LIMITS OF MOVEMENT OF OVERTHRUST BLOCKS 343 



impossible, a priori, that a single intact prism of the earth's crust could 

 move more than a small fraction of a mile by real overthrusting as a 

 mobile block past a passive underlying block. 



In the diagram, figure 3, let AB be the surface of the earth and CD 

 an assumed rupture plane on which the prism ABCD tends to move to 

 the right by a stress from the left. Let EF be the new rupture induced 

 by a stress less than the frictional resistance on CD, but greater than the 

 strength of rocks at the surface. The rupture EF will presumably start 

 at the surface, where the strength is least, and be propagated downward. 

 There will be relative displacement on EF, but this will be limited to 

 elastic rebound and therefore will be small in amount. At first EF may 

 be small, but if the stress be persistent the strain will, after the first 

 relief, again accumulate to an excess of the strength of the rock and the 

 rupture will be extended. Finally, by repetition of this process, the rup- 



c 



Figure 4. — Diagrammatic Representation of a Thrust Block 



AB, surface of the earth ; CD, thrust plane; EF, GH, IJ, successive thrusts clue to failure 



of thrust block. 



ture EF will reach a limit, determined by the angle a, at which friction 

 on this plane will also inhibit movement, and no farther extension of the 

 crack can take place. Further accumulation of strain can then only be 

 relieved by another rupture, as at GH, and so on. 



Here two cases are possible as to the relation of the successive ruptures 

 EF, GH, and so forth, to the original fracture CD : In accordance with 

 engineering experience, these ruptures will make a high angle with the 

 direction of stress — that is, with the horizontal. The horizontal distance 

 between E and F, and therefore EF itself, is a function of the angle AEF, 

 becoming less as the angle is larger. Before reaching the limit of its 

 possible length EF (and similarly GH, and so forth), it may encounter 

 and merge with CD, as in figure 4 ; or, if CD be too far below the surface, 

 EF, GH, and so forth, will terminate before reaching it, as in figure 3 ; 

 but in neither case can there be large movement on the ruptures EF, GH, 

 and so forth. It is to be noted, however, that, in the case where CD is 

 limited in length and is shallow, so that the new highly inclined ruptures 



