Denudation history of the continental margin of western peninsular India since the early Mesozoic ^ reconciling apatite ¢ssion-track data with geomorphology

A comprehensive apatite fission-track (AFT) study of the passive margin of western peninsular India between 12‡ and 16‡N is used to reconstruct the denudation chronology of the continental hinterland. In common with other rifted margins, the morphology is characterised by a low-lying coastal plateau separated from an elevated inland plateau by an erosionally controlled escarpment (Western Ghats). We modelled the fission track data using the commonly adopted annealing algorithm of Laslett et al. [Chem. Geol. 65 (1987) 1^13]. Using the default parametrisation (i.e. an initial track length of 16.3 Wm), it was found that during the Mesozoic denudation rates remained extremely low, increasing sporadically when erosion peaked at 130 Ma (rifting with Antarctica) and 80 Ma (rifting with Madagascar). Denudation rates rose considerably during the Cenozoic, reaching maxima of ca 120 m/Myr. Such values are, however, considered as major overestimates and the effects of the Seychelles rifting at 65 Ma remain suspiciously unrecorded. We explored the consequences of changing the initial track length in this model to a value of 14.5 Wm. In practice, this reduces the rapid Cenozoic denudation artefact, model peak rates during the Mesozoic are much more variable, and during the Cenozoic reach values an order of magnitude lower than with the original initial track length. The response to the Seychelles rifting event is almost immediate. Just as previous model calibrations in AFT analysis have been relatively empirical, this revised approach does not provide insights into the physical mechanisms of low-temperature annealing. However, it is shown to agree much better with independently established geomorphic, cosmogenic, stratigraphic and tectono-magmatic evidence in this and other stable shield regions in terms 0012-821X / 03 / $ ^ see front matter F 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0012-821X(03)00380-7 * Corresponding author. Present address: Laboratoire de Ge¤ographie Physique, CNRS-UMR 8591, Campus du CNRS, 1 Place Aristide Briand, 92190 Meudon, France. Fax: +33-1-46-64-78-81. E-mail addresses: [email protected] (Y. Gunnell), [email protected] (K. Gallagher), [email protected] (A. Carter), [email protected] (M. Widdowson), [email protected] (A.J. Hurford). § Supplementary data associated with this article can be found at 10.1016/S0012-821X(03)000380-7 EPSL 6761 22-9-03 Earth and Planetary Science Letters 215 (2003) 187^201 R Available online at www.sciencedirect.com www.elsevier.com/locate/epsl of both the timing and the magnitude of geological events, and the geomorphic response of the landscape to them. F 2003 Elsevier B.V. All rights reserved.