MESOZOIC TECTONOSTRATIGRAPHIC EVOLUTION OF THE SEYCHELLES MICROCONTINENT

The Seychelles region developed as a unique continental sliver through the three phases of rift tectonism that, since the Late Permian, crea ted the western Indian Ocean. The history of rifting in Seychelles can perhaps be traced back to similar to 700 Ma when the granites that fo rm the central islands were emplaced. The low delta(18)O of the high-t emperature minerals within these slightly alkaline granites on Mahe in dicate an intraplate origin, and the fracturing of the lithosphere con sequent to their emplacement created a locus of weakness for future ri ft tectonism to exploit. Thermally induced extensional rifting, initia ted during the Carboniferous and rekindled in the Late Permian (simila r to 225 Ma), eventually severed Super-Gondwana in the Middle Jurassic (similar to 160 Ma) into East and West Gondwana. A proto-Indian Ocean , the Somali Basin which separated East and West Gondwana, ceased to e xpand in the Early Cretaceous (similar to 115 Ma). This rift/drift eve nt is recorded in Seychelles by block-faulted sequences of distal fluv iatile, through near-shore to open marine deposits along its western m argin and possibly at Farquhar. Within East Gondwana, transform riftin g then separated Madagascar and Seychelles/India in the Late Cretaceou s. At 84 Ma a spreading ridge developed between these blocks forming t he Mascarene Basin as a new arm of the evolving Indian Ocean. A thick progradational wedge of sediment, initially alluvial followed by lacus tro-marine and eventually open marine, resulted from this event along the southwest Seychelles margin. Finally, hotspot-related thermally in duced rifting, causing block-faulting and fluviatile to marine deposit ion, eventually separated Seychelles from India at the Cretaceous/Tert iary boundary. This separation coincided with the Deccan volcanic even t and the development of the Carlsberg Spreading Ridge, which replaced the Mascarene Spreading Ridge and initiated the opening of the northw estern Indian Ocean Basin. Sedimentation on the Seychelles microcontin ent throughout the Tertiary produced predominantly shelf carbonates.