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Deccan volcanism was synchronous with rifting along the west coast of India. Pre- and synmagmatic rifting has been widely reported in the Deccan Volcanic Province, but extension post-dating magmatism, and predating India–Eurasia collision, is less well known. A recent study in the Kachchh area of western India documented weathering of basalts to kaolinite at the base of Cenozoic rift basins, with rift flanks relatively less altered to smectites, and this was attributed to post-magmatic rifting. This study models basalt weathering under open- and closed-system conditions to simulate rainwater interacting with basalts either on topographical slopes (within rifts) or on flat-topped hills (flow tops). Both systems were modelled under [Formula] conditions ranging from low, present-day values to higher values more appropriate for the end-Cretaceous–early Paleocene time, after basalt emplacement. The results show that if [Formula] exceeded values of 10−2.5, basalts would be altered to kaolinite in both open and closed systems. Existing [Formula] estimates in the aftermath of Deccan volcanism fall below this value, implying that the differential basalt weathering was more likely to have been caused by terminal to post-magmatic rifting. This indicates that extensional tectonics along the Indian west coast in the Kachchh region continued even after cessation of Deccan volcanism.

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.