SECT. 1] 



GRAVITY AT SEA 



15-; 



willing to assume that the crustal and mantle densities are constant and of the 

 above magnitude. 



A. Continental Margins 



Heiskanen and Vening Meinesz (1958) conclude that of the 26 coastal 

 margins studied, in all but two cases the regions were most closely in local 

 isostatic equilibrium for an Airy isostatic assumption and T = 30. Near the 

 Canary Islands and the west coast of Africa, Australia's northwest cape and 

 Socotra, a value of T = 20 seemed better but they thought the data inconclusive. 



Worzel and Shurbet (1955a) have derived seven sections across the con- 

 tinental margin of the northeast coast of the United States from gravity and 



300\ 



Distance in ki lomef e rs 



iqOO , 2000 , 3000 



4000 



Bouguer Anomaly 



01 \ \ L_ 



300 



Anomaly 



..^ 



Nares 



Deep^ 



Mid- Atlantic 

 Ridge 



Canary 

 Islands-, 



Topography 



M- Discon tinuity - 



Fig. 15. Crustal profile from Puerto Rico to the African coast near the Canary Islands. 

 (After Koryakin, 1958.) This section was computed from the gravity anomalies 

 assuming a depth to the M-discontinuity just north of the Nares Deep. 



seismic data. These sections show a considerable variation of sedimentary cover. 

 The crust-mantle boundary is computed on the basis of the following assump- 

 tions : (1) a water density of 1.03 ; (2) a mean density of the sedimentary layers 

 of 2.30 ; (3) a mean density of the crustal layers of 2.84 ; (4) a mean density of 

 the mantle of 3.27 ; and (5) the standard sea-crustal section of Worzel and 

 Shurbet (1955). Fig. 16 shows three sections representing the variation en- 

 countered. 



It was concluded that the true edge of the continent occurs at about the 

 1000-fm curve ; the maximum sedimentary thickness occurs at the base of the 

 continental slope and a considerable thickness persists across the continental 

 rise ; the distribution of sediments is probably brought about by the action of 

 turbidity currents; sedimentation, mostly derived from the continents, must 

 have modified the basic structure with the various sections representing inter- 



