Going slowly under water: Tectonic, magmatic and hydrothermal controls on melt production and lithosphere architecture

Slow and ultraslow spread oceanic lithosphere consists of a mixture of magmatic and peridotites rocks with variable alteration degrees. In these environments, crustal architecture and faulting have been attributed to the interaction between magmatism and tectonics. Numerical models have investigated how variations in melt supply influence tectonics, and others have explored how alteration reactions, such as serpentinization, affect faulting modes. However, how tectonics influences melt production and the formation of heterogeneous crust, as well as the extent of serpentinisation, remain not well understood. Here, we use 2D numerical models to analyze the interactions among coupled tectonics, mantle melting, and serpentinization during ultraslow, magma-poor oceanic lithosphere spreading. We focus on ultraslow, magma-poor ridges, in particular the Southwest Indian Ridge at 64°30' East, which exhibits crustal thickening along detachment faults interpreted as a deep serpentinization front. We reproduce the observed bathymetry, seafloor morphology, shaped by alternating flip-flop detachments, and crustal thickness variations, ~3-6 km, associated with deep along-fault serpentinisation. Our model shows that ocean loading and crustal density promote fault offsets and durations. We find that in this magma-poor environment, where serpentinized mantle dominates crustal composition and magmatic effects on tectonics are limited, variations in melt production are the consequence of changes in mantle flow patterns in response to faulting. Early detachment evolution leads to a slow-down in mantle upwelling and melt production decrease. Late in the detachment life-cycle, antithetic faults in its footwall trigger enhanced mantle upwelling, increasing melt production. During the flip-flop detachment life-cycle, tectonically induced spatio-temporal variations in magma emplacement and serpentinisation contribute to the formation of a heterogeneous crustal architecture and magnetic signature of the seafloor.