COMBINED LABORATORY AND NUMERICAL-STUDIES OF THE INTERACTION BETWEEN BUOYANT AND PLATE-DRIVEN UPWELLING BENEATH SEGMENTED SPREADING CENTERS

A combination of laboratory and numerical models are used to examine t he mantle flow beneath a segmented ridge generated by the interaction of a linear, buoyant upwelling source with plate-driven flow. In the a bsence of plate spreading, the linear buoyant source creates a very na rrow (across-axis), two-dimensional upwelling pattern. The plate-drive n flow consists of a quasi-linear sheet-like upwelling that cuts benea th ridge-transform inside corners and is not centered beneath the spre ading segments. When buoyant and plate-driven flows are combined, mate rial rises beneath the inside corners and flows away from the axis asy mmetrically. Near the ends of segments, this results in a geometrical misfit between the center of mantle upwelling and the ridge axis. If a similar pattern of mantle flow occurs beneath a segmented mid-ocean r idge, the result will be a thinner crust toward segment ends and possi bly a negative correlation between extent of mantle melting and averag e depth of melting. These results indicate that even with an essential ly two-dimensional source, in cases where it is oblique to the actual spreading segments, the upwelling beneath a segmented ridge will appea r to be three-dimensional along axis. Since slow spreading ridges are generally more segmented than fast spreading ridges, this effect is li kely to be more important at slow spreading ridges.