Structural underprint and tectonic overprint in the Angavo (Madagascar) and Western Ghats (India) - Implications for understanding scarp evolution at passive margins

摘要

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 homoclinal 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-rift 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.