694 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Cayman trench, along the Cayman trench and to the Gulf of Honduras, 

 and thus it is seen that the Cayman trench of all four basins alone is 

 seismically active. If the other basins have a similar origin, then their 

 formation occurred in earlier times, and the Cayman should be considered 

 in process of formation today. 



The Cayman trench has long been considered a down-faulted trough, 

 and the fault scarplike topography of the trench walls has been cited as 

 evidence. Also, the escarpment of the Sierra Maestra of southeastern Cuba 

 facing the trench, and the fault valley of the Cul de Sac of Hispaniola are 

 taken to mark the eastern extent of the trench faults (Taber, 1922). The 

 other basins have been imagined blocked out by faults (review by Eard- 

 ley, 1954 ) but on a tenuous basis. On any grounds, the downf aulting could 

 not affect the base of the crust which has moved up. 



Ramifications of Tension Hypothesis 



With the tension hypothesis before us several thoughts result: ( 1) How 

 does tension in the western Caribbean relate to the outward flow and 

 peripheral compression of the eastern Caribbean crust, the theory just 

 proposed? (2) If these basins mark lanes of thinning of the entire crust 

 and consequently extension, are we dealing with the drifting of South 

 America apart from North America? (3) If the outward flow theory 

 pertains to the eastern Caribbean, why has not the Gulf Coast of the 

 United States flowed toward the Mexican basin and caused a volcanic 

 archipelago and trench there? Likewise why has not the Rrazilian conti- 

 nental margin overridden the Atlantic Ocean crust? 



Continental drift and the oceanward flow hypothesis of continental mar- 

 gins freshly formed by fragmentation and drifting apart seem logically 

 related, but a serious objection to the oceanward flow hypothesis as noted 

 above, may be an argument against drifting. 



POSTULATED EASTWARD SHIFT OF CARIBBEAN BLOCK 



In 1938 Hess presented a theory of evolution of the Antillean region 

 that involved eastward displacement of the Caribbean block. He regarded 

 major horizontal shortening in the orogenic belt of the Lesser Antilles 



necessary as the crust was rolled down in the tectogene, and accordingly 

 imagined the Caribbean block between Jamaica, Hispaniola, and Puerto 

 Rico on the north and the Leeward Islands on the south to have been 

 translated 50-100 miles eastward, and in the course of this movement the 

 faults of the Cayman trench and the Anegada Passage were formed, and 

 were chiefly ones of horizontal movement. Hess and Maxwell in 1953 

 depict some changes in the original theory (Fig. 42.12), and propose 

 that the areas of metamorphic rock of the Greater Antilles were once 

 joined in a single strip before the strike-slip faulting of great magnitude 

 broke and displaced the strip. They write as follows: 



This reconstructed strip of metamorphic rocks represents the axis of the 

 mid-Cretaceous down-buckle or downbulge. The tectonic axis is not the present 

 negative anomaly strip north of Puerto Rico and Hispaniola as previously be- 

 lieved. ... In all our previous analyses the structure of Puerto Rico appeared 

 to be anomalous. Here the folds are overturned to the north-northeast. If the 

 tectonic axis lay to the north, the overturning should have been southward. In 

 our present analysis the tectonic axis lies to the south-southwest, and the Puerto 

 Rican structures are then in a consistent relation to it. 



A system of faults in northern Colombia and Venezuela are interpreted 

 as wrench faults with movement of the Caribbean block eastward a con- 

 siderable distance ( Rod, 1956; Alberding, 1957 ) . This supports the theory 

 of Hess and Maxwell. 



Bucher (1952) has presented a variation to Hess's theory. He believes 

 that en echelon arrangement of fold axes along the coast of Venezuela on 

 the south and in the islands of the Greater Antilles along the north indi- 

 cates that the crust of the Caribbean Sea basin has moved eastward. 

 Crosswise of these compressional structures is a set of high-angle faults, 

 presumably tensional structures, which completes the picture of a shear 

 zone along the south and north sides of the sea basin. He says: 



In the Greater Antilles, 500 miles to the north, the same combination of 

 features recurs, but with directions reversed. There also, straight east-west 

 trending coast lines are conspicuous in the shapes of the islands from Jamaica 

 and the Sierra Maestra of Cuba through Hispaniola and Puerto Rico. As in the 

 ranges that form their counterpart in the south, the axes of individual folds 

 trend obliquely across the ranges and shore lines. But here they trend east- 

 southeastward, while there they bear east-northeastward. 



A complementary set of northeast-trending fractures finds conspicuous ex- 



