Cenozoic lateritic weathering and erosion history of Peninsular India from 40Ar/39Ar dating of supergene K–Mn oxides

Abstract

Since Deccan Traps extrusion ~65Ma ago, thick weathering mantles have developed over Peninsular India on both the western coastal lowland and adjacent eastern plateau separated by the Western Ghats Escarpment. Manganiferous lateritic profiles formed by supergene weathering of Late Archean manganiferous protores are exposed on paleolandsurface remnants on both sides of the escarpment. Petrological and geochemical characterizations of samples from those Mn lateritic profiles allowed identifying cryptomelane (K–Mn oxide) dated by 40Ar/39Ar geochronology. The ages obtained document major weathering periods, ca. 53–50Ma, and ca. 37–23Ma in the highland, and ca. 47–45Ma, ca. 24–19Ma and discrete weathering pulses at ~9Ma and ~2.5Ma in the lowland. Old ages of the highland (53–50Ma) and the lowland (47–45Ma) indicate synchronous lateritic weathering across the escarpment at a time the peninsula started to drift across the equatorial belt. Intense weathering periods at ca. 53–45 and ca. 37–23Ma are interpreted to reflect the Early Eocene climatic optimum and the onset of Asian monsoon regimes, respectively. The ages further indicate that most of the dissection of the highland must have taken place after ~23Ma, whereas the lowland was weakly incised essentially after ~19Ma. Our results also document divergent erosion and weathering histories of the lowland and the highland after the Eocene, suggesting installation of a dual climatic regime across the Western Ghats escarpment.