Abstract

The eastern flank of the Beartooth Mountains is located approximately 30 miles north of Cody, Wyoming along the steeply dipping northwestern margin of the Bighorn Basin. Along the northeastern and eastern margins of the Beartooth uplift, northeasterly-directed compression during the Laramide orogeny reactivated pre-existing zones of weakness in the Precambrian. Northwest- trending structures developed along the northeast margin, and north-south trending structures developed on the eastern flank. The northeast flank of the uplift has been transported toward the northeast, over the axis of the Bighorn Basin.

Three miles west of Red Lodge, Montana, gravity data, surface data, seismic data, and well data indicate the Beartooth fault is dipping 25° to 35° southwest, and is striking northwest-southeast. A minimum amount of basement overhang is estimated at 16,500 feet along this segment of the uplift.

At the Grove Creek area, the Beartooth fault begins to strike north-south. The change in strike from northwest-southeast to north-south presents an interpretational dilemma as data suggest models of both high-angle and low-angle faulting. The intersection of the trends represents the maximum overhang (26,000 feet) and maximum eastward transport of the Beartooth uplift. As the fault is traced southward, the angle of faulting begins to gradually steepen, decreasing the magnitude of overhang. A well penetrating the Precambrian overhang was drilled by Phillips Petroleum at Ruby Creek. Basement overhang of 8,000 feet is interpreted at this location. The decreased overhang suggests a higher angle of fault dip along this margin of the Beartooth uplift. Displacement and Precambrian overhang continue to diminish southward along the Beartooth fault, until displacement eventually "goes blind" by propagating into the Canyon Mouth Anticline.

The strike of the fault along the eastern Beartooth uplift is somewhat oblique to the direction of tectonic transport. This alignment would suggest lateral motion along the north-south striking fault. Field evidence suggests that lateral motion did occur along the fault, resulting in reverse right-oblique slip along the eastern flank.

High-angle dip on the Beartooth Fault results in decreased shortening along the eastern margin of the uplift. Along the transition zone from low to high-angle dip of the Beartooth Fault, the Line Creek Fault and the BTF(A) increase their displacement and shortening. The increased shortening along these two faults southward along with the reverse right-oblique slip on the Beartooth fault helps to maintain structural balance between the northeast and eastern margins of the Beartooth range.