Abstract Geological strain analysis of sedimentary rocks is commonly carried out using clast-based techniques. In the absence of valid strain markers, it can be difficult to identify the presence of an early tectonic fabric development and resulting layer parallel shortening (LPS). In order to identify early LPS, we carried out anisotropy of magnetic susceptibility (AMS) analyses on Mississippian limestones from the Sawtooth Range of Montana. The Sawtooth Range is an arcuate zone of north-trending, closely spaced, west-dipping, imbricate thrust sheets that place Mississippian Madison Group carbonates above Cretaceous shales and sandstones. This structural regime is part of the cordilleran mountain belt of North America, which resulted from accretion of allochthonous terrains to the western edge of the North American continent. Although the region has a general east–west increase in thrust displacement and related brittle deformation, a similar trend in penetrative deformation or the distribution of tectonic fabrics is not observed in the field or in the AMS results. The range of magnetic fabrics identified in each thrust sheet ranges from bedding controlled depositional fabrics to tectonic fabrics at a high angle to bedding.

Abstract The Teton anticline and adjacent structures, in the Sawtooth Range, Montana, USA, are fractured in such a way that may be taken as a model for fractures propagating during buckle folding. However, advances in understanding both the process of folding in forelands and the evolution of fracture patterns found within these folds suggest that it is time to reinterpret the nexus between fracturing and folding within these classic structures. With the benefit of seismic lines, the Teton anticline is best described as a fault-propagation fold. Joint propagation initiated with the formation of two major sets whose orientation is controlled by pre-folding, regional stresses. Two more joint sets propagated in local stress fields, developed in response to anticline growth. Some early joints were reactivated as wrench faults during amplification and tightening of the anticlines. The fracture sets identified are consistent with: (a) propagation in a regional stress field, which may be related to stretching in the Sawtooth Range orocline; and (b) tangential longitudinal strain of the backlimb and forcing or trishear of the forelimb during anticline development. Thus, we suggest that fracture networks across folded structures should be interpreted and characterized in the light of the geological history of the entire system.

Abstract In sedimetary basins not currently undergoing primary compaction (e.g., Rocky Mountain Basins), p-wave velocities noticeably vary with azimuth, yet the mechanism(s) controlling the anisotropy remain uncertain. Possible geologic causes for azimuthal anisotropy include but are not limited to sedimentary fabrics, steep bedding, changes in local in-situ or residual stress, and open or mineralized fractures. To test these hypotheses, P-wave velocity azimuths (Vfast) from a proprietary seismic survey of a NNW-trending Laramide Anticline on Casper Arch in central Wyoming were compared to image log data from the seismic coverage area and fracture orientations from nearby analog structures.

Quantification of fault-related illite neomineralization in clay gouge allows periods of fault activity to be directly dated, complementing indirect fault dating techniques such as dating synorogenic sedimentation. Detrital “contamination” of gouge is accounted for through the use of illite age analysis, where gouge samples are separated into at least three size fractions, and the proportions of detrital and authigenic illite are determined using illite polytypism (1M d = neoformed, 2M 1 = detrital). Size fractions are dated using the 40 Ar/ 39 Ar method, representing a significant improvement over earlier methods that relied on K-Ar dating. The percentages of detrital illite are then plotted against the age of individual size fractions, and the age of fault-related neoformed material (i.e., 0% detrital/100% neoformed illite) is extrapolated. The sampled faults and their ages are the Absaroka thrust (47 ± 9 Ma), the Darby thrust (46 ± 10 Ma), and the Bear thrust (50 ± 12 Ma). Altered host rock along the frontal Prospect thrust gives an age of 85 ± 12 Ma, indicating that the 46–50 Ma ages are not related to a regional fluid-flow event. These ages indicate that the faults in the Snake River–Hoback River Canyon section of the Wyoming thrust belt were active at the same time, indicating that a significant segment of the thrust belt (100 km 2 +) was active and therefore critically stressed in Eocene time.

A sub–Middle Jurassic unconformity is exhumed at Swift Reservoir, in the Rocky Mountain fold-and-thrust belt of Montana. The unconformity separates late Mississippian Sun River Dolomite of the Madison Group (ca. 340 Ma) from the transgressive basal sandstone of the Middle Jurassic (Bajocian-Bathonian) Sawtooth Formation (ca. 170 Ma). North-northwest–trending, karst-widened fractures (grikes) filled with cherty and phosphatic sandstone and conglomerate of the basal Sawtooth Formation penetrate the Madison Group for 4 m below the unconformity. The fractures link into sandstone-filled cavities along bedding planes. Clam borings, filled with fine-grained Sawtooth sandstone, pepper the unconformity surface and some of the fracture walls. Sandstone-filled clam borings also perforate rounded clasts of Mississippian limestone that lie on the surface of the unconformity within basal Sawtooth conglomerate. After deposition of the overlying foreland basin clastic wedge, the grikes were stylolitized by layer-parallel shortening and then buckled over fault-propagation anticlinal crests in the Late Cretaceous–Paleocene fold-and-thrust belt. We propose that the grikes record uplift and erosion followed by subsidence as the Rocky Mountain foreland experienced elastic flexure in response to tectonic loading at the plate boundary farther to the west during the Middle Jurassic. The forebulge opened strike-parallel fractures in the Madison Group that were then karstified. The sandstone-filled karst system contributes secondary porosity and permeability to the upper Madison Group, which is a major petroleum reservoir in the region. The recognition of the fractures as pre–Middle Jurassic revises previous models that have related them to Cretaceous or Paleocene fracturing over the crests of fault-propagation folds in the fold-and-thrust belt, substantially changing our understanding of the hydrocarbon system.