Alteration and submergence of basalts in Kachchh, Gujarat, India: implications for the role of the Deccan Traps in the India–Seychelles break-up
Souvik Mitra, Kaushik Mitra, Saibal Gupta, Satadru Bhattacharya, Prakash Chauhan, Nirmala Jain, 2017. "Alteration and submergence of basalts in Kachchh, Gujarat, India: implications for the role of the Deccan Traps in the India–Seychelles break-up", Tectonics of the Deccan Large Igneous Province, S. Mukherjee, A. A. Misra, G. Calvès, M. Nemčok
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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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Understanding the Deccan Trap Large Igneous Province in western India is important for deciphering the India–Seychelles rifting mechanism. This book presents 13 studies that address the development of this province from diverse perspectives including field structural geology, geochemistry, analytical modelling, geomorphology and geophysics (e.g., palaeomagnetism, gravity and magnetic anomalies, and seismic imaging). Together, these papers indicate that the tectonics of Deccan is much more complicated than previously thought. Key findings include: the Deccan province can be divided into several blocks; the existence of a rift-induced palaeo-slope; constraints on the eruption period; rift–drift transition mechanisms determined for magma-rich systems; the tectonic role of the Deccan or Réunion plumes; sub-surface structures reported from boreholes; the delineation of the crust–mantle structure; the documentation of sub-surface tectonic boundaries; post-Deccan-Trap basin inversion; deformed dykes around Mumbai, and also from the eastern part of the Deccan Traps, documented in the field.