Crustal velocity structure on the flanks of the Mid-Atlantic Ridge at 24 N

A detailed seismic refraction survey was made over a 70 × 35 km area of young (9 Ma) crust at 24°N 45°W to investigate the detailed velocity structure of a portion of ‘typical’ oceanic crust. An array of eight ocean bottom hydrophones was used to monitor 358 explosive charges and over 750 airgun shots. The crust exhibits normal oceanic structure, with a steep vertical velocity gradient through layer 2 and almost 3 km of layer 3 material with a mean velocity of 6.65 km s-1 overlying probably anisotropic mantle. Ray tracing shows that the major amplitude variations observed in the upper crustal returns are governed more strongly by the seafloor topography than by the seismic velocity structure within the crust. A delay time function method was used to invert the 1613 well-determined first arrival travel times from the explosives. We find that the delay time vanes by only 0.05–0.10s over most of the survey area, but that there are rather larger delay times in the north-west corner which may be associated with crust formed in the vicinity of a ridge jump. We also find a systematic correlation between variations in delay time and in the seafloor depth at the ray entry point, with smallest delay times where the seafloor is deepest. This is consistent with a model of the crust in which the topography of the base of layer 2 and the Moho is a subdued copy of the upper basement surface.