Dependence of mid-ocean ridge morphology on spreading rate in numerical 3-D models

The morphology of natural mid-ocean ridges changes significantly with the rate of extension. Full spreading rate on Earth varies over more than one order of magnitude, ranging from less than 10 mm/yr at the Gakkel Ridge in the Arctic Ocean to 170 mm/yr at the East Pacific Rise. The goal of this study is to reproduce and investigate the spreading patterns as they vary with extension rate using 3-D thermomechanical numerical models. The applied finite difference marker-in-cell code incorporates visco-plastic rheology of the lithosphere and a crustal growth algorithm. The evolution of mid-ocean ridges from nucleation to a steady-state is modelled for a wide range of spreading rates. With increasing spreading rate, four different regimes are obtained: (a) stable alternating magmatic and amagmatic sections (≈ 10 mm/yr), (b) transient features in asymmetrically spreading systems (≈ 20 mm/yr), (c) stable orthogonal ridge-transform fault patterns (≈ 40 mm/yr) and (d) stable curved ridges (≥ 60 mm/yr). Modelled ultraslow and slow mid-ocean ridges share key features with natural systems. Abyssal hills and oceanic core complexes are the dominant features on the flanks of natural slow-spreading ridges. Numerically, very similar features are produced, both generated by localised asymmetric plate growth controlled by a spontaneous development of large-offset normal faults (detachment faults). Asymmetric accretion in our models implies a lateral migration of the ridge segment, which might help explaining the very large offsets observed at certain transform faults in nature.