Fault and fault structures

Article By:

Davis, George H. Department of Geological Sciences, The University of Arizona, Tucson, Arizona.

Last reviewed:November 2019

DOI:https://doi.org/10.1036/1097-8542.251600

Products of fracturing and differential movements along fractures in continental and oceanic crustal rocks. Faults range in length and magnitude of displacement from small structures visible in hand specimens, displaying offsets of a centimeter (1 cm = 0.4 in.) or less, to long, continuous crustal breaks, extending hundreds of kilometers (1 km = 0.6 mi) in length and accommodating displacements of tens or hundreds of kilometers. Faults exist in deformed rocks at the microscopic scale, but these are generally ignored or go unrecognized in most geological studies. Alternatively, where microfaults systematically pervade rock bodies as sets of very closely spaced subparallel, planar fractures, they are recognized and interpreted as a type of cleavage which permitted flow of the rock body. Fractures along which there is no visible displacement are known as joints. These include shear joints, formed by fracturing and imperceptible movement of the walls of the fractures parallel to fracture surfaces, and tension joints, formed by negligible or barely visible displacement of the walls of the fractures perpendicular to the fracture surfaces. Large fractures which have accommodated major dilational openings (a meter or more) perpendicular to the fracture surfaces are known as fissures. Formation of fissures is restricted to near-surface conditions, for example, in areas of crustal stretching of subsidence. When faulting takes place under conditions of high temperature or pressure, zones of penetrative shear flow may develop which are best described as ductile fault zones.

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