Magnetic survey over the Mid‐Atlantic Ridge between 42°N and 46°N

A detailed bathymetric-magnetic survey was conducted over the crest of the mid-Atlantic ridge between 42°N and 47°N (about 300 to 800 km northwest of the Azores). These survey data were analyzed to yield depth to source, intensity, and, to a limited extent, direction of magnetization. In addition, ten unweathered basaltic rock samples were dredged at two sites on the inner slopes of the median rift valley. Eight specimens of dense basalt from one site showed remanent magnetization of 0.002 to 0.011 cgs (average 0.007), and two samples of glass-sheathed basalt dredged from the second site yielded remanent intensities of 0.00055 and 0.00075 cgs. Q ratios ranged from 5 to 120 in the ten samples, with a median of 60. The high Q suggests that induced magnetization plays only a minor role in the large magnetic anomalies observed. The magnetic properties measured here are similar to those previously reported for oceanic basalts. The magnetic field structure in this area has two principal elements: (1) Anomalies of short wavelength (3 to 7 km) and (2) a long-wavelength anomaly (about 15 km) centered over the median rift valley. The short-wavelength anomalies are shown to be of shallow origin, being largely caused by topographic highs. A number of topographic features are either devoid of magnetic expression, however, or are magnetized only in part. Intensities of magnetization computed from the larger anomalies vary from 0.005 to 0.025 cgs and are thus comparable to intensities measured from dredge samples of the first site. Directions of magnetization are roughly ambient; in some cases the remanent declination may be somewhat to the west, and the inclination may be as much as 30° shallower than ambient. The long-wavelength magnetic high is approximated by a computed anomaly due to a vertical north-striking dike about 13 km wide and underlying the central rift valley at a depth of about 7 km below sea level (average deepest sounding over valley was 3 km). A magnetization contrast of at least 0.002 cgs is directed roughly parallel to the ambient field.