RESUME OF STRUCTURAL GEOLOGY OF NORTH AMERICA 





Central America 



Southern Mexico, Guatemala, Honduras, El Salvador, and Nicaragua 

 contain a belt of metamorphic rocks which sweeps from southwestern 

 Mexico in an easterly direction to the Caribbean Sea. A belt of deformed 

 Permian strata with Permian ( ? ) granitic and ultrabasic intrusives makes 

 up part of the crystalline complex. A fold belt of Jurassic and Cretaceous 

 strata borders the crystalline belt on the north. 



The major geologic feature of southern Mexico and Central America 

 is an extensive accumulation of Tertiary volcanic rocks which masks 

 much of the underlying older rocks mentioned above. All the Isthmus of 

 Costa Rica and Panama is made up of an igneous complex, mostly 

 Tertiary, or of sediments derived from the volcanics. 



Antillean Region 



The Greater Antilles, composed of Cuba, Hispaniola, Puerto Rico, and 

 the Virgin Islands, have a late Mesozoic and Ccnozoic history. Thick 

 limestones made up a northern facies in Jurassic and Cretaceous times 

 and a volcanic assemblage a southern facies. Folding, thrusting, and in- 

 1 trusions followed. Tertiary time saw extensive flooding and reuplift of 

 the islands but not much deformation of the strata. 



The Lesser Antilles or Caribbees are a Cenozoic volcanic arc developed 

 on the oceanic crust. 



Precambrian (Plate 1) 



Absolute age determinations on Precambrian rocks are now sufficiently 

 numerous so that divisions of different ages are becoming defined. The 

 ages denote the time of origin of the mineral of which the analysis was 

 made, and this denotes the time of an igneous intrusion or of an episode 

 of metamorphism. In other words, the ages appear to indicate belts of 

 orogeny. They define a continent made up of a rather small central 

 region of greatest age, and belts on the northwest and southeast of 

 progressively younger age. The strange aspect of the belts older than 

 800 million years is that they project out to the Pacific Ocean basin, as 

 if the continent at this time continued to the southwest farther than its 



present boundary. The Beltian basin or geosyncline, about S00 million 



years old, lies unconformably across the older belts, and apparently, for 

 the first time marked a direction subparalle] with the existing mar 



Subsequently, all orogenesis occurred in belts conformable to the present 

 margin. 



Cambrian (Plate 2) 



Cambrian seas and sediments defined major tectonic divisions of the 

 continent which lasted until the end of the Paleozoic era. The Canadian 

 Shield of Precambrian rocks formed the central and northeastern part 

 of the continent, and it probably was a vast region of low relief. By way 

 of an extension to the southwest, the Transcontinental Arch, the United 

 States was divided into western and eastern seaways, and a svinmetrial 

 arrangement of shelves, miogeosynclines, and eugeosvnclines resulted. 

 No Cambrian strata are known along the western margin or the eastern 

 margin south of Maine, and the conditions in these regions in Cambrian 

 times are not well known. The hypothesis that North America was at 

 this time part of a much larger continent, which cracked and spread 

 apart, seems to help most in understanding the paleotectonic elements. 

 Southern Europe, Africa, and South America are postulated by some to 

 have lain close together, and hence, it is suggested that the Atlantic- 

 Ocean and Gulf of Mexico, with associated coastal plains or continental 

 shelves, did not exist at this time. 



Ordovician (Plate 3) 



The broad Williston basin became well-defined during the Ordovician 



and a narrow basin of diick carbonate sediments formed in Oklahoma. 

 and extended to the shallow Colorado sag nearlv across the Transconti- 

 nental Arch. Extensive regions of the Canadian Shield were invaded by 

 epeiric seas. The margins of the continent are still problematical. Flat- 

 lying, unmetamorphosed strata in northern Florida south ot the eastern 

 orogenic belt seem to require continental connections where now is the 

 Atlantic Ocean. The Taconic orogeny of folding and thrusting occurred 

 in eastern New York, Vermont, and southeastern Quebec. 



