THE ROLE OF MATNLE-DEPLETION AND MELT-RETENTION BUOYANCY IN SPREADING-CENTER SEGMENTATION

Numerical experiments are used to examine the structure of mantle flow beneath the axis of a spreading center. Buoyancy results from the dep letion of residual mantle in iron relative to magnesium as melt is ext racted (mantle-depletion buoyancy) and from the presence of low-densit y melt (melt-retention buoyancy). 3-D buoyant mantle flow arises spont aneously from an initially 2-D solution for low mantle viscosity and l ow spreading rate. At high viscosity and high spreading rate, initiall y 2-D solutions remain 2-D. This may explain the fundamentally differe nt structure of fast and slow spreading centers. In a uniform viscosit y halfspace, the along-axis wavelength of 3-D buoyant upwelling scales with the maximum depth of melting, the only length scale in this syst em. For reasonable maximum depths of melting (60-90 km), along-axis wa velengths of 200-300 km are preferred, longer than the 50-100 km segme ntation length of slow spreading centers. In a viscosity-layered halfs pace, the thickness of the asthenosphere introduces another length sca le. A wavelength of segmentation comparable to the asthenosphere thick ness also develops in our numerical experiments. This suggests the pos sibility that a wavelength corresponding to the spacing between gravit y lows may be controlled by the asthenosphere thickness, while the spa cing of major fracture zones corresponds to the longer wavelength (alm ost-equal-to 3 times the maximum depth of melting) intrinsic to the me lting region. The along-axis structure of 3-D flow varies from narrow, focused upwellings, at low spreading rates and mantle viscosities, to broad regions of upwelling at high spreading rates and mantle viscosi ty. To allow an along-axis crustal thickness variation no larger than that which is observed (almost-equal-to 3-4 km), the highly focused up welling and crustal production predicted at slow spreading rates requi res appreciable along-axis transport of melt.