Shear Zone Fabrics Research Articles

The Middle Mountain shear zone, southern Idaho: Kinematic analysis of an early Tertiary high-temperature detachment

The Middle Mountain shear zone is exposed in the Idaho portion of the Albion- Raft River-Grouse Creek metamorphic core complex. The structure strikes north- northeast-south-southwest and dips shallowly to the west. It is defined on the basis of well-developed mylonitic fabrics and had an initial thickness of at least 4 km. At least some movement on the shear zone occurred at depths of 22-26 km during Eocene amphibolite-facies metamorphism, but shear-zone fabrics may be found in some granite bodies of late Oligocene age. The zone may thus have been active in the ductile regime over a period of as much as 10-15 m.y. Mylonitic rocks within the Middle Mountain shear zone display both type I and type II S-C fabrics. Augen structures, pressure shadows, and quartz c-axis orientations are strongly asymmetric. These fabrics and textures indicate that deformation within the zone was characterized by noncoaxial laminar flow, during which the hanging wall of the structure moved down and to the west-northwest relative to the footwall. We interpret the Middle Mountain shear zone as a major detachment that played an important role in middle Tertiary extension of this part of the eastern Great Basin. The top-to-the-west displacement sense contrasts with late Cenozoic, predominantly top-to-the-east extension in this area, implying a major change in extension geometry in early to middle Miocene time.

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.