SECT. 1] 



THE MAGNETIC FIELD OVER THE OCEANS 



199 



leaves little doubt that these displacements represent real displacements of the 

 crust, or at least of those layers with which anomalies are associated. Fig. 14 

 summarizes the displacements across the three faults. A summary of displace- 

 ments on large wrench faults on land and at sea has been given by Menard 

 (1962). 



A subject almost as intriguing as the lateral faults is that of the origin and 

 significance of the magnetic pattern. The lineations of the field are remarkably 

 straight over very large distances. The principal group of north-south-trending 

 anomalies, for example, strikes almost exactly 5°W of N from the southern 

 edge of the majo to the Mendocino Fault, a distance of 900 km. They are also 



Fig. 15. Magnetic profile along line A-A of Fig. 12a and its interpretation in terms of an 

 infinite north-south slab. 



remarkably uniform in magnitude and in width. As far as can be judged from 

 the best available to]30graphic maps, over most of the area the pattern shows 

 little correlation with topography, and there is no indication of any systematic 

 differences between seismic velocities in the anomalous areas and in the areas 

 between, though the amount of data is not yet sufficient to demonstrate this 

 point conclusively. 



As regards their immediate cause, the anomalies are such as might be 

 expected of flat, slab-like structures approximately underlying the areas of 

 positive anomaly. Fig. 15 shows the observed anomaly along line A-A of Fig. 

 12a and superimposed on it the computed anomaly for an east-west crossing 

 of an infinite north-south slab immediately underlying the ocean floor. The 

 agreement is striking. 



