Structural analysis of features on natural and artificial faults AND In situ stresses
Authors: Norris D K , Barron K , Bielenstein H V 
Published: July 2017

Many beds in the coal measures of the eastern Cordillera have been detached from one another because of buckling and of differential translation on thrust surfaces. Striae, steps and polish on bedding faults were compared with similar features produced in the
laboratory to evaluate their usefulness in kinematic analysis. Pitches of striae on the same bed at various 'localities in Upper Marsh seam in the eastern Cordillera approximate a circular normal distribution with a preferred direction of slip essentially perpendicular to the regional strike. This
distribution includes a very large number of discrete directions of slip representing some phase of the kinematic activity on bedding. Statistical tests provide reasonable assurance that samples of pitch of striae from one and more slip surfaces are from the same movement picture. Steps on bedding
and on extension faults are of two basic types: those of mineralized gouge plastered on the slip surface (accretion) and those cut into the solid rock (fracture). Both types may be linear or arcuate in projection on the slip surface; they commonly trend about at right angles to the preferred
direction of slip and they may be modified or erased by later movement. Accretion steps adjacent to Upper Marsh seam are usually localized at irregularities in surface configuration. In any specimen or locality they characteristically occur on the same flank of depressions. Microscopically they
consist of a complex lamination of quartz and carbonate with carbonaceous gouge. Layers of gouge feathered out in the quartz and carbonate, sigmoidal tension gashes, tension fissures filled with mineralization, and fragments of steps plucked from the risers indicate that these steps commonly face in
the preferred direction of slip of the opposed wall. The steps are, therefore, plastered in the lee of irregularities in the rock surface. The fine structure, however, suggests modification of some steps of slip in a reversed direction consistent with observations of some faults which have
themselves been cut and displaced in the coal seam. Fracture steps may originate in at least three ways in and adjacent to Upper Marsh seam: by inheritance from the initial fracture configuration of the slip surface, by plucking of platy fragments, and by faulting of the slip surface. - 137 - Polish
on and within beds is widespread adjacent to Upper Marsh seam. It may be due to relatively large shear displacement over considerable area on individual beds or to microscopic shear on a multitude of slip surfaces of limited area within beds. Steps and striae were created in the laboratory on
prepared and induced slip surfaces in cylindrical specimens of soapstone, mudstone and siltstone loaded axially under confining pressures up to 9,000 psi. The steps were similar to the two basic types observed in the field. Both were oriented more or less perpendicular to the direction of the
striae. Risers on accretion steps were observed to face in opposing directions depending largely on whether the fragments of compressed gouge stuck to one side or other of the specimen when the slip surfaces were separated for viewing, The directions in which the risers faced, therefore, were not
consistent with the known sense of slip. Prominent fracture steps were observed to be due to faulting of the principal slip surface on the conjugate shear set. They consequently opposed motion on the principal surface. Both field and laboratory data, therefore, indicate that accretion steps are
formed as slip surfaces are parted, and depending on which wall retains the gouge, the steps will face in the one direction or other. Because the gouge is more readily bonde to the lee flank of irregularities, however, it commonly sticks there so that risers face preferentially in the direction of
motion of the opposed block. Fracture steps on the other hand can face in either direction and therefore cannot be used indiscriminately as a definitive criterion for sense of slip. Where their nature and origin are understood, however, both kinds of steps may be used with meaningful samples of
pitch measurements to establish directions of slip and therefore to document at least part of the kinematic history of faulting and folding in orogenesis.