimum.[1] Bonnet NJ et al. (2016) Chem Geol, in press.[2] Bonnet NJ et al. (2014) Earth Planet Sci Lett 386:126-137, doi:10.1016/j.epsl.2013.11.002[3] Beauvais A et al. (2016) Geology, in press, doi:10.1130/G37303.1