213 



In summar)', considerable uncertainty exists in placing minimum depths of 

 burial of greenschist facies rocks. However, a minimum value of 2 km beneath an 

 ocean floor at 5 km seems reasonable. The maximum water pressure at such a depth 

 is considerably below some estimates for greenschist facies metamorphism. However, 

 these water pressure estimates are based on very little experimental data and are 

 most likely subject to considerably more uncertainty than the minimum temperature 

 estimates. 



Minimum vertical displacement along faults should be on the order of a few 

 kilometers to expose greenschist facies rocks. Such movements are not unreasonable 

 in view of the relative relief of the ridge crests above the median valley in the 22°N 

 latitude area (Van Andel et al., 1965). However, the greenstones may well have been 

 metamorphosed at considerably greater depths, particularly if the water pressure 

 estimates for greenschist facies metamorphism are correct. In this event, simple rift 

 normal faulting may not have been sufficient to give exposure. More complex tectonic 

 movements would have to be invoked, or, alternatively, the possibility of considerable 

 submarine erosion would have to be considered. 



The structure of the Mid- Atlantic Ridge is thought by some to reflect tension and 

 collapse occurring above a rising convection current which is constantly adding new 

 crust to the floor of the rift valley. If greenschist facies or higher grade metamorphic 

 rocks are found to be abundant in the ridge as a whole, periods of a halt in this 

 forceful upward movement of material are required. Alternatively, rapid burial may 

 occur. More extensive rock dredging programs are required to clarify the importance 

 of metamorphism in the Mid-Atlantic Ridge as a whole. 



Bearing on the M-discontinuity and oceanic crust 



Engel et al. (1965) suggested that the Mohorovicic discontinuity under oceanic crust 

 may mark the transition of tholeiitic basalts to a "schisted" metamorphic equivalent. 

 They postulate temperatures between 100 and 200°C and pressures of between 1 and 2 

 kbar at the Moho and suggest that the metamorphic grade should not exceed green- 

 schist or" blueschist" facies. As they point out, transition of basalt to greenschist is 

 not in accord with the geophysical evidence about rocks below the M-discontinuity; 

 the 22°N greenstones, are, in fact, not as dense as most abyssal basalts (average 

 density 2.74 for analyzed greenstones. Table VI, versus 2.80 for fresh abyssal basalts 

 from 22°N). It is thus unlikely that greenschist facies metabasalts occur at or below 

 the Moho. 



On the other hand, greenschist and perhaps zeolite facies rocks may be com- 

 mon in oceanic crust, particularly beneath the Mid-Atlantic Ridge. Seismic velocities 

 of the Mid-Atlantic Ridge crustal rocks range considerably even in single profiles 

 (EwiNG and Ewing, 1959) and do not exclude the possibility that greenschists are 

 abundant. Rift faulting of the thick volcanic pile which evidently composes the ridge 

 in places would give ready access to water, either form descending heated sea water, 

 or from hydrothermal solutions derived from the mantle. 



