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Abstract
The origin of mid-ocean ridge segmentation--the systematic along-axis variation in tectonic and magmatic processes--remains controversial. It is commonly assumed that mantle flow is a passive response to plate divergence and that between transform faults magma supply controls segmentation. Using seismic tomography, we constrain the geometry of mantle flow and the distribution of mantle melt beneath the intermediate-spreading Endeavour segment of the Juan de Fuca Ridge. Our results, in combination with prior studies, establish a systematic skew between the mantle-divergence and plate-spreading directions. In all three cases studied, mantle divergence is advanced with respect to recent changes in the plate-spreading direction and the extent to which the flow field is advanced increases with decreasing spreading rate. Furthermore, seismic images show that large-offset, non-transform discontinuities are regions of enhanced mantle melt retention. We propose that oblique mantle flow beneath mid-ocean ridges is a driving force for the reorientation of spreading segments and the formation of ridge-axis discontinuities. The resulting tectonic discontinuities decrease the efficiency of upward melt transport, thus defining segment-scale variations in magmatic processes. We predict that across spreading rates mid-ocean ridge segmentation is controlled by evolving patterns in asthenospheric flow and the dynamics of lithospheric rifting.