318 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Fig. 20.15. Sketch of east face of Rocky Mountain trench at latitude 49°08'. Made from 

 photograph by Leech, 1959. 



been an active feature since the oldest Cordilleran disturbances. From latitude 

 50° to 51° 30' or beyond, several longitudinal faults pass into the trench at small 

 angles. Westerly dipping thrust faults cut the rocks of the Dogtooth Mountains 

 into slices, and such a fault or fault zone is assumed to underlie the floor of 

 the trench for many miles. Easterly dipping faults east of this part of the 

 trench have been interpreted as underthrusts. Some long straight steep faults, 

 such as the Redwall, may be dominandy strike-slip faults. North of Finlay Forks, 

 rocks of the Sifton formation that floor the trench have been tilted and cut into 

 long narrow slices by closely spaced faults that strike parallel with the trench. 

 The fault slices transect, at a small angle, the strike of the Sifton strata and 

 that of structures immediately east of the trench. 



The long trench has been little studied until recently when a report on 

 its nature immediately north and south of the international border 

 has appeared (Leech, 1959). Previously it had been postulated to be the 

 result of erosion following Cretaceous thrusting and folding (section B-B', 

 Fig. 20.3 ) , or to be due to normal downfaulting, either of late Laramide 

 or late Cenozoic age (Section K-K', Fig. 20.3). In the first edition of this 

 book it was postulated to be due to late Cenozoic graben-type faulting or 

 rifting, and part of a great belt that extends from southwestern Utah 

 to the Yukon. 



Figure 20.14 is a map reproduced from Leech (1959) which shows 

 the complex Laramide and Nevadan ( ? ) structures on either side of the 

 trench. The following points are mostly by Leech: 



1. The trench in the Cranbrook area is particularly sinuous in contrast to its 

 linear extent farther north. 



2. It is asymmetrical, with the east flank high and of youthful fault-scarp 

 topography (Fig. 20.15). 



3. It contains outcrops of Paleozoic and Belt strata on its floor and does not 

 appear to be as heavily alluviated as are some of the trenches further south 

 in the United States. 



4. It is probably of block fault origin but bounding normal faults are not 

 everywhere apparent, especially in the sinuous section. 



5. The postulated bounding normal faults are commonly disposed acutely to 

 the older thrust faults. 



6. Since the same formations appear on either side of the trench in this 

 southern region, it is evident that here the rift is not an exact boundary be- 

 tween the Nevadan orogenic province on the west and the Laramide on the 

 east. 



In the Yukon. The trench loses its identity north of latitude 59°. 

 The division between Nevadan and Laramide provinces also is difficult to 

 identify, but probably swings northerly to he east of the Selwyn Moun- 

 tains. 



About 100 miles northwest of Watson Lake on the Yukon-British Co- 

 lumbia border a remarkably strait valley, the Tintina, extends for about 

 400 miles northwesterly to the Alaskan border. See Fig. 39.1. Although 

 not connected with the Rocky Mountain Trench it is in alignment with it, 

 and a Tertiary filled valley at Watson Lake helps bridge the gap. Ac- 

 cording to Bostock et al. (1957); 



Major faults mark the course of the valley near Ross River Post and in the 

 Glenlyon and southwest Mayo area, and major geological boundaries coincide 

 with it in other places. Early Tertiary beds, only gently warped, outcrop at 

 intervals along the valley floor, proving its early development as a physio- 

 graphic feature. 



Shakwak Valley, another long straight lineament, extends from the Alaska 

 boundary southeast through Kluane Lake almost to latitude 60 degrees. Through 

 most of its length it forms a major geological boundary and is believed to mark 

 a great fault zone. Evidence of recent movement is found in unconsolidated 

 deposits along the valley floor. Southwest of Shakwak Valley in the Kluane area, 

 a zone of overthrust faults is believed to form, with the Shakwak Valley fault, 

 a graben structure enclosing upper Paleozoic to Tertiary rocks. 



