THE BEAVERHEAD DIVIDE FAULT ON THE IDAHO-MONTANA BORDER – CRETACEOUS CONTRACTION, EOCENE EXTENSION, BUT NOT A TERRANE BOUNDARY

The Beaverhead Divide fault (BDf) is a zone of both ductile and brittle deformation that crosses the Beaverhead Range NNW-SSE. In its central part, east-verging overturned strata to the east had been assigned to the Mesoproterozoic Belt Supergroup whereas those to the west include Gunsight Formation of the Lemhi Group, also Mesoproterozoic. The “eastern” strata include both feldspathic and nearly pure coarse-grained quartzite, and matrix-supported conglomerate with clasts of vein quartz, granitic and gneissic rock, and typically have potassium feldspar in excess of plagioclase. The Gunsight Formation is a similar fluvial sequence, but lacks pebbles and is finer grained and more feldspathic, with plagioclase dominant. Juxtaposition of these strata and at least the ductile deformation likely date from Cretaceous contraction. However, some deformation resulted from Eocene extension. Hornblende diorite bodies, one with a U-Pb age of 46 ± 2 Ma, are common along and near the fault zone; some are sheared with top-to-the-west normal sense kinematic indicators. In the northwestern and southeastern parts of the range, the eastern strata grade downward into Gunsight Formation. A similar contact exists at the type section of the Gunsight Formation in the Lemhi Range where it grades upward into the coarser grained and locally pure quartzite of the Swauger Formation. Thus the eastern strata are anomalously thick Gunsight and Swauger (and perhaps higher units unknown in the Lemhi or southern Beaverhead ranges). These strata appear well into Montana where their extraordinary thickness (>10km) may reflect proximity to the Belt Basin margin. The BDf, then, simply dissected an essentially intact Lemhi Group section. Any contact with more typical units of the Belt Supergroup must be considerably farther north.