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Deformation-band networks at Buckskin Gulch, Utah, and the Big Hole fault, Utah, both formed in the Navajo Sandstone with similar initial porosity and permeability, at similar burial depths, and result in similar reductions in effective permeability. However, the band networks at Buckskin Gulch, which formed in a contractional tectonic setting, appear to be much more areally extensive and are not associated with any discrete faults having displacements greater than at most a few meters and more likely only a few tens of centimeters. In contrast, the bands at Big Hole fault are generally limited to the damage zone of a about 25-m (82-ft) displacement normal fault formed in a locally extensional environment. These results suggest that deformation bands in well core from extensional settings may be indicative of discrete damage zones associated with normal faults, whereas deformation bands in well core from contractional settings may be indicative of much more areally extensive deformation-band networks. The band networks in both cases will affect similar reductions in reservoir effective permeability, but only in the latter case will the affected area be sufficiently large to affect well performance.

Anisotropy of magnetic susceptibility (AMS) is capable of recording finite strain in weakly magnetized rocks. AMS was measured for 228 samples from 20 sites in two mylonite zones with the same deformational history. AMS measurements were compared with finite strains determined from dike rotations and from foliation orientations. In one zone (the Santa Catalina Mountains) the orientations of susceptibility and finite strain ellipsoids are in excellent agreement, and there is a logarithmic relationship between susceptibility difference (ΔKi=[Ki−K¯]/K¯) and finite strain magnitude. In the second zone (the Pinaleno Mountains) minimum susceptibility is perpendicular to the finite flattening plane, but the maximum susceptibility does not parallel the maximum extension direction, and there is no systematic relationship between susceptibility magnitude and strain magnitude. Oriented polished thin sections indicate that magnetite in the protolith of the Santa Catalina mylonite occurs as randomly oriented, elongate grains. With subsequent deformation, the long axes are rotated into the maximum extension direction. In the Pinaleno mylonites, both equant and elongate magnetite grains are present. With deformation, the elongate magnetite grains are rotated into the maximum flattening plane but show no preferred orientation within this plane. AMS in the two mylonite zones appears to be predominantly controlled by the orientation of elongate magnetite grains with respect to the megascopic fabric. The final orientation of the elongate grains is a function of their initial orientation as well as the finite strain. Therefore, despite similar deformational histories, the two zones display different AMS patterns due to the differences in occurrence, initial orientation, and shape of ferromagnetic grains.

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Rock fabrics that result from displacement in extensional orogens provide a means of identifying geometric models responsible for extension of continental crust. Strain compatibility arguments indicate that a finite extension can be accommodated by displacements across (1) planar, nonrotating faults or ductile shear zones, (2) shear zones which rotate above a horizontal detachment (the domino model), or (3) shear zones which rotate as a result of a horizontally oriented, pure shear stretching component (the plastic model). Listric, normal shear zone geometries may develop as the result of a depth dependent change in the pure shear component (in the plastic model) or the area loss component (in the planar shear zone model). Within the geometric framework of these various models, the effects of superposed simple shear, pure shear extension, and area change on the rock fabric are investigated. These displacement components, which may be superposed sequentially or simultaneously, determine the state of finite strain associated with a given magnitude of tectonic extension. The relationships between displacement and strain are expressed as graphs of foliation dip (or strain ratio) versus shear strain and as graphs of foliation dip (or strain ratio) versus tectonic extension. The shear strain graphs illustrate the effects of the different displacement components on the rock fabric in a single shear zone, whereas the tectonic extension graphs better illustrate these effects on the regional scale. The shear strain and tectonic extension graphs can be used by field geologists (1) to determine tectonic extension across a region from measurements of strain ratio or orientation and fluctuation of foliation, (2) to distinguish between possible extension models from field data, and (3) to evaluate the influence of shear zone attitude and thickness upon the amount of displacement. Applications of these methods are illustrated for field areas in metamorphic core complexes of Arizona and in the Basin and Range province of the North America Cordillera.

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