392 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



scale: in miles 



Fig. 25.3. Upper section generalized across the Front Range of Colorado and three of the back 

 ranges, from Boulder to State Bridge. pC, Precambrian undifferentiated; Cf, Fountain formation; 

 CI, Lyons formation; f Lylcins formation; lis, State Bridge siltstone; Cm, maroon formation; Jm, 

 Morrison formation; Kd, Dakota sandstone; Kp, Benton, Niobrara, and Pierre formations; Kl, 

 Laramie formation; Kde, Denver formation; Kmv, Mesa Verde formation; Ti, Tertiary intrusives; 



The period of overthrusting was followed by northeasterly and east-north- 

 easterly faulting on a large scale throughout the mineral belt during and after 

 the intrusion of the porphyritic rocks that dot it. Many of the mineral deposits 

 are localized at the intersection of easterly and northeasterly faults with the 

 earlier persistent northwesterly faults where they cross the mineral belt. 



Faults formed after the Laramide revolution are comparatively local and 

 largely confined to Miocene volcanic areas and Tertiary basins close to the 

 mountain front. 



A group of northeast-trending faults is confined mostly to the western 

 part of the range and seems to mark the western limit of the mineral belt. 

 They appear to be steep and are marked by gougy shear zones 10 to 600 

 feet wide. The largest of the group is the Moffat Tunnel fault (Lovering 

 and Goddard, 1938a), which is intermittently exposed on the surface for 



Tnp, North Park fm.; Tmv, Miocene volcanics. Section A of Fig. 25.1. 



Lower section is a detail of the Front Range near Montezuma, after Lovering, 1935. Ais, Idaho 

 Springs formation; As, Swandyke hornblende gneiss; Asp, Silver Plume granite; Cm, maroon 

 formation; Jm, Morrison formation; Kd, Dakota sandstone, Kb, Benton shale; Kn, Niobrara forma- 

 tion; Kp, Pierre shale; Tqm, Tertiary intrusives. Section B of Fig. 25.1. 



a distance of more than 25 miles and forms a wide zone of "heavy 

 ground" 1000 feet wide in the Moffat Tunnel, 2000 feet below the out- 

 crop. It passes through Berthoud Pass, where the zone of fractured rock 

 is 200 feet wide, and through Loveland Pass, where it is less than 50 feet 

 wide. The badly broken rock is apparently responsible for these depres- 

 sions. 



The Rocky Mountains through central Colorado have generally been 

 regarded as a belt of horizontal compression, but they can also be in- 

 terpreted as a group of closely packed uplifts with the Front Range 

 longer and wider than any of the others in the entire province. The an- 

 cestral Colorado Range was an uplift almost as large as the entire cluster 

 of ranges in the central Colorado belt, but the later Laramide uplifts de- 



