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Terminology for structural discontinuities

¹ Geomechanics-Rock Fracture Group, Department of Geological Sciences and Engineering/172, University of Nevada, Reno Nevada 89557; schultz{at}mines.unr.edu
² Centre for Integrated Petroleum Research/Department of Earth Science, University of Bergen, Allégaten 41, N-5007 Bergen, Norway; Haakon.Fossen{at}geo.uib.no

Richard Schultz received his B.A. degree in geology from Rutgers University (1979), his M.S. degree in geology from Arizona State University (1982), and his Ph.D. in geomechanics from Purdue University (1987). He has worked at the Lunar and Planetary Institute, NASA centers, and in precious metals exploration before joining University of Nevada, Reno in 1990. His interests include rock fracture mechanics, growth and statistics of fracture and band populations, and planetary structural geology.

Haakon Fossen received his Candidatus Scientiarum (M.S. degree equivalent) degree from the University of Bergen (1986) and his Ph.D. in structural geology from the University of Minnesota (1992). He joined Statoil in 1986 and, since 1996, has been a professor in structural geology at the University of Bergen. His scientific interests cover the evolution and collapse of mountain ranges, the structure of rift basins, and petroleum-related deformation structures at various scales.

ABSTRACT

Strain localization structures such as fractures, stylolites, and deformation bands have important effects on reservoir performance but lack a consistent terminology. Advances in the recognition and interpretation of such structures now motivate a comprehensive framework that stresses their similarities instead of their differences. We review and assess the classical terms for localized geologic structures, followed by a comprehensive nomenclature that accounts for joints, faults, fractures, anticracks, shear zones, and deformation bands in compact and high-porosity rocks. Geologic structural discontinuities are defined by their lengths and by the sense and rate of displacement change across them. For example, structural discontinuities having negligible thickness, and consequently, a discontinuous displacement across them, are called sharp discontinuities. Depending on the sense of displacement (opening, shearing, or closing), these structures are called cracks, faults, or anticracks. However, structural discontinuities having a measurable thickness in outcrop or hand specimen and a continuous change in displacement across them are called tabular discontinuities. Correspondingly, these types of deformation bands are called dilation bands, shear bands, or compaction bands. The class of structural discontinuity, i.e., sharp or tabular, depends on the properties of the deforming rock. Consistent characteristics and patterns of these structural discontinuities, and their displacement-length scaling relations, demonstrate the rich yet consistent varieties of strain localization that are manifested in crustal rocks in general, and reservoir rocks in particular.