SHAPE AND STRUCTURE OF OCEAN BASINS 



12 



21 





10 



1000 2000 



Depth 



3000 'km 



Fig. 7. Compressional (P) and shear (5) wave velocities in the earth (after 

 Gutenberg, 1958). 



particularly to results such as those shown in Fig. 8. The increase 

 in compressional and shear velocities at a depth of about 200 km 

 or slightly below was compared with the similar depth found by 

 Benioff for the juncture of two systems of energy release for 

 orogenic regions bordering continents (e.g., Kuriles and South 

 America). On the basis of this comparison, Lehmann suggested 

 that "the 200-300 km level thus represents a tectonic discontinuity 

 which is apparently the lower boundary of the continents." 

 Lehmann's more recent work (1959, 1960) has strengthened the 

 concept of a sharp increase of F and S velocities at a depth of 

 about 220 km below Europe and northeastern North America, 

 which depth would mark the bottom of the low-velocity zone. 



