A NOVEL METHOD TO EXTRACT STRUCTURAL ATTITUDES OF VEINS AND STYOLITES FROM ORIENTED HAND SAMPLES: APPLICATION TO THE RIERDON FORMATION OF SOUTHWESTERN MONTANA

The Sevier fold and thrust belt (FTB) is an essential component of the North American Cordillera understood largely from datasets collected at the outcrop to regional scales and augmented with petrologic analyses. However, important structural features such as veins and styolites are often best observed at the hand-sample scale due to constraints involving aperture size and spacing. This gap in observational scale limits the ability to collect a statistically significant number of structural attitudes pertaining to submesoscopic features. We present a novel method to collect and process data from high resolution scans of oriented hand-samples using GIS software and code written in Python and R. Our approach implements a sample preparation procedure, database schema, digitization routine, and data processing in R. Sample preparation involves cutting three mutually-perpendicular faces along known orientations, which are polished and scanned at high-resolution. Code written in Python generates a set of database tables with a predefined schema that are implemented in QGIS for digitization. Data collected in QGIS is processed in R to extract the orientations of structural features. We have applied this approach to a suite of samples collected from exposures of the Rierdon Formation in the Bridger Mountains of Montana. Previous work involving three-dimensional strain analyses indicates that deformation is localized along regional faults, whereas volumetric strain appears to be controlled by dissolution and reprecipitation associated with styolites and veins. Preliminary results characterize vein orientations, document cross-cutting relationships between vein sets and styolites, and provide information regarding potential stress orientations and kinematic vectors. More than 2,500 vein segments have been digitized from 27 scanned images constraining more than 40 individual vein orientations. These results indicate a spectrum of attitudes displaying both discrete and continuous vein sets. Poles to veins define a great-circle, interpreted to represent a kinematic plane (SD: 238, 72) that corresponds to the development of the Sevier FTB. Future work will address the redistribution of mass, kinematics, and stress during the development of the Sevier FTB using this methodology.