780 Thousand Years of Upper‐Crustal Construction at a Melt‐Rich Segment of the Ultraslow Spreading Southwest Indian Ridge 50°28′E

Melt supply at the ultraslow-spreading Southwest Indian Ridge (SWIR) has been shown to vary from nearly amagmatic, leading ultramafic seafloor, magmatically robust, producing fully volcanic seafloor. The center of SWIR 50°28′E segment represents a robust endmember. High-resolution bathymetry and backscatter, near-bottom magnetic data, visual observations were acquired infer spatiotemporal variations in upper-crustal construction over past 780 kyr. Tectonic strain inferred observed faults <8% total plate divergence during this period. Mapped seafloor lava morphologies show prevalence smooth hummocky (64% mapped area), corresponding dominance high-effusion-rate, sheet-lobate lavas, (29%) that mostly comprises low-effusion-rate pillow lavas. This is most pronounced within ∼5 km-long domain center, while prevails its east west, indicating substantial along-axis decrease average eruption rate. Across-axis changes faulting pattern indicate two successive cycles construction. These last ∼300 kyr, interpreted as waxing waning magmatic phases. During phases, shallow axial melt lenses (AMLs) form beneath feeding narrow diking systems build high-eruption-rate domal volcanos. lower frequencies replenishment may cause AML become deeper ultimately disappear. configuration triggers hummocky-dominated eruptions wider domain, with more faults. We propose modes slow ultraslow mid-ocean ridges are influenced by than spreading