The Pacific-Nazca plate boundary evolves continuously through the frequent, rapid propagation of ridge segments and through the growth or abandonment of microplates. Propagation events can initiate at overlapping spreading centres only a few kilometres wide as well as within large transform faults. This instability of the ultrafast East Pacific Rise (EPR) probably results from the presence of a hot, thin lithosphere in the axial region, coupled with a melt supply that may be temporally or spatially variable. It indicates that along-axis magma transport can be efficient at rates corresponding to propagation rates, up to 1000 mm yr(-1). To a first-order, the tectonic segmentation of the ridge correlates with along-axis variations of the axial morphology and other physical parameters suggesting a diminished magmatic budget near offsets larger than a few kilometres. A similar correlation between axial segmentation and variations in physical characteristics at the Mid-Atlantic Ridge (MAR) is commonly interpreted to indicate that mantle upwelling is focused near mid-segment at slow-spreading ridges (three dimensional). Accordingly, mantle upwelling may be focused at discrete intervals along the ultrafast EPR. However, fluctuations of the along-axis characteristics are considerably more subdued at the EPR than at the MAR. This has been interpreted to reflect smoothing of the structural variations by efficient transport within the shallow crust and upper mantle of the material brought up through focused upwelling. Alternatively, it has been argued that mantle flow is essentially uniform along-axis (two dimensional) at the faster spreading centres. It is proposed here that the actual pattern of mantle flow along the EPR combines aspects of both models. Fast spreading centres may be supplied by vertical mantle flow nearly continuously along-axis, but the intensity of this upwelling can fluctuate both temporally and spatially, hence favouring along-axis transport away from the magmatically most robust areas.
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