Reconstruction of Post-Deccan topographies from mapping of relict lateritic paleosurfaces: Implications for long-term denudation of Peninsular India
Résumé
Cenozoic topographic rejuvenation of divergent continental margins and their cratonic hinterland is a matter of
debate. For instance, continental scale tilt has been advocated as a cause of Late Cenozoic relief growth of the
Indian Peninsula. Our work aims at providing independent geomorphological constraints to test such assertions.
The western margin of the Peninsula bears theWestern Ghats Escarpment (WGE), which carves both Precambrian
rocks and 65 Ma old Deccan Traps. The escarpment separates a narrow coastal lowland plain drained to the Arabian
Sea from a highland plateau drained towards the Bengal Sea. Alternating and coupled chemical weathering
and erosion led to formation and dissection of lateritic paleolandscapes preserved both sides of the WGE. In the
highland, the relicts of three distinctive lateritic surfaces are recognized. They formed successively from 53
Ma to 23 Ma on the basis of ages obtained by Ar-Ar dating of K-rich manganese oxides [1]. The first surface
is topped by a duricrust rich in Al (bauxite) forming mesas at altitudes of 960 to 1400 m. The second surface
called "Intermediate" is mantled by a ferricrete, whose relicts remain 100 m below the bauxite at altitudes of
850 to 1250 m. The third surface corresponds to the relicts of a lateritic pediment, which may be capped by a
ferricrete at altitudes of 600 to 900 m. Three lateritic surfaces have been documented in the lowland. The Ar-Ar
ages obtained indicate common lateritic weathering in both the highland and the lowland between at 53-45 Ma,
then a divergence later on [1]. The oldest remnants of lateritic surface in the lowland are preserved at maximum
elevation of 400-500 m close to the WGE and correspond to a pediment. This ancient landform was re-altered at
24-19 Ma to form residual hills of the “Intermediate” relief (ca. 350-200 m). A younger pediment formed around
the residual hills and is preserved at ca. 150-50 m elevation.
Relicts’ elevations of the three surfaces have been retrieved in both the lowland and the highland to reconstruct
successive topographies across the WGE and estimate relief evolution and erosion budgets for major time steps.
The results suggest that the denudation did not exceed 10 m/my in the Highland since 45 Ma, and is less than 11
m/my since mitigation of chemical weathering ca. 23 Ma ago. In the lowland, denudation was limited to 5 m/my
since 45 Ma and less than 6 m/my since 19 Ma, implying negligible reworking of the WGE, which stabilized
at least 50 Ma ago [2]. These results have major implications for relief evolution of South India. They imply
very low relief growth since the Eocene and do not require a tilt of the Peninsula. They further put constraints on
sedimentary supplies to both margins of the Peninsula over the last 50 Ma.
[1] Bonnet et al., 2016, Chemical Geology 446, 33-53.
[2] Beauvais et al., 2016, Geology 44, 299-302.