ALASKA AND THE YUKON 



629 



demonstrated by the presence of raised beaches along the Arctic Coastal Plain 

 and in the area west and southwest of the Mackenzie Delta. 



Potassium-Argon Dates of Intrusives 



The ages of several intrusives in the Yukon and the District of 

 Mackenzie have been determined by Baagsgaard, Folinsbee, and Lipson 

 (1961). The oldest date, 353 m.y., indicates an Acadian age. Two dates 

 of 220 and 240 m.y. suggest late Paleozoic magmatic activity. Several 

 dates ranging from 94 to 101 m.y. indicate intrusive activity in Mid- 

 Cretaceous time or during the Nevadan orogeny. Figure 39.16 shows the 

 position of the above intrusions and Fig. 39.17 shows the relation of 

 Paleozoic orogenic belts and dated intrusions around the Arctic in 

 Eurasia, Greenland, and northern Canada. 



CENOZOIC TRENCHES AND FAULTS 



Topographic Expression 



The new Army Map Service Relief Quadrangles of Alaska, the Yukon, 

 and northern British Columbia show strikingly five major linear 

 topographic trends. Several smaller ones are also apparent. These linear 

 features consist in part of trenches and in part of bold mountain escarp- 

 ments, but the continuity of one with the other cannot be doubted. The 

 Rocky Mountain, Tintina, and Shakwak trenches mentioned in Chapter 

 37 are especially clear on the maps. Some have been partially described 



i in the literature and mentioned on previous pages of this chapter. The 

 major alignments are so striking and the geomorphic provinces on either 

 side in places so distinct that the writer is prone to consider them major, 



: if not the most important, structural features of central and southern 

 Alaska and adjacent regions. They are emphasized by bold lines on the 



1 map of Fig. 39.2. 



A Tintina-Rocky Mountain Trench Fault Zone 



i It is fairly evident that the Rocky Mountain trench projects to the 



; Tintina trench, and thence to the south side of the Yukon Flats in 



n 



1 Fig. 39.15. Structures of the southern Richardson Mountains. Reproduced from Jeletzky, 1961. 



LEGEND 

 MESOZOIC 



Cretaceous and Jurat*, c 

 I Jurassic only m 

 northwestern corner 



-est ol Peel River 

 and Husky Channel and 

 north ol VitVekwa River) 



PAL/EOZOIC 



Palaeozoic tCambr>an to 

 Perm.an west ol Peel 



River and Husk, 

 mostly or entirely 

 Devonian east the f efromt 



Principal Strike ■ S 3« 

 {?) normal faults (known 



and assumed!, arrows 

 indicate inferred or 

 assumed direction of 



horizontal movement 



Principal thrust faults 

 Iknown and assumed! 



Major anticline f celled. 

 approximate, arrow 

 indicates plunge) 



— i 



Ma/Of synd^ne (defined, 

 approximate, arrow 

 indicates plungef 



Ma/or dome 

 Piercement structure 



Boundary between stable 

 and tectonicatly active 

 regions (approximate 

 assumed f 



G S C 



