Reykjanes ridge crest: A detailed geophysical study

Abstract

A geophysical survey employing satellite navigation was carried out over the Reykjanes submarine ridge southwest of Iceland. Water depth, sediment thickness, and the gravity and magnetic fields were continuously measured. In addition, bottom cores and measurements of sediment and water temperatures were obtained at stations. Expendable radio sonobuoys were used to make seismic refraction measurements. This paper combines these various geophysical data to obtain information about phenomena in the water layer, about details of crustal structure, and about mechanisms operating at the ridge axis. The satellite navigation results and water temperature data are used to deduce current directions and magnitude over the ridge. These currents play a role in the observed distribution of sediment. Variations in these currents are inferred from sediment temperature measurements. Magnetic profiles parallel to the ridge crest are used to demonstrate the presence of a thin, highly magnetized layer (termed layer 2A, since it constitutes the top of layer 2 of refraction seismology), as well as to directly infer the presence of normally and reversely magnetized rocks in bands on the ridge. Seismic refraction measurements reveal: (1) a 6.5-km/sec layer under the ridge; (2) a flankward increase in seismic velocity in the crust; and (3) evidence for a surface layer of relatively low velocity (about 3 km/sec) corresponding to layer 2A. Geothermal measurements revealed two zones of low heat flow, one within 10 km of the ridge axis and the other about 75 km from the axis. The maximum values of heat flow were observed in a zone from 15 to 50 km. The over-all average of heat flow over the ridge is not significantly different from that observed in the adjacent oceanic basins. Free-air gravity anomalies over the Reykjanes ridge range from +25 to +60 mgal. Compared to the mid-Atlantic ridge, the Bouguer anomalies over the Reykjanes ridge are about 60 mgal less, but the gradients are nearly the same. The narrow axial magnetic anomaly can be traced with minor offsets to the Reykjanes Peninsula. On Iceland, positive magnetic anomalies occur over much wider areas, implying that the active zone is much wider in Iceland than over the Reykjanes ridge.