584 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



| I FOLD AND THRUST 



I | BELT, MOSTLY POST- 



1 1 BATHOLITHiC 



Mountains are probably part of this system, and the Richardson Moun- 

 tains are also thought to be made up of folds with a northerly trend. The 

 age in part from map relations appears to be pre- or Early Cretaceous, but 

 elsewhere to be Late Cretaceous or Early Tertiary. As described in fol- 

 lowing paragraphs the age of one of the batholiths in the Selwyn Moun- 

 tains is Mid-Cretaceous. 



BELTIAN GEANTICLINE 



A geanticline of Reltian strata emerges in southeastern Rritish Columbia 

 from the broad region of Reltian rocks in northwestern Montana, and 

 extends northwestward in narrowing fashion almost to Yukon Territory. 

 The Rocky Mountain trench lies along its east side for the most part. In 

 northwestern Montana the deformed Reltian is on both sides of the 

 trench zone, and in fact, forms the entire Rocky Mountain system there. 



Rroad areas of strata, probably equivalent to the Relt series occur in 

 the Yukon, but since the whole region of outcrop is intruded by numer- 

 ous batholiths, it is regarded as part of the Nevadan orogenic belt and, 

 therefore, not shown as geanticlinal. The geanticline of Beltian strata 

 in eastern Rritish Columbia is probably one of Laramide orogeny. In 

 Chapter 6 the geanticline is postulated to have developed as early as 

 Cambrian time in the form of an arch separating the eugeosyncline from 

 the miogeosyncline, but the main rise, evidently, was incident to folding 

 and thrusting of the Laramide orogeny (see Chapter 20). The Beltian 

 geanticline is somewhat of a parallel to the South American geanticlines. 



BATHOLITHIC PROVINCE 



The batholiths of Rritish Columbia, Yukon Territory, and southeastern 

 Alaska are arranged in two divisions or belts. Those on the west are 

 described as follows by Lord et al. (1947): 



k* STOCKS AND SMALL BATHOLITHS 

 % OUTSIDE MAIN BATHOLITHIC BELT. 

 MID- AND LATE CRETACEOUS 



ACTIVE AND DORMANT VOLCANOES 



Fig. 37.1. Major tectonic and igneous units of the Canadian Cordillera. G, Mt. Garibaldi. 

 Numbers such as 102 m.y. are absolute ages in millions of years determined by the potas- 

 sium-argon method (Follinsbee er al., 1957). 



