Landscape evolution of Peninsular India in response to Cenozoic epeirogeny

Abstract:

Peninsular India is a stable cratonic region remote from active convergence yet it exhibits high, strongly asymmetric relief with a clear long-wavelength ($>$1000 km) eastward tilt from the Western Ghats escarpment ($\sim$1000 -- 1500\,m amsl) down to the floodplains of the Mahanadi, Krishna, Godavari and Cauvery rivers. Offshore, oceanic residual depth measurements mirror this asymmetry and show excellent spatial correlation with upper mantle shear wave velocity anomalies identified in tomography models. South Indian topography deviates strongly from isostatic expectations and the $\textrm{9.3}_{-2.2}^{+3.8}$ km effective elastic thickness of the region generates flexural wavelengths considerably shorter than would be required for a flexural driving mechanism. Taken together, these observations strongly suggest that the long-wavelength tilt of the Indian Peninsula is supported by convective circulation in the Earth's upper mantle.

To investigate the evolution of this physiography we have jointly inverted 530 longitudinal river profiles to determine uplift rates as a function of space and time, calibrating our models against independent geological constraints including thermochronologic data, palaeosurface incision and sediment flux histories. Our results suggest that the tilt grew slowly from 40 Ma -- 25 Ma ($\leq$0.02 mm a$^{-1}$) but developed rapidly from 25 Ma onwards ($\leq$0.2 mm a$^{-1}$). This Neogene acceleration is reflected in the large-scale stratal architecture of the Indian margins with the development of regional unconformities on the western margin coinciding with a switch from progradation to aggradation on the eastern margin. The onset of rapid uplift predates the intensification of the Indian monsoon at 8 Ma suggesting that rock uplift rather than climate change is primarily responsible for the modern-day relief of the peninsula, although we note that a positive feedback is likely to exist between Western Ghats relief and precipitation patterns. As a result, we propose that late Cenozoic dynamic topography is responsible for the long-wavelength tilt of the Indian Peninsula.