Increase of seismic velocities in upper oceanic crust: The “superfast” Spreading East Pacific Rise at 14°14'S

One of the most important observations of oceanic crustal evolution is that seismic compressional wave velocities in the upper igneous crust, layer 2A, increase as the crust ages. The most viable explanation for this phenomenon is decreasing porosity due to filling of open void spaces with hydrothermally generated minerals. But the idea that seismic velocity could be a function of plate age, however, depends primarily on airgun/sonobuoy profiles collected more than 20 years ago [ Houtz and Ewing, 1976]. Therefore, in late 1995 new seismic refraction measurements were obtained on the eastern flank of the “superfast” spreading East Pacific Rise south of the Garrett transform. We found that velocities rapidly increase by about 45–50% within only 0.5–1 m.y., double within 5 m.y. or less and remain constant thereafter.