SEISMOLOGICAL EVIDENCE FOR 3-DIMENSIONAL MELT MIGRATION BENEATH THE EAST PACIFIC RISE

The extent to which crustal processes along mid-ocean ridges are contr olled by either the pattern of mantle upwelling or the mode of magma i njection into the crust is not known. Models of mantle upwelling vary from two-dimensional, passive flow(1) to three-dimensional, diapiric f low(2-4). Similarly, beneath a ridge segment bounded by tectonic offse ts, crustal magma chambers may be replenished continuously along the r idge(5-7) or at a central injection zone(2-4) from which magma migrate s towards the segment's ends. Here we present tomographic images that reveal the seismic structure and anisotropy of the uppermost mantle be neath the East Pacific Rise. The anisotropy is consistent with two-dim ensional mantle now diverging from the rise, whereas the anomalous iso tropic structure requires a three-dimensional but continuous distribut ion of melt near the crust-mantle interface. Our results indicate that crustal magma chambers are replenished at closely spaced intervals al ong-axis and that crustal systems inherit characteristics of scale fro m melt transport processes originating in the mantle.