344 A. C. LAWSON ISOSTATIC COMPENSATION CAUSE OF THRUSTING 



merge with it, then the mobile prism becomes successively AEFC, AGHC, 

 and so forth; and if a new rupture arise sufficiently near to C, CD may 

 be extended down ; but a limit to this extension will be reached when CD 

 becomes too deep to be intersected by the later steep ruptures IJ, and so 

 forth. 



It appears, therefore, that in the case of real overthrust, where the 

 lower is considered as the passive block and the upper as the mobile block, 

 there can be no large relative displacement on any single block, owing to 

 the fact that strain is relieved by a succession of limited ruptures and the 

 development of an imbricated structure. It follows, of course, that the 

 assumed lowly inclined rupture with which we started, if it exceed the 

 limit found for AB, could not have been formed by real overthrusting. 



In the diagrams, figures 3 and 4, the assumption is implied that the 

 secondary ruptures EF, GH, and so forth, are curved, with concavity 

 toward the origin of stress. The consideration underlying this assump- 

 tion may be briefly stated : Prior to the inauguration of the rupture EF 

 at the surface, the direction of stress is horizontal. If now this rupture 

 be initiated for a depth, say, of one mile from the surface, and be inclined 

 to the horizon, as engineering experience indicates it should be, then the 

 angle which the new rupture plane makes with the direction of stress 

 tends to be constant; but the moment that the first mile of rupture is 

 opened and the strain in the upper part of the prism is thereby relieved, 

 the direction of stress is changed from the direction indicated by the 

 arrow p to that of the arrow q, in figure 3 ; and, since the angle tends to 

 remain constant, the direction of EF, for the second mile of its extension, 

 will change by the same angular amount. Since both the extension of 

 EF and the change of direction of the stress are coincidentally pro- 

 gressive, EF will be curved and concave toward the origin of the stress. 



Underthrusting 



We may now consider the case in which, for an assumed rupture of 

 low dip in the earth's crust, the lower block is the mobile element and the 

 upper the passive. 



In the diagram, figure 5, let AB be the surface of the earth and CD 

 the rupture plane. By assumption the region below CD tends to move 

 toward the left by a stress from the right. In this case we may think of 

 the stress either (1) as applied to the cross-section of the lower block, 

 pushing it forward, or (2) as a gravitative stress, acting on every particle 

 within the block, or (3) as applied to the bottom of the block by a viscous 

 current. 



