202 BITLLABD AND MASON [CHAP. 10 



strike-slip faults occur in the Santa Barbara Channel islands and in the trans- 

 verse ranges of California which are thought to mark an extension of the fault 

 zone on land. However, again the displacements are generally in the opposite 

 sense to the magnetic displacements. From what is known of the geology of 

 North America, it can be said with some certainty that no east-west strike-slip 

 displacements of the required magnitude can have occurred anywhere at any 

 time since late Pre-Cambrian. 



The second fact is that there is, on the north side of the Mendocino Fault, a 

 strip of the oceanic crust over 1000 km wide from the foot of the continental 

 slope outwards for w^hich there is no corresponding pattern on the south side 

 of the fault. Further, although there is some evidence for a general deepening 

 of the magnetic structures towards the continental slope, over such parts of 

 the continental shelf as have been mapped the field appears to be relatively 

 featureless and, although the east-west anomaly associated with the Mendocino 

 Fault can be followed shoreward, there is no evidence that the north-south 

 structures continue under it. Even if their depth increased several-fold, they 

 should still produce an observable effect at the surface. An explanation of the 

 disappearance of the magnetic pattern under the continent might be that 

 although the structures continue under it, the pattern has been erased by the 

 combined effects of temperature and pressure ; the conversion of maghemite to 

 haematite referred to in the previous section would provide such a mechanism. 



These facts might be explained in a number of ways: (1) the major fault 

 movements may have taken place at some time earlier than late Pre-Cambrian ; 

 the displacements in the opposite sense may reflect more recent movement 

 along the line of the faults ; (2) the continent may not have been anywhere near 

 its present position relative to the oceanic crust when the fault movements took 

 place ; (3) the displacements may be comparatively recent, but they may have 

 taken place so slowly that they were continuously absorbed by tectonic and 

 other processes still at work in the continental crust, particularly if the 

 continental crust has lieen advancing over the oceanic. 



Menard (1960, 1961) has drawn attention to some similarities between the 

 anomalous crustal structure of the ridge and trough province and that of the 

 East Pacific Rise, an elevated region of the eastern Pacific sea floor which forms 

 part of the mid-oceanic ridge system and disappears under the continent in a 

 northerly direction towards the Colorado Plateau. For example, the crust is 

 apparently thin and seismic velocities abnormal in both areas, and along the 

 crest of the East Pacific Rise and in parts of the ridge and trough province 

 heat -flow values are abnormally high. Menard seeks to link these facts by the 

 hypothesis of a rising convection current which produces high heat flow, uplift, 

 tension cracks, lateral movement and thinning of the crust by normal faulting, 

 and differential stresses, leading to the strike -slip faults. The tension cracks and 

 normal faiflts might have controlled the magnetic structures. 



If the East Pacific Rise and the ridge and trough province were once part of 

 a single continuous structure, then the strike-slip displacements must continue 

 under the continents, presumably within the deeper layers. The explanation of 



