Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Yanni Gunnell ¹, David Harbor ²

Affiliations
1 Department of Geography, Universite Paris-Diderot, CNRS UMR 8591, 2 Place Jussieu, 75205 Pans Cedex 13, FR
2 Department of Geology, Washington and Lee University, Lexington, VA 24450, US

Abstract

The land surfaces on either side of receding escarpments at continental passive margins are formed under the competing influence of drainage systems controlled by two separate base levels of erosion proximal sea-levels in the coastal belt, and more distant and diverse base levels in upland river systems landward of the escarpment. Unequal erosion is therefore the engine that supports the asymmetric topography and persistence of local relief. A comparative study of the passive margin escarpments of Western India and Eastern Madagascar suggests that after a certain time has elapsed since initial continental breakup, steep landforms are either underpinned by resistant bedrock or recently formed by tectonics. Lithological controls, through which geological structure receives its topographic expression as the landscape erodes, are dominant. A morphostructural analysis of the Western Ghats escarpment in its Archaean section shows that its relief and morphology today are largely controlled by lithological heterogeneity and form a diverse gallery of homochnal structures eroded out of the metamorphic and igneous fabric of the craton. Although in Madagascar late Cenozoic faulting locally affords an additional contribution to topographic expression, the growth and persistence of escarpment supporting bedrock landforms is shown to be strongly pre-ordained by the pre-nft geological fabric. It also depends on the capacity of geomorphic agents in a particular climatic environment to reshape the landscape in response to drainage integration across the hinterland. It is argued that mature passive margin escarpments are essentially residual escarpments underpinned by a diverse assemblage of structurally controlled bedrock landforms.

Keywords

Tectonics, Geological Structure, Scarp Evolution, Passive Margin, Western Ghats, Madagascar.

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David Harbor ¹, Yanni Gunnell ²

Affiliations
1 Department of Geology, Washington and Lee University, Lexington, VA 24450, US
2 Department of Geography, CNRS UMR 8591, Universite de Paris 7,2 Place Jussieu, 75251 Paris, US

Abstract

We present the first synthetic and systematic attempt to fingerprint topographic attributes of the Western Ghats passive margin escarpment using newly avaliable SRTM digital elevation data. Spanning 12 degrees of latitude, the escarpment is shown to exhibit contiguous segments where scarp sinuosity and relief, but also drainage basin attributes such as stream orientation, spacing of scarp, coastline and continentai divide, basin shape, basin hypsometry and stream longitudinal profile covary in ways that suggest differences in the process of retreat of the Western Ghats as a continuous yet heterogeneous landform. The methodology presented here could serve as an improvable template applicable to other escarpments around the world for comparative purposes. It can be potentially standardized as a tool designed to construct inferences about the variability of scarp retreat processes under a range of conditions tied to damage and geological structure. For the Western Ghats, we suggest that site-specific feedbacks between climate, drainage and geologic structure are key to understanding scarp dynamics and the manner in which the evolution of drainage boundaries across strike affect morphology and evolution along strike.

Keywords

Escarpment, Drainage, Topography, Western Ghats.

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