Mantle upwelling and melting beneath slow spreading centers: effects of variable rheology and melt productivity

We examine the effects of non-uniform rheology and melting models on the 3D structure of mantle upwelling beneath spreading centers. Our numerical mod els identify a range of conditions for which 3D upwelling can be considered as a possible mechanism for magmatic segmentation of slow spreading ridges . In a highly viscous shallow region due to dehydration of the solid by mel t extraction, flow results essentially from the plate spreading. This contr asts with a deeper, buoyant, low viscosity region, where melt is present bu t upwelling solid is not yet dehydrated, and where both solid and melt flow s acquire their 3D nature. The thickness of this buoyant region, depending mainly on the temperature interval between the wet and dry solidi, results in a specific characteristic length scale for the upwelling. Smaller segmen tation wavelengths occur for smaller values of this thickness. In contrast to isoviscous models where minimum segmentation wavelengths were large (> 1 50 km), minimum wavelengths obtained in models with nonuniform rheology and melt production (40-70 km) are comparable to smallest observed segment len gths. A single wavelength may be preferred for a given set of parameters, b ut such a preferred wavelength is difficult to achieve because a range of i nitially prescribed wavelengths can persist for long times. The persistence of 3D solutions for tens of millions of years of model time indicate that segment lengths observed at slow mid-ocean ridges may be inherited from the initial stages of spreading. (C) 2001 Elsevier Science B.V. All rights res erved.