Change in crustal thickness due to localised rotation caused by a long-distance tectonic event in the Indian Ocean

Mid-oceanic ridges are the sites of oceanic crust formation, accommodating plate divergence through a combination of tectonic extension, magmatic accretion, and hydrothermal circulation. The thickness of the oceanic crust produced at these ridges is a first-order indicator of mantle melting processes and melt supply, and is traditionally linked to spreading rate, mantle temperature, mantle composition, and the efficiency of melt extraction. Fast-spreading ridges are typically associated with relatively uniform crustal thicknesses due to a 2D sheet-like mantle upwelling, whereas slow- and ultraslow-spreading ridges exhibit greater spatial variability due to enhanced tectonic strain and heterogeneous melt focusing. Despite the well documented observations and geodynamic modeling of mantle upwelling, the role of short-lived or transient changes in ridge geometry on melt production and crustal thickness remains poorly constrained. Using high resolution seismic reflection data from the Wharton Basin in the Indian Ocean, we show that the crustal thickness decreases smoothly from a normal crustal thickness of ~ 6 km to ~ 4 km and then back to ~ 6 km over a distance of ~120 km. This distance corresponds to a time span of 1-2 Myrs for a crust formed at the super-fast Wharton spreading centre. The dramatic change in crustal thickness is associated with an anticlockwise rotation of the magnetic anomaly Chron 29 (64.4 - 65.1 Ma), which is temporally coincident with the separation of Seychelles from the Indian sub-continent and the Deccan flood basalt volcanism caused by the La Réunion mantle plume. It is likely that this major plate tectonic event in the Indian Ocean caused a temporary change in the spreading rate and spreading direction. We suggest that a rapid rotation in the spreading direction could divert the melt focusing away from the ridge axis, decreasing the melt delivery and thus decreasing the crustal thickness. Within a span of 1 - 2 Myr, the spreading ridge returned to its original geometry and the regime stabilised to a uniform upwelling directly beneath the ridge axis, giving rise to a normally thick crust of 5.5 - 6 km. Our findings show that changes in ridge orientation can significantly influence melt fluxes on relatively short geological timescales, without requiring large-scale changes in mantle temperature or composition. This underscores the sensitivity of magmatic systems at spreading ridges to evolving plate kinematics.