THE OVERLOOKED THRUST BELT: NEW GEOLOGICAL MAPPING REVEALS SEVIER THRUST BELT STRUCTURES IN THE SOUTHERN BEAVERHEAD MOUNTAINS, IDAHO

Geologic mapping at 1:24000 in eleven 7.5-minute quadrangles in the southern Beaverhead Mountains in east-central Idaho reveals Cretaceous thrust belt structures, including a previously undocumented thrust herein named the Copper Mountain thrust. The Beaverhead Mountains are a Basin and Range horst with range-front faults on the west side. Strata are dominated by carbonates, and range in age from the Proterozoic Swauger (?) Formation to the upper Pennsylvanian to Permian Snaky Canyon Formation. The study area exhibits a distinctive structural style of ENE- to E-vergent overturned fault-propagation folds, present from single-outcrop to whole-range scales. The folds and thrusts trend N-S in the southern study area, and NNW-SSE in the northern area. The Copper Mountain thrust is part of a N-S trending fault propagation fold with stratigraphic throw decreasing at shallow exposure levels, and significant stratigraphic throw at deeper levels. Over most of its length, the thrust places Middle Canyon Formation. over the Big Snowy Formation, omitting the Surrett Canyon Formation and, in places, the South Creek Formation. The western range front is overturned at Deadman, Bloom, Spring, and Bare canyons, with overturned dips as low as 25 degrees west. A minor thrust in Snaky Canyon, a minor thrust has a hanging wall flat connected to a hanging wall ramp. On the east side of the range, chevron-style folds have amplitudes up to 1000 feet (305 meters) in spectacular exposures. Overturned limbs on the east side of the study area in the thin- to medium-bedded Snaky Canyon Formation are laterally continuous up to 10 km. In contrast, overturned folds on the west side in the thick-bedded Scott Peak Formation. have lengths less than 3 km. and amplitudes of less than 100 m. In the Nicholia mining district, the ore host rock is exposed in five domes, one of which hosts the highly productive Viola mine. Strata previously interpreted as Dinwoody and Phosphoria in Snaky Canyon are reinterpreted as Juniper Gulch member of the Snaky Canyon Formation. Much of this mapping was done by BYU-Idaho field camp students.