Rapidly growing information regarding the rocks of the Mid-Atlantic Ridge has led Melson & Thompson to postulate a basement consisting of an upper layer of oceanic tholeiites with subsidiary hydrothermal metabasalts, and a lower plutonic layer of peridotites, serpentinites and gabbros. These layers may correspond to the second and third layers of refraction seismology. The same data permit the assumption that the oceanic tholeiites are underlain by an intermediate layer of metabasalts resulting from iso-chemical regional metamorphism under increased temperature and pressure. Laboratory measurements of magnetic properties have shown that the Koningsberger ratio of oceanic metabasalts is generally less than unity. Thus, such a regional metamorphic layer would place a floor under the zone responsible for the observed geomagnetic anomaly patterns.
Reflection/Refraction Seismology
