324 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



EXPLANATION 



Monzonite stocks 



Areas of outcrop of 

 Prichard forma- 

 tion, represent- 

 ing positions of 

 large upwarps 



Movement patterns 







10 MILES 



Accompanying a major reorientation of the stress system, the axes of the 

 folds began to bow (Fig. 21. 3B), the southern part of the region moved 

 relatively westward, and incipient strike-slip faults developed. The Mill Creek 

 and Deadman syncline was separated from the Granite Peak syncline and 

 wrapped around the truncated end of the Granite Peak syncline. The northern 

 flank of the Lookout-Boyd Mountain anticline was sliced off by one of the 

 antecedent fractures of the Osburn fault. 



Monzonite stocks intruded the structural knot thus produced (Fig. 21.3C), 

 and the principal period of ore deposition followed. Most of the veins are 

 included in spatial groups that define distinct linear belts trending slighdy 

 more northwesterly than the Osburn fault system. The concentration in such 

 belts of veins, which are subparallel but differ in size and orientation, suggests 

 that linear feeders for the mineralizing solutions existed at depth, although no 

 through-going structural elements reflect these feeders in the upper crust. 



After the principal period of ore deposition, strike-slip movement along the 

 ancestral Osburn zone of weakness became more through-going than previously, 

 and apparently deep-seated stresses were accommodated at this time by dis- 

 placement on relatively few faults, most of which were in or parallel to this 

 zone. The Osburn fault offset the major folds and early reverse faults, and 

 separated the northern segment of the ore-bearing area from that to the south. 

 The Thompson Pass fault also offset the major folds, and the Placer Creek fault 

 offset the Pine Creek anticline and vein system. The Dobson Pass fault came 

 into existence concurrently with the Osburn fault. The small stocks a few miles 

 west of the Dobson Pass fault may represent cupolas displaced from the main 

 part of the Gem stocks by dip slip on the Dobson Pass fault. 



Some of the early-formed tight folds and strike-slip faults were flexed as later 

 rotational stresses were accommodated along newly developed slip planes. Thus, 

 the east end of the Savenac syncline and the adjacent north branch of the 

 Osburn fault were sharply bent and later movement was "short-circuited" along 

 the south segment of the fault. Likewise, the Polaris fault may have accommo- 

 dated strike-slip deformation after the Placer Creek fault buckled. 



Late normal faults, some resulting from the final stages of strike-slip deforma- 

 tion, and others possibly of Quaternary age (Pardee, 1950), have affected the 

 area. 



The fault and fold pattern of the map of Fig. 21.2 suggests immediately 

 that the Idaho batholith has moved eastward as a rigid mass, and that 

 the thrusts along its east side are a direct compressional result. But this 

 idea seems incorrect when it is realized that the strike-slip movement on 

 the Osburn fault zone was in the wrong direction. 



Fig. 21.3. Stages in the development of the Osburn fault zone in the Coeur d'Alene district, 

 Idaho. Reproduced from Wallace ef a/., 1960. 



