Abstract

Analysis of fractures associated with an anticline in upper Paleozoic rocks in Pole Canyon, Madison County, Montana, defines four persistently developed fracture sets: Ji, Jii, Jiii, and Jiv- These sets can be interpreted either as two systems of orthogonal regional fractures (Ji–Jii, Jiii–Jiv) or as two systems of conjugate shear fractures (Ji–Jii, Jiii–Jiv) related to the folding process. Interpretation of the systems as shear fractures is favored on the basis of their relationship to the fold axis.

Compression and tension (C and T) axes calculated from calcite petrofabric analysis of limestone samples collected from the anticline correspond to σ1 and σ3 directions inferred separately from fracture analysis and confirm a shear origin for both fracture systems.

The Ji–Jiii system is related to the stresses responsible for initiating folding in which σ1 acted normal to, and 0–3 parallel to, the fold axis. Reorientation of stress axes during the folding episode produced extension normal to the fold axis and parallel to the folded layers and is believed to be responsible for the Jii–Jiv system, σ2 was essentially vertical during deformation.

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