118 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Cambrian , •'' ,. 



aol v /s / Ch.lho^ee gr 



Chilhowee gr 



I oudoun fm 

 c °TocTin ~~ '9 re er>srone 



Fig. 8.25. Blue Ridge near Elkton, Va. After P. B. King, 1950b. 



somewhat metamorphosed during the later Taconian or Acadian 



Batholiths 



A number of plutons, most of batholithic proportions, occur in the 

 Piedmont province. Their distribution is shown on the Tectonic Map of 

 the United States, on the Geologic Map of the United States, and on the 

 Geologic Map of North America. Major differences in distribution appear 

 on the three maps; the later one shows a far less extent of the plutons in 

 South Carolina and Georgia than the earlier one. According to Keith 

 (1923) most of the plutons are granite and are little deformed or non- 

 deformed. According to Jonas (1932) the Petersburg granite of Virginia 

 is not deformed; it cuts across older structures without disturbing them 

 and enters the rock by replacing those already there. 



The plutons are known today, however, to be both concordant and 

 discordant. The former are foliated, and in the older reports are con- 

 sidered early Precambrian. The more or less discordant plutons are the 

 massive ones, and according to the older reports ( Keith, 1923, and others) 

 are of late Paleozoic age and associated in time with the folding and 

 thrusting of the Valley and Ridge province. The separation into two 

 vastly different time groups is now held to be unwarranted for two 

 reasons: (1) A similar complex is well-worked out in New England 

 (Chapter 11), and on the basis of fossils and stratigraphic succession the 

 intrusions range in age from Late Ordovician to Carboniferous; (2) 

 isotope age determinations now date the intrusions as Paleozoic. It seems 

 probable that the metamorphism of the Blue Ridge and crystalline Pied- 

 mont developed progressively during Paleozoic time as a result of 

 orogeny, possibly several phases of orogeny. The silimanite schist and 



gneiss zone of the inner Piedmont evolved as a result of the invasion 

 of the vast granitic plutons. 



Structure of the Piedmont 



From within the central metamorphic and plutonic belt northwestward 

 to the Great Valley nearly all the faults, folds, and cleavage are steeply 

 inclined but have a northwestward asymmetry; i.e., the fault planes, fold 

 axial planes, and cleavage planes dip to the southeast. Toward the 

 Coastal Plain a tendency is noted for the opposite asymmetry. The 

 northwest asymmetry of the inner zone (Fig. 7.2) is more one of folia- 

 tion than major displacement along a few discrete faults, with relatively 

 slight movement along an infinite succession of foliation planes ( Bloomer, 

 1950). 



-J L 



Fig. 8.26. Geologic map of Greaf Smoky AAountains and vicinity. After King ef a/., 1958. A, 

 Early Precambrian granitic and gneissic rocks; b,c,d,e, groups of the Ocoee series (later 

 Precambrian); P. Chiihowee group (Cambrian and Precambrian(?)); h, Mississippian, Ordovician, 

 and Cambrian rocks. 



