MID-ATLANTIC RIDGE NEAR 45° NORTH: I. THE MEDIAN VALLEY

Detailed maps of bathymetry, free air gravity anomaly, and total field magnetic anomaly are presented for an area approximately 50 × 20 mi along the crest of the ridge. The median valley and the associated belt of large positive magnetic anomalies are continuous and display a striking lineation in direction 019°. The free air gravity anomaly shows a strong resemblance to topography. This correlation disappears when the Bouguer anomaly is calculated, indicating that the intrusive body immediately underlying the median valley is not significantly different in density from those bodies beneath the elongated sea mounts which overlook the valley. Small variations in the Bouguer anomaly indicate that there is an increase in density in a northwest direction across the survey area. Magnetic anomalies within the surveyed area can be simulated by a two-dimensional model in which steeply dipping contacts separate blocks of rock having different magnetizations. These blocks could be entirely within the volcanic layer extending to a depth of 5 km below sea level, but the central block, underlying the median valley, must be much more strongly magnetized than those adjacent to it. The mechanism by which the valley was formed remains obscure.