BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 



Vol. 34. pp. 649-660 DECEMBER 30. 1923 



BOUDINAGE, AN UNUSUAL STEUCTURAL PHENOMENON^ 



BY TERENCE T. QUIRKE 



(Presented in abstract before tJie Society December 29, 1922) 



CONTENTS 



Page 



Parallel folds 649 



The boudins 650 



The quartz veins 651 



Elastic and non-elastic compression 653 



Elastic recovery of folded rocks 655 



Non-elastic recovery of folded rocks 655 



Formation of boudinage 656 



Peculiarities of boudinage 656 



Parallel Folds 



Common parallel folds with liorizontal axes die out upward and down- 

 ward at a distance which is theoretically infinite. However, the curva- 

 ture becomes almost zero at a distance about twenty times the radius of 

 perfect curves, at the center of the structure. Thus a semicircular 

 parallel fold 200 feet in diameter would die out at a depth of 2,000 feet. 

 A fold of the same type, to persist for a depth of 10 miles, w^ould require 

 a diameter of one mile at the surface. This theoretical structure involves 

 enormous shortening of the central line of the folds and a diminishing 

 shortening of the flanks, until at the place where it dies out th'3 shorten- 

 ing becomes zero (figure 1). Such a feature would be a remarkable case 

 of rotational strain 6i a type in nature unknown to the waiter. 



Certain folds which die out in depth are described by Van Hise." 

 None of the cases cited appears to be a case of parallel folds; uniformly, 

 the folds are in highly metamorphic rock characterized by similar fold- 

 ing. However, this structure is usually recognized as a probability and 

 is commonly cited as an explanation of how folds die out at great depths. 



^Manuscript received by tlie Secretary of the Society- December 30, 1022. 



- C. R. Van Hise : U. S. Geol. Siu-A^ey, 16th Annual Report, part 1, 1890, p. 601. 



XLIII- Bill. Geol. Soc. Am., Vol. :_]4. 1922 (649) 



