Asymmetric mantle dynamics in the MELT region of the East Pacific Rise

The mantle electromagnetic and tomography (MELT) experiment found a surprising degree of asymmetry in the mantle beneath the fast-spreading, southern East Pacific Rise (MELT Seismic Team, Science 280 (1998) 1215–1218; Forsyth et al., Science 280 (1998) 1235–1238; Toomey et al., Science 280 (1998) 1224–1227; Wolfe and Solomon, Science 280 (1998) 1230–1232; Scheirer et al., Science 280 (1998) 1221–1224; Evans et al., Science 286 (1999) 752–756). Pressure-release melting of the upwelling mantle produces magma that migrates to the surface to form a layer of new crust at the spreading center about 6 km thick (Canales et al., Science 280 (1998) 1218–1221). Seismic and electromagnetic measurements demonstrated that the distribution of this melt in the mantle is asymmetric (Forsyth et al., Science 280 (1998) 1235–1238; Toomey et al., Science 280 (1998) 1224–1227; Evans et al., Science 286 (1999) 752–756) at depths of several tens of kilometers, melt is more abundant beneath the Pacific plate to the west of the axis than beneath the Nazca plate to the east. MELT investigators attributed the asymmetry in melt and geophysical properties to several possible factors: asymmetric flow passively driven by coupling to the faster moving Pacific plate; interactions between the spreading center and hotspots of the south Pacific; an off-axis center of dynamic upwelling; and/or anomalous melting of an embedded compositional heterogeneity (MELT Seismic Team, Science 280 (1998) 1215–1218; Forsyth et al., Science 280 (1998) 1235–1238; Toomey et al., Science 280 (1998) 1224–1227; Wolfe and Solomon, Science 280 (1998) 1230–1232; Evans et al., Science 286 (1999) 752–756). Here we demonstrate that passive flow driven by asymmetric plate motion alone is not a sufficient explanation of the anomalies. Asthenospheric flow from hotspots in the Pacific superswell region back to the migrating ridge axis in conjunction with the asymmetric plate motion can create many of the observed anomalies.