Lithological and structural control on landscape evolution in the western ghat in peninsular India

Abstract

High elevated erosional escarpments are common feature of the rifted passive margin, but the role of external controls in their evolution is poorly understood. Very few of the existing studies have enumerated the variability of geomorphic matrices to narrate the role of lithology and climate in an extensively large regional scale. We investigated the potential role of lithological diversity and rainfall distribution on the landscape of N-S trending tectonically quiescent Western Ghat escarpment. We observed that low to moderately high local relief (~200 - 500 m) are mostly constricted to Cenozoic basalt in the north. Contrary high grade metamorphic rocks combined with the Proterozoic structures in the south sustain high relief (~1200 - 2500 m). Fewer westerly-flowing streams are following Precambrian fold geometry or joint plains. In most of the cases, the regional joint plain direction (N 115°) define the face of the prominent knickpoints. Most of these knickpoints are primarily retreated by block disintegration process which is highly stochastic. We interpreted that high weathering rate in basalt lowers the interfluve regions that subdues the relief in the northern part. The cause of higher relief in the south is attributed by low weathering rate in the Precambrian lithology (e.g. primarily granite, gneiss and charnockite) and accentuated incision of the channel following the older structure. North to south decrease in the sediment yield (0.11 – 0.06 tons/month/km2; June-November) is by and large prominent which corresponds to a transition between volcanic to metamorphic lithology. Kernel density estimates suggests that spatial distribution of rainfall characteristics, on the contrary, have negligible or no contribution to variability in the reach-wise channel steepness index. The overall analysis suggested that the strength and weatherability of rocks and structural characteristics goes hand in hand to sustain or decay the relief with little or no effect from the spatial heterogeneity of rainfall.