288 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



southeastern Alaska is concerned, the writer is aware of no evidence to disprove 

 the assumption that all the Mesozoic intrusive rocks may be of Lower Cretace- 

 ous age, but the data given for adjacent territory suggest that they may be in 

 part of Upper Jurassic and in part of Lower Cretaceous age. 



Follinsbee et al. (1957) report a potassium-argon age of the Coast 

 Range batholith near Vancouver of 105 m.y. This would be at least mid- 

 way up in the Lower Cretaceous, and corresponds well with the strati- 

 graphic evidence above. 



Relation of Batholitlis to Folding. Regarding the relation of folding 

 and intrusion, Ruddington states: 



There is a most pronounced increase in the degree of crumpling, plication, 

 foliation, and isoclinal folding as the border of the batholith is approached from 

 the west, suggesting that the batholith has exerted a tremendous thrust. The 

 manner in which the batholith has peeled off great slabs of schist constitutes 

 further evidence. On the other hand, in the vicinity of the adjacent oudying 

 stocks, sintering and compacting of the phyllite and slate as the contact is 

 approached indicates that the cleavage and foliation are in part older than the 

 intrusion. 



The data are inadequate for a solution of the problem. But if we assume that 

 the intrusion of the batholith took place within the same general period as the 

 Jurassic or Cretaceous folding, then it is probable that at least two factors were 

 involved — an increased local intensity in the dynamic metamorphism above the 

 location of the rising magma and a thrust exerted by the magma itself during 

 its emplacement at horizons equivalent to those now exposed. Under the same 

 stress and with other conditions the same, rocks will be much more highly 

 deformed under higher temperature. Thus it might be that though stresses of 

 essentially similar orders of magnitude affected beds both far to the east and 

 far to the west of the present highly folded zones, the beds to the east and 

 west, relatively much cooler, yielded by close folding and development of cleav- 

 age, whereas those in the intensely folded zone, at a higher temperature due to 

 the rising magma with its advance wave of escaping highly heated vapors, 

 yielded far more extensively. A preliminary foliated or cleaved character had 

 thus already been induced before the arrival of the magma, which accentuated 

 the dynamic effects by its own thrusting pressure and aided recrystallization by 

 heat, vapors, and solutions. 



Another important factor appears to have been the structural relations which 

 the invaded formations bore to the magma. For example, where they were in 

 steeply dipping attitudes above the rising magma, conditions for penetration by 

 magmatic solutions and vapors were favorable and metamorphism was corre- 

 spondingly facilitated; such seems to have been the condition in the belt ad- 

 jacent to the southern part of the batholith in southeastern Alaska. Where the 

 contact plunges steeply the transfer of solutions and vapors was markedly 



Fig. 17.22. Belts of the Laramide orogeny in the Rocky Mountains and the folded Upper 

 Cretaceous trough of Oregon, Washington, and British Columbia. 



