Ridge extinction in the Mascarene Basin due to the Réunion hotspot: preliminary results of the MASC Cruise

The Mascarene Basin, between Madagascar, the Seychelles Plateau, and the Réunion hotspot track, is an ocean lithosphere whose geodynamic evolution remains enigmatic in many aspects. Part of the enigma concerns the unexplained extinction of the Mascarene mid-ocean ridge ca. 62 Ma ago and the shift of oceanic accretion to a new ridge (i.e., the Carlsberg Ridge) further north. The presence and timing of the Amirante aborted subduction trench (between Madagascar and the Seychelles) is another enigmatic aspect of the regional geodynamics (e.g., Rodriguez, CRGEOS 352, 235-245, 2020).

To shed light on these conundrums, we investigated the architecture of the Mascarene Basin during the MD245 “MASC” oceanographic cruise onboard the Marion Dufresne II research vessel. Bathymetric surveying revealed numerous seamounts at the axis of the paleo-ridge, along paleo-transform faults, and some also on the southern flank of the paleo-ridge. Interestingly, all the seamounts are located in the continuity of the Amirante paleo-subduction trench. Dredging operations on the seamounts recovered a suite of highly differentiated magmatic rocks ranging from biotite-rich basalts to rhyolites and granodiorites. Zircon and apatite (U-Th)/He data from these igneous rocks suggest that the seamounts formed during a protracted period between ca. 67 Ma and ca. 43 Ma.

Does this highly differentiated magmatism at 67-43 Ma reflect a residual activity of the ridge under extinction? Or, a nascent arc magmatism associated with the Amirante subduction? Further geochemical analyses are required to answer this question. Regardless, we note that the 67 Ma date coincides with the first magmatic manifestation of the Réunion plume as the Deccan traps, whilst the 43 Ma date corresponds to the deceleration of India and the passage of the Somalia Plate over the Réunion plume. We thus posit that differentiated magmatism, ridge extinction, and subduction initiation and abortion could be all related to the Réunion plume. Indeed, the Réunion plume is suspected to have pushed the Indian Plate toward Asia, causing its drastic acceleration and slowdown from 67 to 43 Ma (Cande and Stegman, Nature 475, 47-52, 2011). We further propose that the Réunion plume had a symmetric push effect on the Somalia Plate, converting oceanic spreading into compression, hampering spreading of the Mascarene Ridge, and eventually leading to the Amirante subduction. Compression (and differentiated magmatism) vanished when the Somalia Plate passed over the Réunion hotspot.