REGIONAL STUDY OF LARAMIDE TECTONICS, NORTHERN BIGHORN/PRYOR UPLIFTS, SOUTH-CENTRAL MONTANA

The greater Bighorn uplift in Montana comprises a 160 km wide, north-plunging anticline with up to 4,800 m of structural relief relative to the adjoining Powder River and Bighorn basins. The area of study is divisible into five domains (Bighorn uplift per se, Pryor uplift, Billings arch, Hardin platform, Powder River basin). The north margin of the uplift terminates against the east-trending, right echelon normal fault system of the Lake Basin fault zone. Other east-trending fault zones separate the northern and southern pairs of structural blocks of the Pryor uplift (Nye-Bowler/Sage Creek) and are present in the Powder River basin (Black Gulch dome/Ash Creek). Previously unmapped en echelon folds within the Powder River basin between the Lake Basin and Black Gulch dome/Ash Creek fault zones, identified through computer-based analysis algorithms, suggest a wrench model origin. A component of left slip along the fault zones facilitated greater crustal shortening, and uplift, on the south side.

Structure contours (1:150,000 compilation) indicate that domains of the more structurally elevated portion are dominantly planar, rotated blocks separated by monoclines (e.g., the 100 km long Wildhorse monocline) with structural relief to 1,100 m. At domain margins block rotation was partially accommodated by back thrusts, normal faults, and local folding. Monoclines, as reconstructed in 1:150,000 and 1:37,500 scale cross sections, appear to represent fault-propagation folds over blind basement thrusts at various stages of maturity. In contrast, the structurally lower Hardin Platform is broadly folded by the Soap Creek and Two Leggins anticlines and the Muddy Creek syncline. Kinematic models of the east flank of the Bighorn uplift indicate a sequential development of thrusts eastward into the Powder River basin.