The tectonic setting of the Seychelles, Mascarene and Amirante plateaus in the western equatorial Indian Ocean

A granitic crustal fragment occurs within a system of the marine plateaus in the western equatorial Indian Ocean. The plateaus form an arcuate series of wide, shallow banks with small islands in places. In spite of the geographical proximity and the morphological similarity, the geological structure and origin of the Seychelles Bank, Amirante Arc and Mascarene Plateau contrast sharply. The Seychelles Bank is founded on Precambrian granite basement. The Mascarene Plateau, from Saya de Malha Bank to Mauritius Island, is founded on Paleocene basalts, and Upper Cretaceous basalts prevail in the Amirante Arc. Geological and geophysical data suggest that the Precambrian granite of the Seychelles Bank is a fragment separated from the Indian continental plate during the Paleocene. This break-up was associated with the volcanic activity that built the Mascarene Plateau. The occurrence of Middle Eocene syenite, diorite and microgranite in the western part of the Seychelles Bank could indicate that crustal accretion across the northwestern Indian Ocean Ridge apparently drove the Seychelles block southwestwards, causing it to collide with the northeastern edge of the Mascarene Basin. This collision could have led to an Early Tertiary subduction, and subsequently to the development of the Amirante Trough. The absence of post-Eocene igneous suites in the Seychelles could indicate that a change in the spreading pattern of the northwestern Indian Ocean during the Late Eocene probably terminated the Amirante subduction.