Phanerozoic surface history of southern Peninsular India from apatite (U‐Th‐Sm)/He data

Abstract

Quantifying bedrock cooling history is crucial for understanding the long‐term landform evolution across passive margins and its control onto the sediment routing system. To constrain the low‐temperature cooling history and its relationships to the Phanerozoic tectonic events of southern Peninsular India, we present new apatite (U‐Th‐Sm)/He (AHe) analyses of 39 Precambrian basement samples. The new AHe ages range from 38.1 ± 6.8 to 364.2 ± 44.6 Ma: they are younger than 50 Ma in the Palghat Gap region and older than 200 Ma in the interior of the Deccan Plateau. Thermal modeling based on AHe data indicates enhanced cooling and exhumation in the interior of the Deccan Plateau by Permian‐Triassic times followed by gradual cooling up to the Present. This discrete episode of Permian‐Triassic cooling is associated with continental extension that preceded the Early Jurassic breakup of Gondwana. Bedrock cooling and exhumation on the southeastern and southern limits of the Deccan Plateau was likely accomplished by Late Cretaceous drainage reorganization. The distribution of old (>200 Ma) AHe ages over the >2600 m high Nilgiri Plateau reflects very low erosion/exhumation rates and adds to examples of long‐lived postorogenic topography. The relatively younger AHe ages from the ∼30 km wide low mountain pass (Palghat Gap) within the Western Ghat Mountains attest for intense Cenozoic erosion likely facilitated by the erodible lithological backbone of the Neoproterozoic shear zone. AHe ages across the western coastal plain challenge the widely hold notion of ∼3 km of post‐breakup isostatic rebound in response to erosion of the margin. Instead, the new AHe data are more compatible with less than 1–1.5 km of crustal denudation along the coastal strip.

Geomorphic fluvial markers reveal transient landscape evolution in tectonically quiescent southern Peninsular India

“…The sedimentary record of eastern pericratonic basins indicates an increase in sediment accumulation and grain size during the Late Cretaceous–Early Cenozoic (Manmohan et al ., ; Gupta, ; Shanmugam et al ., ), arguably associated with Late Cretaceous climatic shifts triggering drainage reorganization (Mandal et al ., ). The increasing supply of terrigenous detritus has been interpreted to source from the cratonic hinterland (Madhavaraju et al ., ; Bakkiaraj et al ., ).…”

Cenozoic epeirogeny of the Indian peninsula

“…We note that significant Neogene uplift disagrees with the conclusions drawn by Mandal et al . []. Based on the apparent consistency between millennial, 10 Be erosion rates and Cenozoic, AFTA rates, they suggested that the landscape of the southern Indian Peninsula is in steady state (i.e., slow isostatic rebound is balanced by landscape erosion).…”

“…It cannot easily be reconciled with 1–1.5 km of denudation following eruption of Deccan basalts that is required to form the Konkan coastal plain. It also disagrees with thermochronologic evidence for significant post‐40 Ma denudation at rates of m Ma −1 across the Kerala‐Konkan lowlands since Late Miocene times [ Kalaswad et al ., ; Gunnell et al ., ; Campanile et al ., ; Mandal et al ., ]. These rates are consistent with cosmogenic nuclide studies and with evidence for ongoing escarpment retreat [ Mandal et al ., , ].…”

“…Cosmogenic nuclide studies suggest that erosion rates are less sensitive to precipitation but that they primarily scale with channel gradient and relief. This suggestion implies that rock uplift, and not climate per se, is primarily responsible for the switch [ Mandal et al ., ].…”

“…We have not included variable precipitation in our stream power formulation for three reasons. First, cosmogenic nuclide 10 Be studies suggest that no clear link exists between precipitation and erosion rates [ von Blanckenburg , ; Mandal et al ., ]. Second, it has been argued that no appreciable change in spatially averaged precipitation rates occurred over the Indian peninsula since Eocene times [ Reuter et al ., ; Shukla et al ., ].…”