COUPLED NUMERICAL MODELING OF PASSIVE MARGIN ESCARPMENT EVOLUTION AND OROGRAPHIC PRECIPITATION

Great escarpments are prominent geomorphic features of passive margins which can persist over timescales of tens of millions of years. The maintenance of such steep topography over these timescales has been attributed to lithospheric flexure and spatial co-incidence of the escarpment rim with the drainage divide. Ongoing research in India’s Western Ghats suggests an additional role for climate, specifically spatial variability in precipitation rates, in sustaining this great escarpment. Precipitation in the Western Ghats is dominated by the South Asian Summer Monsoon which produces frequent shallow convective precipitation during summer months. Precipitation rates are maximized at low elevations within the foothills of the range and decrease rapidly with elevation along the escarpment. One and two dimensional idealized numerical models can simulate the observed precipitation pattern along the Western Ghats. A numerical landscape evolution model which includes fluvial erosion and landsliding is coupled with these climate models to simulate long-term co-evolution of a passive margin escarpment with the monsoon climate. We include lithospheric flexure and also examine the effects of pre-existing fluvial network geometry and bedrock hardness variability on landscape evolution. Spatially varying precipitation enhances the slope and lifespan of an escarpment in a similar manner to a resistant bedrock ridge parallel to the escarpment crest. Spatial variability in precipitation also influences planform development of the range including sinuosity of the escarpment front. Precipitation variability is a viable mechanism for maintaining a steep escarpment, indicating that the lifetime of such geomorphic features may reflect climate and precipitation mechanisms.