Rift valley/no rift valley transition at mid‐ocean ridges

A simple mechanical model for the axial topographic and gravity variations with spreading rate is developed. The model emphasizes the rapid transition from one mechanism to another with changing spreading rate. Our model has a brittle layer over a ductile layer which are being pulled by forces away from the ridge axis. There exists a potential failure zone of width Lf where the stresses within the brittle plate exceeds its yield strength. If the yield strength of the brittle plate is nearly constant, this width is largely determined by the strain rate and the viscosity of the “strong” mantle. The high ductility of oceanic crust at high temperatures relative to that of mantle at the same temperatures leads to the concept of a decoupling region, a critical feature in this model. The size of the decoupling region Ld is determined by the thickness of the oceanic crust and temperatures within the crust. If the decoupling region is small (Ld Lf), as shown to be the case for fast spreading ridges, then each of the two layers responds separately to the horizontal stretching. The brittle plate will be broken only at the axis, and any buoyant force of the region would lift it up to form the rise crest high as a result of local isostasy. The changing size of the decoupling region gives rise to the transition from Atlantic type axial topography to Pacific type, as well as the transition from a high amplitude, highly variable gravity signature to a low amplitude, very uniform gravity signature. Parameters can be chosen so that such a transition occurs at a half-rate of 35 mm/yr, which is the critical half-rate suggested by both topographic and gravity data.