Imaging the transition between the region of mantle melt generation and the crustal magma chamber beneath southern East Pacific Rise with short-period Rayleigh waves

Beneath the southern East Pacific Rise resides a crustal magma chamber confined to the immediate vicinity of the rise axis, yet the mantle melt-production region is broad (>200 km wide) and asymmetric. Short-period (10-20 s) Rayleigh waves propagating along the rise from regional earthquakes provide a means of probing the transition from the asymmetric melt production region to the rise-centered crustal magma chamber. Low velocities beneath the rise act as a waveguide for surface wave energy. We model this complex wave propagation with a Gaussian beam representation as part of our non-linear inversion for velocity structure. Rayleigh wave results are generally similar to those of our previous Love wave study and show an asymmetry low-velocity zone in the mantle that is present at the crust-mantle interface, or Moho, and increases in width and asymmetry downward. The lowest mantle velocities are located beneath the rise at the Moho; thus, beneath the rise there is a gap in the high-velocity "lid" at the top of the mantle indicating a gap in the lithosphere. In addition there are lower asthenospheric velocities beneath the Pacific plate than the Nazca plate. The transition from the broad upwelling region to the narrow crustal magmatic system occurs over a 15 km depth interval at the top of the mantle. The low-velocity region is much wider in the Rayleigh wave solution than the Love wave solution, indicating that seismic anisotropy is present. One explanation, that requires further quantitative analysis, is that horizontal alignment of olivine in the upper 100 km increases with increasing distance from the rise axis, which retards the increase in Rayleigh wave velocity while advancing the increase in Love wave velocity. Our observations indicate higher temperatures and greater melt production beneath the Pacific plate. East of the rise melting appears to abruptly shut off at shallow levels, perhaps due to a component of downward mantle flow. We suggest that melt percolates upwards, increasing in concentration, until reaching a permeability barrier beneath the lithosphere, it then moves up along the lithosphere to accumulate at the Moho beneath the rise.