ALASKA AND THE YUKON 



631 



ruin Bay Fault 



Along the northeast side of the Cook Inlet depression is the Bruin Bay 

 fault (D. J. Miller, 1959). Between it and the Mt. Logan fault is the 

 Hook Inlet Tertiary province, described on a previous page. 



faults of the Kuskokwim Region 



The Iditarod-Nixon fault, the Holitna fault, and the Farewell fault 

 lave been described on previous pages and are part of the great fault 

 'ystem. 



Mature and Age of Faults 



The faults of southwestern Alaska are undoubtedly high-angle faults, 

 Imd Cady et al. apparently sees no evidence of horizontal movement on 

 'he faults. An old erosion surface has been arched and faulted, and certain 

 ;treams are regarded as antecedent to the vertical uplifts. 



The Bruin fault is a major high-angle thrust that dips to the north- 

 vest (D. J. Miller, 1959). The Chugach-St. Elias fault is also a thrust 

 ivhich dips northward 30° to 60° and is estimated to have a throw of 

 1.0,000 feet. The 1958 movement on the Fairweather fault at the head of 

 Lituya Bay produced scarps in which the vertical displacement was 3/2 

 ieet, and the horizontal movement 21/2 feet. The southwestern down- 

 thrown block moved to the northwest in the horizontal movement (D. J. 

 (tiller, 1959). 



| In general the benches suggest tension, but their straightness or 

 l.moothly curved nature suggests, if not demands, the interpretation of 

 'major horizontal displacement. The Farewell-Shakwak fault seems to 

 'have cut the Alaska Bange and displaced the south side westward (Mt. 

 McKinley block) and the north side eastward, leaving the opposite facing 

 'escarpments in nice alignment. These topographic features remind one 

 bf features of the San Andreas fault. 



The pronounced and fresh-looking topographic features can only mean 

 jthat major movements on the faults have occurred in Late Cenozoic time. 

 If the faults are like the San Andreas system, then we might suspect a 

 beginning of displacement as far back as the Cretaceous. 



The Farewell fault has been connected with the Shakwak trench fault 



Fig. 39.17. Absolute dates from granites in the orogenic belts around the Arctic. Map kindly 

 supplied by R. E. Folinsbee. See Baadsgaard, Folinsbee and Lipson, 1960. 



zone by St. Amand (1957) and the entire system, including the projection 

 down the Lynn Canal, called the Denali fault, for Mt. Denali, the Indian 

 name for Mt. McKinley. He extended the fault down the Alexander 

 Archipelago, along the outer side of the Queen Charlotte Islands, and 

 connected it with the San Andreas fault off Cape Mendocino. Pursuant 

 to the thesis that the Pacific block is in counterclockwise rotation, as 



