163 REID ET AL. REPORT OX NOMENCLATURE OF FAULTS 



As we pass from, one part of a fault to another, we find that certain 

 characteristics vary. Tor instance, the two walls of the fault may be, in 

 some places, in contact; in others separated; one wall may be elevated 

 relatively to the other in one part of the fault's course and depressed in 

 another part, etcetera. Definitions descriptive of characteristics which 

 vary locally must, therefore, be considered as referring to the parte of the 

 fault to which they are applied and not necessarily to the fault as a whole. 

 A closed fault is one in which the two walls of a fault are in contact. 

 An open fault is one in which the two walls of a fault are separated. 

 The same fault may be closed in one part, open in another. 



The fault space is the space between the walls of an open fault. 

 A fault surface is the surface of fracture; it is rarely plane, but where 

 it is without notable curvature over any area it may be called a fault 

 plane for that area. The word "surface" should not be restricted to its 

 accurate mathematical meaning. The shear zone, if sufficiently thin, 

 would in general be referred to as the fault sur- 

 face; but if it is wide and has definite bound- 

 aries, these may constitute two fault surfaces. 



A fault line is the intersection of a fault sur- 

 face with the earth's surface. The same word 

 might also be used to denote the intersection of 

 the fault surface with any artificial surface of 

 reference, such as the floor of a tunnel, mine 

 level, etcetera. 



Figure l.-A Horse. The shmr ZQne . ^ llere a fault ig made up of 



slips on closely spaced surfaces, with more or less deformation of the 

 intervening rock, the portion of the earth's crust containing these minor 

 faults is called the shear zone. This name would also apply to the brec- 

 ciated zone which characterizes some faults. 



Multiple fault: This term may be used to designate a group of parallel 

 faults fairly near together, the intervening rock not being distorted. The 

 term shear zone would hardly be applicable to this case. 



An auxiliary fault is a minor fault ending against the main fault. It 

 is often the boundary of a dropped wedge. 



The outcrop of the fault is the area along which the shear zone comes 

 to the earth's surface. In the case of a sharply defined fault the outcrop 

 and the fault line become synonymous. 



The fault breccia is the breccia which is frequently found in the shear 

 zone, more especially in the case of thrust faults. 



Gouge is the pulverized rock, generally claylike when moist, which is 

 often found between the walls of a fault. 



