220 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



bounds the east side of the Carboniferous basin of the St. Georges Ray 

 area of southwestern Newfoundland, it would also be one of vertical dis- 

 placement. The wedge of sediments of the 3.80-km/sec layer suggest a 

 vertical fault. The major structural elements from Newfoundland to Nova 

 Scotia may be drawn across to Nova Scotia with reasonable continuity 

 and without a horizontal offset, as shown in Fig. 13.12. Although none of 

 these is compelling evidence against horizontal movement, they lead the 

 writer to conclude that considerable transcurrent movement has not 

 occurred. 



MAJOR TECTONIC RELATIONS OF GREATER ACADIA 



Definition 



Greater Acadia has been defined by Schuchert and Dunbar ( 1934 ) as 

 the combined regions of New England, the Maritime Provinces, the St. 

 Lawrence-Gaspe area of Quebec, and Newfoundland. Much of the area is 

 now covered by shallow waters, and from an historical point of view 

 Greater Acadia includes all the lands of the past in the great geosynclinal 

 and orogenic belt seaward to the continental shelf slope. 



Major Geocynclinal Characteristics 



Numerous series of beds in Greater Acadia have thicknesses of 5000 to 

 15,000 feet, and the total thickness in places ranges up to 100,000 feet. 

 Thick and coarse elastics in every stratigraphic system of the Paleozoic 

 and numerous unconformities within and between systems attest long- 

 continued crustal unrest in the geonsyncline and at times in belts 

 adjacent to it. A dominant lithology of the materials in the geosyncline 

 is volcanic rocks of all descriptions. They consist chiefly of andesites and 

 basalts, but other varieties, especially rhyolites, are by no means absent. 

 A very thick accumulation of Ordovician rhyolite marks the central part 

 of the geosyncline in Newfoundland. The volcanics occur as flows, in 

 large part submarine, and as various pyroclastics. They are especially 

 concentrated in the medial part of the geosyncline, if the Precambrian 

 rocks of Nova Scotia and the Avalon peninsula of Newfoundland mark 

 the site of the outer or southeastern portion. The inner belt of the Taconic 



Mountains-Lake Champlain-St. Lawrence-Gaspe region was compara- 

 tively free of volcanics until late Ordovician and Silurian time when the 

 igneous activity spread to the Gaspe Peninsula and to western New- 

 foundland in the western belt. Aside from Devonian volcanic activity 

 in the Gaspe Peninsula the western belt was again free of volcanism 

 after Silurian time. Eruptive activity had died out in all Newfoundland 

 by late Mississippian time but not in the Maritime Provinces and in the 

 eastern part of New England. Volcanism continued exceedingly active 

 there in places, and was accompanied and followed in the Carboniferous 

 basins of New England by intrusive activity. 



Batholiths 



The central zone of the geosyncline, along with tumultuous volcanic ac- 

 tivity, was the site of great batholithic intrusions. Where better known, as 

 in New Hampshire, four magma series are recognized, the first about of 

 Taconic age and the other three of Acadian, which there started in mid- 

 Devonian and lasted probably until early Mississippian. Of the three Aca- 

 dian magma series, the first preceded the major compressional orogeny, 

 the second was synorogenic, and the third followed the orogeny. 



As studies progress in the Maritime Provinces and in Newfoundland, it 

 is becoming clearer that most of the dioritic to granitic batholiths there 

 are Acadian also. The batholiths are not limited to the medial volcanic 

 zone of the geosyncline but some have intruded the inner, less volcanic 

 complement of geosynclinal sediments and others in great volume, 

 the outer zone, now mostly of Precambrian rocks. 



Metamorphism 



A striking character of the stratified rocks of the geosyncline of Greater 

 Acadia is their metamorphism. Where distant from the batholiths they 

 are generally slates, phyllites, argillites, quartzites, and metavolcanics. 

 Where close to the altering influence of the intrusions they are schistose 

 and gneissic. The very-low-grade and low-grade metamorphism is more 

 characteristic of the inner belt, and also the outer where Paleozoic sedi- 

 ments are preserved, as in the Conception Ray area of Newfoundland. 

 Medium-grade metamorphism is more characteristic of the central belt. 



