Tectonics controls on melt production and crustal architecture during nearly amagmatic seafloor spreading

At ultra-slow ridges, tectonics, hydrothermalism, serpentinization and magmatism interact to build the oceanic crust. How this heterogenous crust forms and relates to faulting remains poorly understood, but is key for elucidating hydrothermal flow patterns and their implications for ocean-lithosphere element exchange. Along the melt-poor Southwest Indian Ridge (SWIR) at 64°30' East, crustal thickness varies across the ridge strike, with crustal thickening attributed to serpentinization extending downward along detachment faults, DFs. This observation calls into question the commonly assumed relationship between local crustal thickening and magma-supply increase. Here we use 2D numerical models to analyze how coupled tectonics, mantle melting, magma emplacement and serpentinization interact. Our model includes hydrothermal cooling, ocean loading, and the oceanic crust density. We reproduce the observed bathymetry at SWIR, 64°30'E, which is shaped by alternating DFs formed in flip-flop mode. Our results show that the offset and duration of DFs are controlled by ocean loading and crustal density. Importantly, shallow faulting and deeper mantle flow are coupled: long-lived DFs result in relatively slower mantle upwelling, lower melt supply, but crustal thickening due to deeper serpentinization, ~5 km, consistent with the observed thick ultramafic crust in nature. In between alternating DFs, mantle upwelling is faster, melt supply higher, and serpentinization shallower, < 2km. Since magmatic crustal thickness is overall very small, 1.5-2 km, changes in faulting-induced serpentinisation depth, are the main cause for observed variations in crustal thickness, 2-7 km. We conclude that, at melt-poor ridges, tectonics controls both crustal thickness variations and melt supply oscillations.