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Abstract

The Deccan Trap flood basalt volcanism has commonly been considered to have initiated the break-up of India from the Seychelles (c. 62.5 Ma). In Kachchh, Gujarat, western India, the sedimentary succession in the Paleocene Matanomadh Formation was deposited on highly weathered Deccan Trap basalts that were altered to kaolinite before basin formation. This contrasts with the weathering pattern on flat-topped hills of the Deccan Traps outside the Cenozoic rift basins in the Kachchh region and other parts of India, where basalt is dominantly altered to smectitic minerals. As basalts that are altered to smectite and kaolinite occur just across the faults that bound the Matanomadh Basin, the differential weathering cannot be attributed to climate. Geochemical modelling shows that kaolinite stabilizes in preference to smectite if CO2- and O2-buffered rainwater interacts with well-drained basaltic rock at high water:rock ratios. Such conditions can be accomplished by rainfall on a slope created by Cenozoic rifting that exposes the graben flank and basin floor basalts to continuously flowing water, the composition of which is buffered by equilibration with the atmosphere. As the rift post-dates both the eruption of the basalts and subsequent smectite formation, the associated extensional tectonics must be unrelated to flood basalt volcanism, and is most likely to correspond to the India–Seychelles break-up.

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