ABSTRACT

The Red Rock fault in Beaverhead County is a strikingly fresh-looking range-front normal fault along which the Red Rock basin to the northeast has been downthrown relative to the adjacent Tendoy Mountains. Triangular faceted spurs dipping 30-33°, fresh scarplets cutting modern alluvial fans, and recent seismicity in the area suggest the fault is presently active. A detailed geomorphic analysis of fault line scarps and slope profiles indicates several periods of activity along the fault. The most recent activity occured west of Lima, between Chute Canyon and Little Sheep Creek. This indicated by incised alluvial fans, scarp slopes greater than 40°, and nick points in streams crossing the inferred trace of the fault.

The Red Rock fault, for most of its length, parallels the regional northwest trending structures characteristic of southwestern Montana. However, southwest of Lima, its trend becomes more easterly. This trend, parallel to that of the Centennial fault mapped 35 km to the east of Lima, implies that two tectonic regimes are responsible for the formation of the Red Rock fault. These are: northeast-southwest extension characteristic of the Basin and Range; and north-south extension characteristic of the Snake River Plain.