NEW INTERPRETATIONS OF SEVIER THRUST BELT STRUCTURES, SOUTHERN BEAVERHEAD MOUNTAINS, IDAHO

Geologic mapping by faculty and Field Camp students at BYU-Idaho reveals previously unrecognized complex thrust faulting and overturned folding involving a thick Mississippian to Permian stratigraphic section. The study area is the southern end of the Beaverhead Mountains between the Idaho-Montana border and the Snake River Plain in the Medicine Lodge thrust sheet. The Copper Mountain thrust trends north-south the length of the study area, and consistently places Mississippian Scott Peak and Surrett Canyon formations over the Mississippian Big Snowy to Arco Hills interval. The thrust varies in character north to south from a fault propagation fold at to a small scale thrust with minor displacement. The Copper Mountain thrust sheet is markedly more complexly folded than strata below the thrust. Fold axes in the thrust sheet are rarely more than a mile long, and folds are typically asymmetric and east-vergent. Minor thrust faulting accompanies folding locally.

On the east side of the range below the Copper Mountain thrust, an unnamed thrust fault places shallowly dipping Lower Pennsylvanian to Upper Mississippian Bluebird Mountain Formation over steeply dipping Upper Mississippian to Lower Permian Snaky Canyon Formation. The thrust can be traced for about six miles, and is cut by a subvertical, NW-striking fault within a subvertical fold limb.

The range-bounding normal fault is on the west side of the range, and steps eastward at the latitude of Copper Mountain. An intrablock normal fault at the southern end of the range displaces the Snaky Canyon formation by about 5000 feet, and displacement decreases northward to zero near Copper Mountain. A pair of west-striking normal faults at Deadman Canyon cuts the Copper Mountain thrust and creates a distinctive horst of the Scott Peak formation. The west-striking faults are the youngest structures within the range, and may have formed in relation to passage of the Yellowstone hotspot near the end of the Miocene.