Constraints on melt in the lower crust and Moho at the East Pacific Rise, 9°48′N, using seafloor compliance measurements

Seafloor compliance measurements across the East Pacific Rise at 9 degrees 48'N reveal low shear velocities throughout the crust and at the crust-mantle boundary, with the lowest shear velocities centered beneath the rise axis. The compliance method uses the seafloor deformation under the loading of long wavelength ocean waves to probe the oceanic crust. The shape of the compliance function as a function of frequency is primarily controlled by regions of low shear velocity within the crust. At 9 degrees 48'N, the shear velocity is less than 20 m/s in the shallow on-axis melt lens located 1.4 km beneath the seafloor, demonstrating that the melt lens at this site is fully melt rather than a connected crystal mush. The compliance data also require a second on-axis melt lens 5.4 +/- 1 km beneath the seafloor with shear velocities slower than 50 m/s. This "deep" melt lens may be created by melt pooling at a permeability or density barrier at the crust-mantle interface. The shear velocity in the lower crust between the two melt lenses averages 1.7 km/s, indicating 2.5-18% melt. Melt persists in the lower crust to at least 10 km off-axis, where the top of the lower crustal low-velocity zone is approximately 4 km beneath the seafloor. In seismic layer 2B, the ratio of shear to compressional velocity increases from 0.41 on-axis to 0.58 by 10 km off-axis, indicating that there are abundant thin cracks in the sheeted dikes on-axis and that these cracks close away from the rise axis. High on-axis porosity in layer 2B may allow hydrothermal circulation down to near the shallow melt lens.