SECT. 1] 



GRAVITY AT SEA 



159 



mediate stages ; all of the sections are closely in isostatic balance ; and the M- 

 discontinuity varies considerably in steepness but continental crustal thickness 

 changes to ocean crustal thickness within 200 km. 



Drake et al. (1959) computed various structures for the Portland, Maine 

 section on various assumptions of crustal layering and densities covering the 

 gamut of seismological and geological possibilities. They found that none of the 



CJrOSS SECTION OF CRUST BASED ON DIRECT METHOD 



■3i60 



-3.20- 



Vertical Exaggeration 

 10 I 



' Theoretical . 



Mohorovtc'ic 



Dtscontinvity 



ZOO 



400 600 ^ 9O0 



DISTANCE IN KILOMETERS 



1000 



Fig. 17. Crustal section across continental margin and trench near Antofagasta, Chile, 

 computed from gravity data. (After Wuenschel, vmpublished thesis.) 



possibihties that they considered reasonable appreciably modified the section 

 from those computed in the earlier work of Worzel and Shurbet except for a 

 different choice of density for the mantle. 



Wuenschel, in his unpublished Ph.D. thesis, derived three sections 

 across the western continental margin of southern South America. Fig. 17 

 shows one of his jirofiles, which is quite similar to the other two. The analysis 

 was complicated by the deep trench near the continental slope and the high 

 mountains on the shore side, and the lack of detailed geological knowledge in 

 the region. The densities he used are shown on the figure. Once again one sees 

 the abrupt change from continental crustal thickness (mountainous here) to 

 oceanic thicknesses in a distance of less than 200 km. 



Fig. 18 shows a first approximation crustal section along the west coast of 

 the United States near San Diego. No seismic data were available so all the 

 crust was considered to have a density of 2.84. It is known that there are a 

 number of basins with some sedimentary fill in the region but so far no estimate 

 of the density or the sediment thickness can be made. It is not expected that 

 the section will be appreciably modified when these sediment accumulations 

 are known. Once again, one is struck by the abrupt shallowing of the mantle 



