Three‐dimensional numerical simulations of mantle flow beneath mid‐ocean ridges

Abstract

We perform three‐dimensional (3‐D) numerical simulations of an oceanic lithosphere cooling above a convective mantle to investigate mantle flow geometry and its associated lithospheric features. A constant horizontal velocity (half spreading velocity) of 2 (or 4) cm/yr is applied at the box surface to mimic plate motion. In all cases, because of the temperature‐dependent viscosity, a rigid conductive lithosphere cools progressively beneath the two plates separated by the ridge. After 16–40 Ma, cold downgoing instabilities develop at the base of the lithosphere. Small‐scale convection is then superimposed on the large‐scale circulation. It produces, and then interacts with, a slowly evolving short‐wavelength isotherms topography at the base of the lithosphere. The influence of the imposed ridge geometry on the mantle flow appears by comparison of simulations with case A, a linear ridge perpendicular to the plate motion; case B, a linear ridge strongly oblique to the plate motion; and case C, a ridge with transform faults along the spreading center with a mean orientation strongly oblique to the plate motion. All simulations reveal complex interaction between the isotherm topography within the base of the lithosphere, the small‐scale flow generated at or just below the base of the lithosphere, and the large‐scale flow dominating in the mantle below. The large‐scale flow appears always controlled by the mean ridge axis direction and thus may be oblique to the imposed plate motion direction. It is enhanced by the instabilities development. We argue that the large‐scale flow orientation results from the interaction with the small‐scale velocity field which flows down the isotherm topography at the base of the lithosphere. The latter is either inherited (lithospheric cooling, transform faults) or develops as boundary layer instabilities.