160 



\VORZEL AND HARRISON 



[chap. 9 



STRUCTURE SECTION THROUGH SAN DIEGO 



100 



Kilometers 

 300 400 



Computed Anomaly 



500 



NE 



SW 



10 



35 



COMPUTED 



P'I03 



p'28') 



P'3Z7 



STRUCTURE SECTION 



I J \ 



Fig. 18. Crustal section across continental margin near San Diego, California. (After 

 Worzel and Talwani, unpublished.) 



at the continental edge, which here occurs about 200 km from the shorehne at 

 about the 1000-fm curve. 



Thompson and Talwani (1959) have computed a section across the continental 

 margin along the west coast of the United States, near San Francisco. The 

 densities used are shown in the section illustrated in Fig. 19. The section is 

 complicated by the various mountain ranges in that coastal region. The abrupt 

 change of crustal thickness at about the 1000-fm curve is again evident. 



B. Mid-Ocean Ridges 



The only mid-ocean ridge examined in detail is the Mid-Atlantic Ridge at 

 about latitude SO'^N. Fig. 20 shows, at the top, a continuous gravity curve 

 obtained with the Graf Sea Gravimeter in crossing this feature, and. at the 

 bottom, the topography. After the effect of the topography (Talwani. Heezen 

 and Worzel, 1961) has been removed, an anomaly (Bouguer) is obtained which 

 must be explained by densities within the crust or the mantle. There were some 

 seismic measurements available to provide some control for the upper part of 

 the crust (Ewing and Ewing, 1959). Fig. 21a shows the resulting section for 

 the conventional view that the variation of the thickness of the crust is primarily 

 responsible for the remaining anomaly. Even with the abrupt change of depth 



