Geometric and mesofabric analysis of the Hossfeldt Anticline - Eustis Syncline and the Lombard Thrust Zone near Three Forks, southwestern Montana

Folding and faulting are inseparable crustal shortening processes. Fold development requires a fault or detachment at the base of the affected strata (Dahlstrom, 1970). In this study, the structural relationship between folding and thrust faulting is investigated for the Hossfeldt anticline - Eustis syncline pair in the Montana fold and thrust belt.

Geometric modeling (Suppe, 1983 and 1985, and Jamison, 1987) provides a means of defining fold-fault relationships from the final fold form and its position relative to a thrust fault. In some instances final geometry is not conclusive, in which case kinematic constraints of the different fold-fault models can be used to differentiate between them. This approach requires mesofabric data for the fold structures in question.

Mesofabric analysis suggests the Hossfeldt anticline - Eustis syncline pair formed as kink folds with deformation concentrated in the shared, overturned limb. The axial plane of the Hossfeldt anticline appears to have remained fixed during folding. Bedding thicknesses remain roughly constant around the folds despite locally intense mesofaulting, veining and fracturing.

These kinematic constraints and the mechanical properties of the folded stratigraphy suggests the Hossfeldt anticline - Eustis syncline formed as a detachment fold pair. These folds formed as a result of movement on the regional decollement inferred at or near the base of the Precambrian Belt Supergroup (Schmidt and O'Neill, 1982).

Mesofabric analysis also reveals the cataclastic nature of the Lombard thrust and associated imbricates in the carbonate lithologies of the study area.