THE MECHANICS OF GEOLOGIC STRUCTURES 



515 



sheet thus deformed develops characteristic overthrust faults of 

 the type shown in Figure 3, which are most numerous along the 

 center line and decrease in abundance and displacement toward 

 the margin. 



We have in these two types of deformation limiting cases, 

 neither of which is probably realized under natural conditions. In 

 the first case there is a net increase in area; in the second case a 

 decrease in area. We may now consider an intermediate case in 

 which the total area remains constant. This in terms of the 



Fig. 8. — ^lUustrating changes in area due to torsional warping of a rectangular 

 surface. In A the line 0-P on the axis of torsion has remained constant in length 

 resulting in a net increase in area from A BCD to A'OB'C'PD' . In B the lateral 

 edges have remained constant in length and the result of warping is a decrease in 

 area from ABCD to AO'BCP'D. In C the area has remained constant during warping 

 resulting in elongation of the margins, shortening along the axis, and no change along 

 the neutral lines E-N and F-M. 



rubber sheet would result in tensional stresses along the free 

 margins and compressional stresses along the center line with a 

 neutral zone of neither compression nor tension on either side of 

 the center. These three cases are illustrated in Figure 8. In 

 each case the rectangular area represents the outline of an unde- 

 formed surface. The area with curved ends represents the surface 



