IDAHO BATHOLITH AND THE OSBURN FAULT ZONE 





According to Wallace et al. ( 1960 ) pronounced strike slip is indicated 

 by the following features: 



(a) the offset of large upwarped blocks more or less delineated by areas of 

 outcrop of the Prichard formation, the oldest unit of the Belt series; (b) the 

 offset of major folds and faults, and the dissimilarity of structural features adja- 

 cent to one another on opposite sides of the fault; (c) large-scale drag features; 

 (d) offset of the same sense along parallel or subparallel faults; and (e) the 

 position of major mining areas on opposite sides of the Osburn fault and the 

 pattern of ore and gangue-mineral distribution within the areas. A maximum 

 of about 16 miles of right-lateral strike slip is indicated on the segment of the 

 Osburn fault east of the Dobson Pass fault and about 12 miles displacement in 

 the same sense is indicated west of the Dobson Pass fault. The difference in 

 displacement on these two segments is believed to be principally the result of 

 contemporaneous dip slip on the Dobson Pass fault, which has effectively 

 lengthened the block north of the Osburn fault relative to the block south. 

 A few miles east of the area shown in Fig. 21.3, in the vicinity of Superior, 

 Mont., the cumulative lateral movement in the Osburn and the related Boyd 

 Mountain fault, as shown by stratigraphic displacement, appears to be ap- 

 proximately 16 miles, which strongly corroborates the suggested displacement 

 on the Osburn fault in the Coeur d'Alene district. 



The age of the Osburn fault is known only within broad limits. It cuts 

 rocks of the Belt Series of Precambrian age and is capped by flows of Colum- 

 bia River basalt of middle Miocene age. The probably contemporaneous Dob- 

 son Pass fault cuts the Gem stocks, which have been dated as about 100 million 

 years old. Other geologic evidence indicates that a lineament in the general 

 position of the Lewis and Clark line may have been in existence since early 

 Precambrian time. 



Ages obtained from uraninite from the Sunshine mine indicate that uranium 

 mineralization occurred about 1,250 million years ago. Thus tight folds, such 

 as the Big Creek anticline (Fig. 21.3), that are cut by the uraninite veins, 

 must have been developed before that time. In contrast, the principal ore- 

 bearing veins are younger than the Gem stocks of about 100-million-year age. 



The same authors outline the history of development of the structural 

 complex as follows: 



During an early stage of deformation (Fig. 21. 3A), the principal folds were 

 developed and overturned to the northeast, and reverse faults that strike north- 

 west and dip southwest were formed. A large domelike structure, the Moon 

 Creek-Pine Creek upwarp, was formed west of the reverse faults. 







( Fig. 21.2. Detail of the belt of Laramide orogeny and the Idaho batholith. Both major fold 

 f axes and thrust faults of the Laramide orogeny shown by lines. The main batholith is stippled, 



and the plutons of known Laramide age are black. Compare with Tectonic Map of the United 



Sfates, 1945. 



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