3-DIMENSIONAL MANTLE UPWELLING, MELT GENERATION, AND MELT MIGRATION BENEATH SEGMENT SLOW-SPREADING RIDGES

In contrast to the along-axis uniformity observed at the East Pacific Rise (EPR), crustal accretion at the Mid-Atlantic Ridge (MAR) appears to be a highly complex and heterogeneous process. Besides spreading ra te, one of the first-order differences between the EPR and the MAR is the much higher degree of ridge segmentation observed in the Atlantic. Circular lows in the mantle Bouguer anomaly (MBA bull's-eyes) are com mon at centers of spreading segments of the MAR, suggesting crustal th ickness variations of up to 4 km along individual segments. We use a t hree-dimensional numerical model of mantle flow to examine the effect of ridge segmentation on mantle upwelling and the resulting overall cr ustal production and along-axis variations in crustal thickness. Mantl e flow in our model is driven by both buoyant forces and segmented pla te spreading. Various asthenospheric viscosity structures, plate sprea ding geometries, and mantle potential temperatures are explored. We fi nd that a combination of buoyant mantle flow and three-dimensional mel t migration can reproduce crustal thickness variations similar to thos e inferred from gravity. Buoyant flow gives rise to variations in upwe lling velocity at along-axis wavelengths greater than 150 km but does not contribute to short-wavelength variations. However, three-dimensio nal melt migration may greatly enhance crustal thickness variations al ong all segments, independent of the wavelength of buoyant upwelling. We present an idealized model, in which melt first rises vertically an d then flows along the base of the lithosphere toward the ridge axis, that easily produces crustal thickness variations greater than 4 km. T he models also predict that the average crustal thickness should decre ase with increasing amount of segmentation and decreasing spreading ra te. Therefore the thinner, more heterogeneous crust observed at the MA R may result from the combined effects of slower spreading rate and mo re pervasive ridge segmentation.