OFF-FAULT DISTORTIONAL STRAIN ANALYSIS WITHIN THE OOLITIC RIERDON FORMATION OF THE HELENA SALIENT, SOUTHWEST MONTANA: IMPLICATIONS FOR THE MECHANICS OF DEFORMATION AND STRAIN LOCALIZATION

Fold and thrust belts are important features concerning economic resource extraction and seismic hazard assessment. One fundamental question regarding the structural development of fold and thrust belts involves the role of off-fault deformation. By quantifying the distribution of off-fault deformation, constraints may be placed in numerical models to better address the potential for seismic rupture, as well as in the characterization of material properties including porosity and permeability. This contribution presents the results of three-dimensional distortional strain analyses applied to samples collected from the Bridger mountains of southwest Montana. Samples were cut along three mutually perpendicular orientations and polished. The traces of ooids were digitized from scanned images of the samples and fitted with ellipses for strain analysis following a modification of the normalized Fry method. A bootstrap statistical approach was applied to two-dimensional sectional analyses and used in fitting three-dimensional strain ellipsoids to each sample. This allows for an assessment of confidence intervals to be applied to the results of the three-dimensional strain analyses.

Preliminary results indicate that distortional strain is minimal throughout the study area, however, detailed observations of samples clearly illustrate features indicative of volumetric strain including styolites and veins. Mesoscopic observations from outcrop analysis and local geologic mapping indicate a number of small-scale faults, often in association with local folding, as well as the presence of regional scale faults. Our results suggest that deformation recorded within the study area is primarily localized along major fault zones such as the Cross range fault, the Ross Peak fault, and the Pass fault. Folding at the local scale is typically associated with small fault systems and accounts for a secondary mechanism accommodating deformation. The lack of distortional strain indicates that shear is not distributed pervasively but restricted to fault zones and slip within less-competent bedding planes. The presence of features indicative of dissolution suggest that changes in volume may be important in the accommodation of deformation at the currently exposed crustal level of the Helena Salient.