690 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Anomaly 

 curve 



Axis of deep 



Light Upper Crust 



Convection | c^ d 



heavy sub- 

 materia/ 



Axis of down-fold 



and of negative 

 J strip 



Fig. 42.10. The hypothetical tectogene and its relation to the axis of the deep and the negative 

 anomaly strip. After Hess, 1933. 



Serpentinite Intrusions and the Negative Strip 



Serpentinized peridotite intrusions occur all along the great negative 

 strip of the East Indies, and are present similarly in the West Indies. Hess 

 ( 1937b ) has pointed out this relation, and believes they come up along 

 each side of the tectogene, but only in a few places are both sides exposed 

 and not covered by younger rocks. He believes also that they are intruded 

 only during the first great deformation producing the strip. It will be re- 

 called that most of the serpentinized ultrabasic rocks in the West Indies 

 previously described are part of the Late Cretaceous and Early Tertiary 

 orogenies. Hess believes also that the serpen tinites are a water-rich prod- 



uct of partial fusion of the peridotite substratum squeezed off as a result 

 of the downbuckling. Conditions are favorable for its migration to the 

 surface along the vertical limbs of the tectogene. Any later deformation 

 does not produce such intrusion, because the bottom of the tectogene is 

 sealed by fusion, or all the low-melting constituents of the underlying 

 peridotitic material have been removed during the earlier cycle. 



[The serpentinized intrusions] . . . thus become useful guides in the interpre- 

 tation of any region such as the West Indies. The serpentinites clearly indicate 

 the former extension of the negative strip from the west end of Haiti along 

 the north coast of Cuba, and thence probably to Guatemala. 



CARIBBEAN REGION AND SEISMIC PROFILES 

 Seismic Data and General Observations 



Figures 42.11 and 41.15 give the principal results of seismic exploration 

 of the crust in the Puerto Rico region, the Lesser Antilles, the Venezuelan 

 basin, and the Colombian basin. This significant work has been under 

 the general direction of Maurice Ewing. 



It will be noted first, that the crust of the general Caribbean region is 

 thicker than the typical oceanic crust, but not as thick as typical continen- 

 tal crust; second, the Caribbean crust appears to contain no silicic 

 basement complex characteristic of the continents, but instead, rocks 

 interpreted mostly as volcanics. In short, the crust of this mediterranean 

 region was once typical oceanic crust but has had unusual amounts of 

 volcanic rocks spread widely but irregularly on it, and has suffered certain 

 deformation. It will also be noted that under four of the basins the crust 

 is thinner (Mexican, Yucatan, Cayman, and Venezuelan), but that under 

 the Colombian basin, and perhaps under the Puerto Rico trench, it is 

 thicker. The rises, ridges, and land elements are supported by thicker 

 crust. From general isostatic considerations this is to be expected, but the 

 negative anomaly belt and postulated tectogene under the Puerto Rico 

 trench pose a problem. 



Puerto Rico Trench 



Following a widespread acceptance and intensive development of the 

 tectogene concept, the seismic refraction survey seemed to refute the 



