COMPLEX FOLDING IN THE ANACONDA EXTENSIONAL TERRANE, WESTERN MONTANA

The Anaconda extensional terrane, western Montana exposes exhumed high-grade rocks beneath an unmetamorphosed cover. In the study area three crustal levels are recognized based on structural style and metamorphic grade. In the region south of Anaconda, Montana all three levels are exposed, providing the ideal opportunity to study fold development in an extended terrane.

Lower level rocks, exposed in Mill Creek canyon, are comprised of complexly deformed amphibolite-grade paragneiss and orthogneiss intruded by a synextensional suite of two-mica granite and hornblende granodiorite. This lower level consists of two structural domains. The structurally lowest domain 1 contains class 2 mesoscopic and microscopic scale folds with hinge trending 100°-110°. Rootless isoclinal folds and boudinage indicate high strain. Domain 2 exhibits concentric, class 1B folds at km and microscopic scales with hinges oriented at 114°/30°. The Mill Creek nappe, a km-scale, south-verging recumbent fold is bound on its base by a high-temperature fault that separates domains 1 and 2.

Overlying the lower-level rocks is a 500-800 m thick greenschist-grade mylonite zone deforming mostly two-mica granite and granodiorite. Throughout the sinuous trace of the shear zone top-to-the-east shear-sense and mineral stretching lineations of 102°-108° are consistent. The mylonite is folded by kilometer to mesoscopic, concentric style folds with hinges subparallel to the stretching direction. These folds are continuous with those found in lower domain 2. Above the mylonite, rocks consist of rotated volcaniclastic and tuffaceous deposits, dissected by high-angle brittle faults. It is difficult to conclude whether hanging wall folds are rooted with those found at structurally deeper levels.

Significant evidence shows that folding occurred during extension and not during thrusting events. Timing relationships are resolved by dikes that demonstrate emplacement during folding. Moreover, hinge orientations in the footwall and the mylonite coincide with the stretching direction. Thus we conclude that folding occurred during extension. This illustrates that folding is not restricted to contractional tectonism, yet it is an important component during extension.