Large-scale asymmetry in thickness of crustal accretion at the Southeast Indian Ridge due to deep mantle anomalies

Hot mantle plumes and ancient cold slabs have been observed beneath modern mid-ocean ridges, but their specific and detailed effects on mid-ocean ridge crustal accretion are poorly understood. The oceanic lithosphere beneath the Southeast Indian Ocean displays unique morphological, geophysical, and geochemical characteristics, which may reflect the influence of both mantle anomalies and upwelling plumes on seafloor spreading. In this study, we combined gravity-derived oceanic crustal thickness with plate tectonic reconstructions to investigate patterns of asymmetry in thickness of crust accreted at the Southeast Indian Ridge over the last 50 m.y. Our results reveal several distinct features: (1) small-scale, short-lived asymmetries in the thickness of crustal accretion of up to 0.75 km are alternatively distributed on the southern and northern flanks of the 90°−120°E Southeast Indian Ridge segment. These can be explained by variations in mantle depletion or mantle temperature. (2) Two large-scale, long-lived (duration of ∼50 m.y.) asymmetries in crustal accretion of >2.5 km are observed around the Kerguelen Plateau and Balleny Islands, which we attribute to excess crust from the off-axis Kerguelen and Balleny mantle plumes. (3) Two large-scale, long-lived (duration of ∼50 m.y.) asymmetries in crustal accretion of 0.75−2.5 km are observed on the northern flank of the westernmost (70°−80°E) Southeast Indian Ridge and the southern flank of the eastern (120°−140°E) Southeast Indian Ridge segment, respectively. We attribute these to asymmetry in mantle temperature of up to 20−53 °C. We suggest these asymmetric temperatures across the Southeast Indian Ridge are associated with the foundered lithospheric fragments of the Indian Craton triggered by the African Large Low-Shear-Velocity Province during the breakup of Gondwanaland and an intraplate subducted slab of the Paleo-Tethys Ocean, respectively. The remnant craton fragments and subducted oceanic slab may have moved north in concert with the northward-migrating Southeast Indian Ridge beginning at 50 m.y. ago.