LOW-ANGLE FAULTING 
is not uniformly distributed, but is 
represented as somewhat stronger at 
the upper end than at the lower, as 
indicated by the relative length of the 
arrows. The fracture plane (0) in con- 
sequence of the rotation of the ellipse is 
inclined at approximately 40° to the 
force. In ellipse C, with stronger rota- 
tion, the active shear plane (6) has been 
lowered to an angle of 25°. In ellipse 
D, because of still stronger rotation, the 
fracture plane is only 15° from the hori- 
zontal, and in £, the limiting case of ex- 
treme rotational strain, the shearing 
plane has reached: horizontality. 
Applying these principles to the 
earth, horizontal thrusts may therefore 
theoretically produce shearing planes at 
any angle from 45° down to horizon- 
tality, depending upon the relative 
strength of the rotational element in the 
strain. Actual faulting, however, would 
probably not take place exactly in these 
planes of maximum shear, but would, 
as shown on pages 17-19, be modified 
somewhat further by the component of 
stress acting normal to the shearing 
plane. A marked rotational strain de- 
rived from horizontally directed stresses, 
if it can be shown that such are likely 
to be developed with sufficient frequency 
in earth dynamics, may form a working 
hypothesis to explain the great low- 
angle overthrusts. It therefore becomes 
necessary to seek the conditions which 
might result in the development of such 
strongly rotational strains. 
Ellipse A, representing the cross-section of a sphere deformed 
pure non-rotational strain, is the limiting case at one end. The planes of no distortion (fracture planes) are here inclined at 45°. 
‘llipse EH, representing extreme rotational strain, is the limiting case at the other end. The active shear plane (}) is here horizontal. 
Fic. 7.—Theoretical deformation of a sphere by compressive stress. 
by 
llipses B, C, and D represent cases intermediate between the two limits. 
al teal 
