David W. Stearns

Tectonic Deformation of Wingate Sandstone, Colorado National Monument

Along the northeast flank of the Uncompahgre Plateau, forced folds developed in a 2-km-thick (6,500 ft thick) sequence of sandstones and shales above high-angle basement faults during the Laramide(?) orogeny. The structures developed in the aeolian Wingate Sandstone, which lies near the base of this section of sedimentary rocks, are well exposed in many of the canyons within the Colorado National Monument. Within these structures, Wingate beds have been substantially flexed and attenuated with little attendant fracturing or major faulting. The primary deformational features producing the observed strain within the Wingate are microfaults. Microfaults are roughly planar zones across which small, but discernible, amounts of shear displacement have occurred. These features r rely continue through major bedding-unit contacts. Microfaults are conspicuous in outcrop by virtue of a light-colored and relatively resistant gouge zone. The gouge consists of a 0.3-mm-wide (0.01 in. wide) zone of mechanically comminuted and compacted sand grains. The porosity of this initially high-porosity sandstone is substantially reduced both within and along the boundaries of the gouge zone. Shear displacement along a microfault of greater than ~5 mm (0.2 in.) produces additional gouge zone segments, which results in a braided or anastomosing texture. Microfaults form in conjugate shear systems that intersect at 20° to 40°, roughly in accord with the predictions of the Mohr-Coulomb criterion. This organized arrangement of low-porosity gouge zones produces a reduction in the permeability of this sandstone, perpendicular to the microfaults, by as much as three orders of magnitude. Microfaults forming parallel to cross-beds and cross-bed-set boundaries occur locally within the Wingate structures, as do microfaults arranged in a Riedel shear-zone fabric. Assessment of mechanisms involved in gouge-zone development, generation of the anastomosing gouge-zone fabric, and offsetting relationships suggests that deformation by microfaulting is a strain-hardening process. This strain hardening terminates when a major fault zone has developed through the Wingate Formation.

Experimental folding of rocks under confining pressure: Part III. Faulted drape folds in multilithologic layered specimens

Drape folds and reverse faults are produced experimentally at confining pressures to 2.0 kb and shortening rates of 10 −3 to 10 −6 sec −1 by displacing a block of brittle sandstone (2 by 3 by 12.6 cm) along a lubricated saw cut into one to five initially intact layers (0.2 to 1.0 cm thick and as much as 12.6 cm long) of limestone, sandstone, and rock salt. The saw cut is inclined at from 30° to 90° to the layer boundary. The deformation is characterized from studies of fault geometry, displacements and sequence, bedding-plane slip, layer-thickness changes, and the development of fault gouge, fold hinges, microfractures, calcite twin lamellae, and dimensional orientations of grains (in the rock salt). Stress trajectories are inferred from faults, microfractures, and calcite twin lamellae, and strains are calculated from layer-thickness changes and from calcite twin lamellae. Reverse faults curving concave downward propagate upward from the saw cut in the forcing block. With increasing displacement along the precut faults, the faults and associated gouge zones in the layer steepen and become progressively younger toward the upthrown block as displacement increases. The faults are preceded by swarms of extension microfractures that form throughout the deformation and that are the best clues to the stress trajectories. The downthrown layers are thickened by uniform flow and by repetition caused by the faulting. They are displaced away from the faults by bedding-plane slip. Trajectories of the greatest principal compressive stress (σ 1 ) are inclined at low angles to the layer boundaries near the faults and become perpendicular to these boundaries away from the fault. The maximum deformation of the downthrown block occurs when the saw cut is inclined at about 65° to the layering. The upthrown layers are all extended parallel to the layering and perpendicular to the fold axes, as indicated by extension fractures, thinned layers, and calcite twin lamellae and the development of graben zones and low-angle normal faults that are conjugate to the reverse faults. The layers are translated by bedding-plane slip away from the fault zone. Trajectories of σ 1 are inclined from 45° to 90° to the layering. The fabric data are internally consistent, and inferred stresses are in good agreement with those calculated from an elastic solution of the experimental boundary conditions. Principal strains calculated from calcite twin lamellae are within an average of 0.01 of those calculated from layer-thickness changes and permit clear resolution of individual events in domains of superposed deformations.

Relations between Stresses Inferred from Calcite Twin Lamellae and Macrofractures, Teton Anticline, Montana

Compression axes inferred from calcite twin lamellae in eleven samples of Madison Limestone from both flanks of Teton anticline are remarkably similar. They indicate that the greatest principal stress (σ 1 ) was subparallel to the dip at one time during the folding and inclined to the bedding at other times. In three of the four samples from the plunging nose, compression axes are more diffuse than are those from the flanks. The compression axis pattern for the flank stations agrees with one of the two prominent macrofracrure patterns. The second prominent fracture assemblage indicates that at some time during the folding σ 1 was parallel to the fold axis. This orientation of the compression axis is never seen in the calcite data, even though some samples were collected immediately adjacent to macrofractures of this group. However, the fabric of a flank specimen, experimentally deformed such that the compressive load was parallel to the strike direction, clearly shows that compression axes inferred from e 1 lamellae are subparallel to the experimental σ 1 , and compression axes inferred from e 2 lamellae reflect the original natural fabric. That is, the original e 1 lamellae are demoted to e 2 lamellae by the superposed deformation. Moreover, the twin lamellae index is much higher in the experiments than in the naturally deformed rocks. Thus, in cases of superposed deformations, the calcite technique statistically maps the compression axis associated with the largest strain. Therefore, for the specimen studied, the shortening associated with σ 1 perpendicular to the fold axis probably is greater than that parallel to the fold axis.

Fault Analysis in Wichita Mountains: ABSTRACT

Analysis of a large population, but small displacement, fault array in the Wichita Mountains of southern Oklahoma strongly supports the hypothesis of left-lateral wrench faulting as a major tectonic control for the region. Middle Cambrian granites make up most of the exposed core of the Wichita uplift. Because these granites were implaced prior to the development of the Anadarko basin structures, they should reflect Anadarko tectonics. In addition, the granites would have behaved in a brittle manner so that abundant faulting is practically the only mechanism of deformation within them; this permits uncomplicated structural analysis. Offset and trend measurements were made both in the field and from aerial photographs, and the collective data show statistically significant groupings with respect to trend and sense of shear. The fault fabric is consistent with a left-lateral wrench system that trends N70°-80°W, but also contains strong elements of the entire Riedel system (R, R^prime, and P shears). In addition to the wrench motions indicated by the analysis of small displacement faults, there is also a large component of vertical displacement in the region. A fault system known as the Wichita front, separates the Wichita uplift from the Anadarko basin and has 9 km (5.5 mi) of differential vertical relief across a zone 10 to 20 km (6 to 12 mi) wide. The relationship between the lateral and vertical motion is essential in understanding the types and distribution End_Page 511------------------------------ of structural traps in the Anadarko basin, and perhaps, even in neighboring basins. End_of_Article - Last_Page 512------------