692 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Yucatan basin, Cayman trench, Nicaraguan rise, and Colombia basin. It 

 appears that the refractive seismic data are to be sought, and that the 

 gravity data are to be considered afterward in light of the seismic data. 

 Later refractive recordings and computations by Officer et al. (1957) 

 and Talwani et al. (1959) in the Puerto Rico trench suggest a thickening 

 of the crust there. Examine the crustal structure under the Puerto Rico 

 trench of Fig. 42.11, section A-A'. The gabbroic and volcanic layers seem 

 to be dropped down by a series of faults under the trench with the Moho 

 at 20 kilometers. The gabbroic layer under Puerto Rico thickens greatly, 

 and the Moho discontinuity reaches to a depth of about 30 kilometers 

 there. 



Lesser Antilles and Barbados Ridge 



A section through the Lesser Antilles and Rarbados Ridge is given in 

 R-R', Fig. 42.11. 



Refore proceeding with the interpretation of the specific velocity layers 

 of the Lesser Antilles it will be pointed out that, in general, velocity layers 

 are interpreted to designate rock types as follows : 



1.8-3.7 km/sec 



Unconsolidated and semiconsolidate sedi- 





ments 



4.7-5.2 km/sec 



Semiconsolidated and consolidated sedi- 





ments 



5.2-5.5 km/sec 



Extruded porous volcanic material or lime- 





stones and dolomites 



5.8-6.1 km/sec 



Granitic basement 



6.5± km/sec 



Basaltic or gabbroic subcrust 



7.5± km/sec 



Mixture of mantle and gabbroic subcrust 





or mantle with a higher temperature 





than general 



In commenting on the velocity layers of Fig. 42.11 J. I. Ewing et al. 

 (1957) conclude that a low velocity sedimentary layer consisting of an 

 upper unit with velocity of about 1.7 km/sec and a lower unit of about 

 2.4 km/sec extends across most of the section. Underneath this is a layer 

 of about 4 km/sec which also extends across all the section except under 

 the Atlantic Rasin. It could be identified as lithified sediments or porous 

 volcanic rocks. The next higher velocity layer is one having velocities of 



4.9 to 5.2 km/sec, and in interpreted to be intruded volcanic rocks. This 

 layer is lenticular in cross section and principally under the Rarbados 

 Ridge; it does not extend under the ridge of the Lesser Antilles. However, 

 in the Puerto Rico region a layer having velocities of 4.9 to 5.8 km/sec 

 seems similar and has been interpreted as the Cretaceous basement of 

 folded shales, tuffs, and agglomerates which have been extensively in- 

 truded. The layer has velocities up to 6.1 km/sec in places, and it is those 

 parts with velocities between 5.7 and 6.1 particularly that are shown in 

 the cross sections of this book as having considerable intrusive rock. Meta- 

 morphism incident to orogeny as well as intrusions may contribute to 

 higher velocities in the layer, such as is evident under Puerto Rico. The 

 most likely interpretation is that the Rarbados Ridge is a large synclinal 

 structure as far as the gabbroic layer is concerned and the low-velocity 

 layers above the volcanic lens of the Rarbados Ridge are anticlinal. The 

 Lesser Antillean Ridge is a large anticline, or at least a belt of uplift of 

 the gabbroic crustal layer. It appears to J. I. Ewing et al. (1957) that 

 the uplift is due to extensive intrusions from below of gabbroic material. 

 Some of this activity has penetrated through to the surface to form the 

 present volcanoes and extrusive rock. During the course of magmatic 

 activity differentiation to more silicic types has occurred, which are 

 observed at the surface. 



The thick lens of volcanic ( ? ) material in the syncline under the Rar- 

 bados Ridge, by the same reasoning, would be the result of an older phase 

 of intrusive activity, but the belt has subsided incident to the new adjacent 

 intrusive activity. The cause of subsidence is not clear. The Rarbados Ridge 

 is not active volcanically or magnetically whereas the Lesser Antillean 

 Ridge is highly active in both respects. 



Grenada Basin, Aves Swell, and Venezuelan Basin 



Layers of volcanics and sediments are spread under the Grenada basin, 

 the Aves swell and the Venezuelan basin as illustrated in section R-B', 

 Fig. 42.11. The gabbroic layer becomes thinner westward of the Lesser 

 Antilles uplift and the overlying volcanics thicker. The Aves swell is ap- 

 parently an especially thick pile of volcanics, and like the Barbados ridge, 

 may have been the site of a previous uplift with intrusive activity. 



