NOTES ON THE MECHANICS OF GEOLOGIC 

 STRUCTURES 



WARREN J. MEAD 



Structural Geology Laboratory, University of Wisconsin 



INTRODUCTION 



Since an early date in the development of the science of geology 

 it has been recognized that secondary structural features are the 

 results of failure or yielding of rocks under deformative forces, and 

 students of geology have attempted to interpret these secondary 

 features in terms of the forces and movements which produced 

 them. Because of the great size and heterogeneity of the earth 

 masses involved, the analysis of the casual mechanics of a given 

 major structural feature is never a simple matter. The geologist, 

 not having witnessed the production of the structural features at 

 hand, or of any similar features, finds it difhcult to view the prob- 

 lem in perspective and in proper relationship to associated structural 

 features. 



Discussions of the mechanics of deformation in geologic litera- 

 ture on the whole indicate a rather elementary conception of the 

 factors involved and a tendency to assume more or less arbitrarily 

 a simple set of mechanical conditions, when the structures observed 

 may be susceptible of several alternative explanations. A single 

 structural feature or group of similar features is not necessarily 

 indicative of the type of deformation involved. A group of inter- 

 secting faults may be looked upon as a sequence of unrelated 

 events, when, with equal or better reason, they might be con- 

 sidered as essentially simultaneous and due to a single deformation. 

 The two interpretations require a widely differing structural 

 history of the region involved. 



An open fissure obviously due to tensional stresses (so far as 

 the fissure itself is concerned) may be an incident in simple elonga- 

 tion, shear, cross-bending, compression or shortening, or torsional 



SOS 



