Is Skew of Mantle Upwelling Beneath Spreading Centers Globally Significant? Observations (EPR), Hints (MAR) and Predictions (Juan de Fuca)

One popular and long-held view for the origin of spreading center segmentation is the magma supply hypothesis, wherein magma is centrally injected at mantle depths into a ridge segment bounded by long-lived tectonic discontinuities and ridge parallel flow of magma governs segment-scale variations in magmatic and tectonic processes. An alternative hypothesis is that tectonic forces transmitted through oceanic plates are responsible for the creation and migration of ridge axis offsets. These two hypotheses are often considered as the end- member alternatives for the origin of ridge crest offsets. We have recently proposed a new hypothesis - that does not fit neatly between the two end-member alternatives noted above - for the origin of segmentation along fast-spreading ridges. In our model, segmentation of the East Pacific Rise (EPR) is a result of a skew between the axes of mantle upwelling and plate spreading. As a result of this skew, mantle upwellings organize into en echelon segments and large-offset overlapping spreading centers (OSCs) provide an accommodation zone that moves the lithospheric plate boundary from one segment of mantle upwelling to another. Between transforms and OSCs, the intrasegment variability in ridge crest processes is related to the cross-axis distance to a center of mantle upwelling: Where mantle upwelling intersects the plate boundary, volcanic eruptions are frequent, seafloor hydrothermalism is intense and persistent and the average lava composition is more primitive. By contrast, where mantle upwelling is offset from the axis of spreading the seafloor is more tectonized, eruptions are less frequent and the average lava composition is more evolved. Here we address the following question: Is skew of mantle upwelling a feature peculiar to the EPR near 9°N, or is there evidence that it is a global phenomenon? A key observation supporting our hypothesis is the azimuthal rotation of seismic anisotropy, and by inference mantle flow, beneath the spreading axis. We present models supporting our interpretation that the azimuth of mantle divergence controls the observed seismic anisotropy. We then reinterpret results from the Mid-Atlantic Ridge (MAR) that also reveal an azimuthal rotation of seismic anisotropy beneath the ridge axis. A synthesis of results from the MAR further suggests that the relation of skewed mantle flow to tectonics is similar to that observed along the EPR and that variations in lithospheric thickness augment focusing of melt toward the segment center. Lastly, we make predictions for the orientation of mantle anisotropy beneath the Endeavour segment of the Juan de Fuca ridge. An upcoming seismic experiment to test these predictions is scheduled for 2008.