Tectonic controls on melt production and crustal architecture during magma-poor seafloor spreading

Slow and ultraslow spreading oceanic crust consists of a heterogenous mixture of serpentinised mantle and magmatic rocks. In these environments, both crustal architecture and faulting mode have been attributed to the interplay between magmatism and tectonics. Numerical models have investigated how variations in melt supply influence tectonics. However, how tectonics influences melt production and the formation of a heterogeneous crust is not well understood. Here, we use 2D numerical models to analyze how coupled tectonics, mantle melting, melt emplacement and serpentinization interact. Our model shows that ocean loading and crustal density promote fault offsets and durations. We focus on ultraslow, magma-poor ridges, in particular the Southwest Indian Ridge at 64 degrees 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, similar to 3-6 km, associated with deep along-fault serpentinisation. 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.