704 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



"wT^ — — — — ' N — — _ - _ — 



Fault >\'"/ N /\/», /y/w w s / \~ i \ / \ i \ / \ i 



Fig. 43.6. Generalized section across Isthmus of Panama, after MacDonald, 1919. Intrusions 

 are granodiorite, diorite, andesite, rhyolite, and basalt. Bedded rocks: 1, Bas Obispo volcanic 

 breccia; 2, Las Cascades agglomerate (Oligocene); 3, Bohio conglomerate; 4, Culebra fm.; 5, 

 Cucuracha fm.; 6, Emperador Is.; 7, Caimito fm. (Miocene); 8, Panama fm.; 9, Gatun fm. 

 (Pliocene); 10, Toro Is.; 11, Pleistocene. 



pressed than on either side with the basement complex showing to a less 

 extent. A somewhat different fault interpretation of the Canal Zone has 

 been rendered by Woodring (1957). 



The Isthmian link has the shape of an S curve and the faulted area of 

 the Canal Zone is in the middle, which may indicate that deformation 

 has been in one direction on one side and opposite on the other (Terry, 

 1956). 



The basement complex of Cretaceous (?) age with dioritic intrusions 

 has been taken to continue the Nevadan orogenic belt into South America 

 (Eardley, 1954). The broad nature of the platform upon which the ex- 

 posed narrow isthmus rests has been regarded as wide enough to contain 

 a major belt of deformation. Evidence to the contrary may be cited 

 as follows. The intrusions are probably not batholithic in size and some 

 may be Oligocene or Miocene in age. Also it is evident now from seismic 

 refraction work that modified oceanic crust lies on the Caribbean side 

 of the link, and true oceanic crust on the Pacific, so if the isthmus repre- 



sents an orogenic belt, it has evolved from oceanic crust, and should 

 not be similar to the Nevadan which in general evolved from a great 

 eugeosynclinal complex along the margin of the continents of North 

 and South America. 



RELATION TO GREATER ANTILLES 



Projection of Crystalline and Fold Belts 



The Crystalline Belt through the coastal ranges of northern Honduras 

 and Nicaragua passes out into the Caribbean Sea, and it is inferred 

 from exposures of the crystalline rocks on the Isla Roatan that the belt 

 continues toward Jamaica, bounding the Cayman trough on the south 

 (Roberts and Irving, 1957). The seismic traverse of Fig. 41.15, reveals a 

 layer of 5.2 to 6.1 kilometers per second, and although this has been 

 interpreted as a volcanic layer, the parts with higher velocities could 

 be the rocks of the Crystalline Belt. It seems hardly thick enough, how- 

 ever, to represent an old orogenic belt of metamorphic rocks. Meta- 

 morphic rocks of the Crystalline Belt varieties are not exposed on 

 Jamaica, but rather the oldest core rocks are folded volcanics of Late 

 Cretaceous age. It is tentatively concluded, therefore, that the Crystal- 

 line Belt as a crustal layer, wedges out not far east of the east coast 

 of Nicaragua. 



The fold belt has commonly been projected north of the Yucatan Basin 

 to Cuba, and the Cockscomb Mountains in British Honduras project east- 

 ward toward the Misteriosa and Cayman banks (Roberts and Irving, 

 1957). If connections north or south of the Yucatan Basin ever existed 

 with Cuba, they must have been very transitory, according to vertebrate 

 paleontologists (Schuchert, 1935). Yucatan has about seventy species of 

 verterbrates which are of the fauna of the Atlantic neotropical realm. If 

 it was united with Cuba at any time during the late Cenozoic, it is in- 

 conceivable why tortoises, pit vipers, Opisthoglypha, and Cnemidophorus 

 should not have crossed over in to Cuba. Since Yucatan was beneath the 

 sea during most of the Cenozoic era, land connections through the penin- 

 sula, at least, seem impossible. 



