200 BtTLLARB AND MASON [CHAP. 10 



Altliough magnetic data are not capable of unique interpretation, some 

 general conclusions can be reached about these slabs. In the first place, even if 

 they were at the shallowest possible depth in the crust, and if they were of a 

 highly magnetic material such as basalt, they would have to be almost 2 km 

 thick in order to account for the magnitudes of the anomalies. At the most, 

 their u])])er surfaces could lie no more than about 1 km below the sea floor, 

 otherwise they would not produce anomalies of the observed sharpness. They 

 would have to lie at least partly in the "second layer" of the crust. They could 

 be thicker slabs, extending to the base of the crust ; in that case their upper sur- 

 faces would have to be even shallower. 



The geological possibilities fall into three categories: (1) isolated bodies of 

 magnetically anomalous material, for example basic lava flows, within the 

 "second layer", (2) elevated folds or fault blocks of the main crustal layer, and 

 (3) zones of intrusion of highly magnetic material from the mantle. The first 

 would fit in well with current views about the somewhat enigmatic "second 

 layer" ; for the second, one could draw a parallel with the block-fault topo- 

 graphy of the neighbouring Basin and Range Province. Objection could be 

 raised to all three on the grounds of lack of topographic and seismic expression ; 

 these objections would be removed if the rocks involved were as higlily magnetic 

 as those from the Mendocino Fault discussed earlier, in which case the amount 

 of rock involved would be correspondingly less. Fig. 16 illustrates the three 

 cases in terms of two-dimensional structures each of which has been adjusted 

 by trial and error to fit the anomaly exactly (Mason, 1958). 



A more fundamental question is, how did such an orderly pattern come 

 about in the first place? There seems to be little doubt that the magnetic 

 structures are related in some way to lines of weakness in the oceanic crust ; in 

 view of the approximately orthogonal relationship between them, one might 

 suspect that the pattern and the faults had a common origin. However, from 

 the closeness with w'hich the pattern on opposite sides of the fault can be 

 matched, and in the absence of any magnetic structures cutting across them 

 (except towards the foot of the continental slope, where there is evidence for 

 complex faulting with north-south components of slip), it must be concluded 

 that tlie faults are more recent than the pattern ; in that case, there seems to 

 be no good reason why they should be related in any way. 



Irrespective of how the disj)lacements came about, the oceanic crust seems 

 to be sufficiently rigid for sections of it covering hundreds of thousands of 

 square kilometres to be capable of moving distances of the order of hundreds of 

 kilometres relative to one another with comparatively little distortion. Although 

 some blocks have suff"ered rotation, the magnetic-anomaly map shows little 

 evidence of distortion within individual blocks, exce])t in the fault zones and 

 near the eastern edge of the map. However, Menard (1959) has drawn attention 

 to a distinctive pattern of minor lineations, characteristic of tension, which 

 borders the faults, suggesting that all the crustal blocks have been under 

 tension at one time or another. The systematic deflection of the lineations in a 

 clockwise sense near the eastern edge of the map south of the Mendocino Fault, 



