New model for ultraslow-spreading ridges

Abstract

Abstract The oceanic crust formed at mid-ocean ridges constitutes seventy percent of the earth's solid surface 1–3. The crust in ocean basins is approximately seven km thick 4,5, but when the spreading rate drops below 15-20 mm/a (ultraslow-spreading), crustal thickness decreases rapidly 6,7. The paradigm view is that thickness depends on the spreading rate 6–8. However, current models for the ultraslow-spreading ridges are not based on direct imaging 9, and it is not well understood how tectonic processes, melt dynamics, 2,3,10,11, lithospheric structure, and crustal formation 12–15 interact along ridges 12–14,16,17. New electromagnetic (EM) data across the Mohns and Knipovich ridges show that, outside the volcanic centres, the lithosphere appears closed to melt migration, and instead of normal thinning, the lithosphere is unusually thick (35-45 km) beneath the ridges. Crustal thickness varies along the strike and is thinnest where the spreading rate is the highest, contrary to the prediction of conventional models6–8. In the new model, ambulatory volcanic centres, forming along weak zones, and fault-induced ultra-deep direct drainage of melt from the asthenosphere explain the EM data. Volcanic centres are point sources of melt supply and both centres, feeder channels and volcanism are episodic, relatively short-lived, and random in time and place. In this model with a thick and brittle lithosphere, the plate motions (rate and direction), local tectonics, lithology, weak zones, and deep faults associated with the deep drainage, control the development. Melt dynamics and crustal formation are passive buoyancy-driven responses to the tectonic development. The fact that the proposed ridge model is closely connected to fundamental tectonic processes support the idea that the model can also be applied to ultraslow-spreading ridges in general.