ABSTRACT

The Late Cretaceous Laramide Orogeny has created a series of northeast-to-east trending, right-reverse faults and related northeast-plunging folds in upper Paleozoic and Mesozoic sedimentary rocks at the northern end of the Tobacco Root mountains in southwestern Montana. One of the more complexly deformed parts of this belt is the "J" fold which lies adjacent to two major splays of the Sappmgton fault as defined by Schmidt, 1975. Detailed plane table and alidade mapping of the core of the "J" structure, and measurement of structural elements as fold axes and axial planes, and slickensides on fractures reveals four faults and related drag folds. The faults strike northeastward, dip 43° to 84°NW, and have suffered right reverse slip of unknown magnitude. Folds adjacent to the faults plunge northeast at angles of 10°-40°. The orientation and asymmetry of the folds are useful in determining both the slip direction and slip sense of the related faults.

Analysis of principal stress using the Ragan single fault method indicate a uniform stress field for the stucture with σ₁ 12°, N. 2°E; σ₂ 6°W, and σ₃ 76°, S. 32°E.

The faulting is contemporaneous with the two major splays. The stress between the major splays were first relieved by folding. As folding progressed, faults were generated along bedding planes and appear to cut up section to the northeast.