ANTILLEAN-CARIBBEAN REGION 



691 



CARIBB 

 SEA 



AN \ H 



*t 





li^iO '^ /v / \ / w\ irw*~ \/\*#0 w w \""/ w w \~ w \~ w V-T^i-'^""' W W W ~/\ /W w W \ / \~V\ / WV/WW - /\7 \/\"/\ / \ / \ /\~/>>— T 

 MODIFIEO MANTLE MiNIMUM I "TT i ? ppi" L>"", 'J ^LCS Of ' 7 1 I '. 



WANTte 







5 



K 



•10 u 



K 

 H 



ail.- a 



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20 " 



SEMI- TO CONSOLID- 

 ATED SEDIMENTS 



VOLCANIC ROCKS 



INTRUSIONS (?) IN 

 VOLCANIC ROCKS 



SABBROIC LAYER 



25 



NAUTICAL MILES 



Fig. 42.11. Seismic sections and speculative geologic interpretations of the Venezuelan basin 

 and Greater and Lesser Antilles. After Officer ef a/. (1957) and J. I. Ewing ef al. (1957). Rocks 

 having velocities of 4.5 km/sec, if volcanics would be porous or more silicic than basalt; those 



theory in the Puerto Rico area, one of its classical localities. Ewing and 

 Heezen (1955), Worzel and Shurbet (1955), and Shurbet and Ewing 

 (1956) conclude from topographic profiles and sediment samples that 

 the trench has been partly filled with several thousand feet of light, un- 

 consolidated sediments, and that these and a thin crust can account for 

 the belt of negative gravity anomalies equally as well as a great downfold 

 or tectogene. The sediment fill may range up to 7 kilometers thick, de- 

 pending on the thickness of the gabbroic layer assumed. They pursue the 

 idea further by bringing to bear on the subject earthquake seismology, 

 magnetism, and seismic refractions. No refractions had been obtained 

 from the Moho discontinuity, or top of the mantle, under Puerto Rico or 

 the trench, but an excellent profile had been determined across the Cay- 

 man trench (Fig. 41.15), which indicates there a marked thinning of the 



having velocities of 5.8 to 6.0 would be volcanic with intrusions of basic rock or mixtures of 

 basalt and more silicic type, or perhaps andesite. For explanation of "modified mantle" see text. 

 Partially modified under Puerto Rico and the Puerto Rico trench according to Talwani ef al., 1959. 



crust. The Cayman trench was considered by Ewing and Heezen (1955) 

 the same kind of tectonic feature as the Puerto Rico trench, and hence. 

 the crust should be conspicuously thin under the Puerto Rico trench. The 

 concept of thinning under the trench is diametrically opposed to the tec- 

 togene concept. From gravity calculations the crust under Puerto Rico 

 should be about 20 to 25 kilometers thick (Worzel and Shurbet. 1955). 

 The gravity map of Fig. 42.9 indicates a general positive gravity area 

 across the Cayman trench with a negative belt on the south through the 

 northern part of the Nicaragua!) rise and a mild negative belt on the north 

 in the Bartlett deep strip. These gravity data do not agree with the com- 

 puted values of Ewing and Heezen (1955), from which they deduce a 

 conspicuous thinning of the crust. The gravity data of Fig. 42.9. are 

 difficult, in fact, to reconcile with the refractive seismic data across the 



