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Landscapes in passive continental margins occupy a large part of the Earth’s surface but their evolution remains under-studied. Despite its antiquity and location on a passive continental margin, the present-day elevated topography of the southern Peninsular India’s Western Ghat Mountains has raised important questions concerning the long-term mechanism of topographic evolution. I use a combination of topographic analysis based on digital elevation model data, catchment-averaged and local erosion rates quantified from 10Be concentrations in river sands and bedrocks, and low-temperature thermochronometry, in particular apatite (U-Th-Sm)/He with inverse t-T history simulations. Specifically, I investigate (1) the magnitude and distribution of millennial-scale erosion rates across the southern Peninsular Indian escarpment, (2) the main controls on landscape erosion in southern Peninsular India, (3) timing and magnitude of late-stage rock cooling across the escarpment, and (4) impacts of Mesozoic extensional tectonics on the landscape. 10Be-derived catchment-averaged erosion rates vary from 9.6 ± 0.8 m Ma-1 in the inland plateau to 114.3 ± 25.4 m Ma-1 on the western escarpment side. Bedrock erosion rates range from 2.4 ± 0.2 m Ma-1 in the ridge-top to 143.4 ± 2.4 m Ma-1 in active channel beds of the inland plateau. Catchment-averaged erosion rates derived from the across-escarpment, westward-draining catchments are significantly higher than those derived from the eastward-draining, over highland catchments. This marked difference suggests that long-term down-wearing of the highland proceeds at lower rates than in the escarpment zones. The relationship among topography, climate, and erosion is accomplished through the establishment of regional millennial-scale erosion pattern and topographic, rainfall, and vegetation analysis. Catchment-averaged erosion rates reveal moderate correlation with mean hillslope angles and local relief whereas they reveal strong correlation with channel steepness, demonstrating that topographic steepness is the major control on the spatial variability of erosion rates. The strong rainfall gradient plays a minor role in this area. 10Be-derived averaged erosion rates in highlands are consistent with long-term erosion rates previously estimated from apatite fission track (AFT) thermochronometry. These results collectively point to large-scale steady-state topography, only decaying slowly with time. Steady state likely reflects the balance between erosion and isostatically driven uplift of the southern Peninsular India. The new AHe ages range from 38.1 ± 6.8 to 364.2 ± 44.6 Ma, are younger in the Palghat Gap region and older in the Deccan Plateau. Thermal history modeling based on AHe data indicates enhanced bedrock cooling and exhumation in the interior of the Deccan Plateau by Permian-Triassic times, followed by gradual cooling to the present. The old (>200 Ma) AHe age distribution over the > 2600 m high Nilgiri Plateau indicates very slow erosion/exhumation rates during the last ~200 Ma, which is consistent with the lowest spatially integrated erosion rates measured by 10Be anywhere in southern Peninsular India. This result serves as a cautionary remainder of the dangers of attempting to infer tectonics from topography without additional supportive evidence. The AHe data indicate no more than 1-1.5 km of crustal unroofing along the western coastal strip.
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