CONCERNING THE PROCESS OF THRUST FAULTING 437 



where friction is greatest, at the steepest part of the plane, affect- 

 ing both the footwall and the hanging wall. During early dis- 

 placement the fault will appear at the surface as a steeply dipping 

 plane; by the time displacement has continued for a mile, the 

 angle of the fault plane must have been worn flatter by the friction 

 of the moving load; finally, by the time displacement has attained 

 a few miles, the low-angled sole will have reached the surface, and 

 the fault plane must have been reduced to a low angle by abrasion, 

 leaving no trace of the steep parts of the original break. 



In this manner it is suggested that all great thrust faults which 

 are steep-angled near the surface, if persistent in depth, follow 

 the low-angle form at the depth of a few miles, and that in a general 

 case the angle of faulting at the surface may depend as much 

 upon the amount of displacement subsequent to rupture as upon 

 fundamentally different conditions in the application of earth 

 forces or in the character of the members affected. In support of 

 this it may be recalled that, so far as is known regarding thrust 

 faults, all faults of great displacement have low-angle fault planes, 

 and no faults of high angles exceed a few miles in displacement. 



WilHs^ has described rotated, high-angle thrust faults of rela- 

 tively small throw in the coast ranges and the Sierra Nevada, 

 which he considers to have some such shape as is here suggested 

 for thrust faults of small displacement; however, his explanation 

 of their character differs considerably from the general argument 

 of this paper. 



CONCLUSION 



In conclusion some repetitions seem pertinent. Certain parts 

 of the earth's crust which are deformed by regional compression 

 are held to be analogous to sheets and comparable in mechanical 

 analysis to long columns. The normal terrestrial stress is said to 

 be an unequally distributed thrust of rotational type. The trans- 

 mission of stress as a vector quantity supposedly extends from the 

 surface to a depth scarcely exceeding fifteen miles. There is a 

 zone of potential separation between the frangible crust and the 

 rigid interior which is nearly parallel to the surface. In some 

 cases thrust faults of very great displacement may be extensions 



' Bailey Willis, Geol. Soc. Amer. Bull., XXX (1919), 84-86. 



