Axial Intrusion Zone beneath the Median Valley of the Mid-Atlantic Ridge at 37°N Detected by Explosion Seismology

Four seismic refraction lines, three of which had shots every 250 m, were shot across, along and parallel to the median valley of the Mid-Atlantic Ridge at 37° N. A method has been developed for calculating the effect on the travel times of the rough sea-floor relief beneath the profiles and has been used to correct all the travel times for this effect. Most arrivals were from a main refractor of apparent velocity 5.4 to 6.3 km s−1; only beyond 35 km were faster arrivals observed from an 8.09 ° 36 km s−1 refractor. The main refractor corresponds in depth, at least approximately, to the top of Layer 3 of the ocean basins but its velocity is significantly less than normal for Layer 3, perhaps due to dip. A study of time residuals along two profiles across the median valley indicates the presence of a 2 to 3 km wide low velocity zone (about 3.2 km s−1) beneath the median valley floor. This zone extends over the upper 2.5 km of the crust and is believed to represent a zone of intrusion through which magma passes on its way to the sea floor.