A Model for the Horst and Graben Structure of Mid-ocean Ridge Crests Based upon Spreading Velocity and Basalt Delivery to the Oceanic Crust

The East Pacific Rise crest from 17 " N to 40 " S is characterized by a central horst. This feature is suggested to be the steady-state product of constant spreading about a ridge with the standard deviation of dike injection rate smaller than the standard deviation of lateral strain rate. A model is developed, based only upon these two parameters, which adequately explains not only the central horst topography, but also that of central grabens. The latter feature is shown to occur within the model when the spreading velocity is so small that the standard deviation of the dike injection rate is larger than the standard deviation of the lateral strain rate. Such is believed to occur only when constant freezing and breaking of the central intrusion zone produces intermittent spreading. Thus, a ' normal ' spreading centre has a central horst. Put simply, when material is intruded at the ridge crest over a distance greater than that required to accelerate the crestal material from zero horizontal velocity to the spreading velocity, a central graben forms. When material is intruded over a narrower distance than that required to accelerate to spreading velocity, a central horst forms