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STRUCTURAL GEOLOGY OF NORTH AMERICA 



tinues southeastward through the Island Ranges of British Columbia. The 

 isoclinal folds, the prevailing northwestern strikes, and the steep north- 

 eastern dips are similar. The belt of metamorphic rocks that flanks the 

 western margin of the batholith seems to continue a considerable distance 

 southward into British Columbia. The synclinoria and anticlinoria do not 

 seem to have been worked out, but perhaps the variety or number of such 

 features is not exposed or not existent. 



The Keku-Gravina synclinorium in Mesozoic rocks extends southeast- 

 ward across the international border to Pitt Island, but seems to end in 

 the great batholith before reaching Douglas Channel. The Dall-Long 

 Island and Dolomi-Sulzer anticlinoria in the Paleozoic strata either die 

 out southeastward or are covered by the waters of Hecate Strait, because 

 on the west is a wide Mesozoic belt of Queen Charlotte Islands and an- 

 couver Island. Lacking information, it can only be assumed that this belt 

 is a broad synclinorium. It seems to correlate with the Sitka Mesozoic belt 

 200 miles to the northwest, but if so it must bulge westward around the 

 Paleozoic anticlinoria of Dall and Prince of Wales islands. 



There seems to be plenty of room for an anticlinorium and another 

 synclinorium under Hecate Strait and Queen Charlotte Sound. 



The Paleozoic belt striking nearly east-west on the southern end of 

 Vancouver Island may mark another anticlinorium outside the Queen 

 Charlotte- Vancouver Mesozoic belt. 



Concordant Fracture System. The pattern of the great batholiths of 

 the Upper Jurassic and Lower Cretaceous of western Mexico, the United 

 States, Canada, and southeastern Alaska remind Peacock ( 1935) of the 

 arc-and-cusp plan of the circum-Pacific orogenic belts. This feature has 

 already been referred to. Since the batholithic rocks show little evidence 

 of deformation, the curved plan appears to have originated during the 

 emplacement of the igneous rocks and during the preceding orogenic 

 events. The grain of the coastland, in British Columbia and southeastern 

 Alaska, as defined by the folds and foliation, is longitudinal to the arcs, 

 and therefore is intimately associated with the arcs in origin. Deformation 

 during Cenozoic time has had little effect on the Mesozoic pattern. 



Peacock (1935) recognizes the fiords and straight stretches of coastline 

 to be the result of erosion controlled by a fracture system composed of 

 two elements, viz., a concordant one and a discordant one in relation to 



the arc. The concordant system is composed of fractures parallel to the 

 grain and normal to it, and the discordant of a north-south and east-west 

 system. See Fig. 17.21 and compare with Fig. 17.18. The first was formed 

 shortly after the solidification of the batholith; the second at the close of 

 the Cretaceous. Dikes and mineralized veins follow the transverse frac- 

 tures of the concordant system. Because of the fissure type of vein fill, the 

 transverse fractures appear to be of tensional origin (Balk, 1937) and 

 associated with the batholith. The main faults thus far recognized along 

 which the fiords have been eroded are the Lynn Canal and Chatham 

 Strait, but these apparently belong to the younger discordant system. 



Peacock's (1935) analysis of the mechanics of the great fracture system 

 is as follows: 



If the coastiand be regarded as a tabular body of rigid material undergoing 

 deformation by dominating horizontal forces acting from the northeast, causing 

 differential horizontal displacement toward the southwest, with the develop- 

 ment of an arc bent away from the dominant pressure, then tensile stresses, as in 

 a bent beam, would develop in the advanced part of the arc. These stresses 

 would be relieved by tension fractures running normal to the directions of 

 maximum tensile stress and, therefore, transversely to the grain or radially to 

 the arc. It is not to be expected that such tension fractures should follow 

 strictly radial directions. Although generally transverse, such fractures might 

 deviate considerably from directly transverse courses, because of irregularities 

 in the mechanical strength of the region; they might also change abruptiy from 

 the transverse to the longitudinal direction, the weak direction of the grain, and 

 thus develop a cranked course with rectangular elbows. 



With the mode of deformation suggested, shearing stresses would also be set 

 up along vertical planes parallel to the grain, and these would be relieved by 

 longitudinal shear fractures of the shear type. 



If formed in the manner oudined, the transverse fractures would be open 

 fractures, and when mineralized they would appear as fissure veins. The longi- 

 tudinal fractures would be closed fractures along which some horizontal differ- 

 ential movement would occur to relieve the shearing stresses. Mineralization of 

 such ruptures would result in mineralized shear-zones such as Schofield has 

 found usually to lie in the longitudinal direction. Both sets of fractures would 

 provide ready-made planes of faulting when subsequent crustal unrest affected 

 the region and caused differential movement between the already separated 

 blocks. 



It is also possible that thrust faults would form. If relief or elongation is 

 easiest in the vertical direction, then shear planes would form which strike 

 longitudinally and have dip-slip movement. Buddington mentions shear 

 zones or faults with strikes of N 38° W. to N 60° W. 



