Small-scale convection induced by passive rifting: the cause for uplift of rift shoulders

Areas adjacent to rifts, or rift shoulders, are often observed to be uplifted as much as a kilometer or more. In some of these regions geologic data indicate a passive origin for the rifting itself (i.e. there was no anomalous heating of the regions before rifting). Purely conductive heat transport between the rift, where the lithosphere has been thinned, and the rift flanks cannot account for the magnitude of the uplift. Small-scale convection will be induced in the mantle beneath a rift due to the lateral temperature gradients there. Numerical experiments show that convection increases the amount of heat transported vertically into the rift and laterally out of it. In these calculations, the viscosity is taken to be dependent on temperature and pressure and, in some cases, stress. The mantle flow results in thinning of the adjacent lithosphere causing flanking uplift as well as slowing of the subsidence of the middle of the rift. The magnitude of the uplift is dependent on the geometry of the rift and the importance of stress-dependence in the rheology of the mantle. For viscosity parameters which are consistent with the pre-rift temperature structure small-scale convection can produce uplift at least twice as great as would be produced by lateral conduction alone.