322 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



intrusion it shouldered aside the adjacent surficial crust and formed the 

 Laramide structures. In the first case the discordant structures would 

 have to be due to later intrusions. 



Lewis and Clark "Line" 



About 30 miles north of the north end of the Idaho batholith is a zone 

 of large high-angle faults which trends slightly north of west. The chief 

 ones are called the Hope, Osburn, Rurnt Cabin, Placer Creek, and St. Joe, 

 and the whole zone referred to as the Lewis and Clark "fine" (Wallace 

 et ah, 1960). They dominate zones of complexly fractured rock and are 

 the chief localizers of ore in northern Idaho. 



The Hope is the most northerly of these great earth fractures. It has a more 

 northwesterly trend than the others, averaging N. 55°-60° W., and dips steeply 

 southwest. It closely parallels the lower course of the Clark Fork of the Colum- 

 bia River for about 65 miles, then extends through the north arm of Pend Oreille 

 Lake and through a notch across the Selkirk Mountains, giving it a total length 

 of not less than 95 miles. It has an impressive vertical component of movement 

 and stratigraphic throw, but the horizontal component is about 12 miles, the 

 northeast side having been displaced southeast relative to the southwest side. 

 Along the fault zone are many associated fault fractures — low-angle thrust, 

 high-angle reverse, high-angle normal, and two sets of strike-slip faults — all re- 

 lated to the Hope and resulting from the tensional and compressional com- 

 ponents of the horizontal shearing stresses which produce the Hope. The 

 faulting, intrusion, and mineralization are closely related events and are re- 

 garded as products of the Laramide orogeny. 



The Osburn fault of the Coeur d'Alene district is of even greater magnitude 

 than the Hope and has been mapped for 90 miles east-southeast of Coeur 

 d'Alene Lake. Its length is probably much greater, for its course approximately 

 coincides with an old valley extending from Spokane, Washington, to Deer 

 Lodge, Montana, a distance of 300 miles. Its course is N. 70°-80° W. and its 

 dip is steeply south. It also has many associated faults of variable magnitude, 

 some of which are mineralized. Igneous intrusion and mineralization in the 

 Coeur d'Alene district are largely localized along the course of the Osburn 

 fault (Anderson, 1948). 



Fig. 21.1. The relation of the Idaho batholith to the Nevadan and Laramide orogenic belts. The 

 Nevadan belt is white and the Laramide belt is dotted and lined. The bold lines in the Laramide 

 belt are axes of prominent folds, thrust faults, and major trends. The Nevadan and Laramide 

 belts overlap; in fact, the geosynclinal division of the Laramide belt was strongly deformed in 

 places in Early and Mid-Cretaceous time. 



