348 



A. C. LAAVSON ISOSTATIC COMPENSATION CAUSE OF THRUSTING 



ure, innumerable shear ruptures would be formed and the rock would be 

 caused to flow. This means that the boundary MN would be moved up 

 and that the prism CDXM would finally become thin enough to insure 

 the transmission of a stress to all points on CD sufficient to overcome 

 friction, when the lower block would move past the upper. 



Figure 7 



-Diagram illustrating Thrusting by a Current moving from Right to Left 

 below the Crust 



But if a deep viscous current be competent to cause displacement on 

 CD against friction, it is also competent, for reasons previously advanced, 

 to inaugurate and extend the rupture CD, and probably in this case the 

 rupture would be started in depth and extended toward the surface, as in 

 figure 8. With the initiation of the rupture, one of two things would 

 happen. Either the mobile wedge would be driven under the passive 



M 



Figure 8. 



-Diagram illustrating the Initiation of a curved Thrust, D. in Depth and its 

 Propagation toward the Surface. 



block, causing its uplift, or the point of the wedge would be driven down- 

 ward into the viscous current and be incorporated in the latter, also caus- 

 ing uplift beyond the intersection of the rupture with the top of the cur- 

 rent. Inasmuch as the rupture would be concave to the right, it seems 

 probable that the wedge would be forced down into the current, that a 

 rotary motion of the mobile block would develop, and that the condition 

 of transfer of mass would be analogous to those set forth in. case 2. 



