Role of inherent geological and climatic characteristics on landscape variability in the tectonically passive Western Ghat, India

Abstract

How does lithological variation affect the landscape and its evolution is a fundamental issue in modern and post-orogenic terrains. In the absence of active tectonic uplift, spatial variation of rock resistance and rainfall distribution are thought to primarily control the fluvial and hillslope processes. However, these lithologic and climatic controls are difficult to decouple owing to the fact that these post-orogenic terrains endure long and complex geomorphic history. To examine the relative importance of lithology and rainfall in a post-orogenic setting, we analyzed west-flowing drainage basins in the Western Ghat (WG) with a multi-proxy analysis. We relied on digital topography, sediment yield and hydrological data. We evaluated topographic relief, hypsometry, river-long profiles and sediment yield to elaborate the importance of spatial variability of rock resistance in the long term evolution of these drainage basins. The dominant increase of relief (∼200–1500m) and decrease of sediment yield (3.32–1.91ton/month/km2) from north to south of the escarpment is directly attributed to the rock erodibility and structural characteristics. Field verification suggests that despite high rainfall condition, the channels are mostly detachment limited and therefore lack the abrasive tools. We further showed that the harder lithologies sustain higher steepness. On the contrary, the spatial distribution of satellite-derived rainfall showed a negligible effect on the long profile evolution. Our analysis is also supported by the existing knowledge of chemical denudation rate and implicates that rock hardness and weatheribility defines the topographic characteristics with little or no effect of rainfall in this passive margin setting.