BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 

 VOL. 33, PP. 337-352 JUNE 30, 1922 



ISOSTATIC COMPENSATION CONSIDERED AS A CAUSE OF 



THKUSTING 1 



BY ANDREW C. LAWSON 



{Presented in abstract before the Society December 29, 1921) 



CONTENTS 



Page 



The limits of length of overthrust blocks 337 



The limits of movement of overthrust blocks 341 



Underthrusting 344 



Compensatory flow and underthrusting 349 



The Limits of Length of overthrust Blocks 



If a rupture plane of low dip exist in the earth's crust emergent at the 

 surface, and if a stress exist in the overlying prism which tends to move 

 it toward the emergent side, the movement is restrained by the friction 

 of the surfaces in contact on the rupture plane. If, further, the stress 



Figure 1. — Diagrammatic Representation of a Thrust Block 

 AB, surface of the earth ; BC, thrust plane. 



be due to a pressure applied from without to the cross-section of the 

 prism, then, in order that the prism may move, and so be thrust over the 

 underlying rock-mass, the pressure must exceed the friction on the rup- 

 ture plane. If the stress be tangential in direction and uniformly dis- 



1 Manuscript received hy the Secretary of the Society November 14, 1921. 

 This paper is one of a series composing a symposium on isostasy. In the absence of 

 the author, it was presented by H. F. Reid. 



XXII— Bull. Geol. Soc. Am., Vol. 33, 1921 



(337) 



