NEWLY IDENTIFIED COMPRESSIONAL STRUCTURES ALONG THE WESTERN MARGIN OF THE SOUTHERN BEAVERHEAD MOUNTAIN RANGE, IDAHO

Geological mapping by the BYU-Idaho Advanced Field Methods course during the summers of 2005 through 2010 has revealed several previously un-identified structures along the western flank of the southern Beaverhead Mountain Range. Dominant structural features are mostly compressional and include thrust faults, one of which appears to form a tear fault along the margin of a minor thrust sheet, high-angle reverse faults at the cores of fault propagation folds, large-scale recumbent folds, and a major fault-bend fold.

The southern Beaverhead range is a structurally-complex terrain involving a thick succession of Middle Proterozoic through Permian strata dominated by Mississippian and Pennsylvanian carbonates. These strata have been highly deformed by Sevier thrusting and later Basin-and-Range extension, as well as normal faulting from passage of the area over the Yellowstone Hot Spot, resulting in a highly complex setting both stratigraphically and structurally. Lithological similarity between carbonate units together with the structural complexity have led to major differences in map interpretations by various workers over the past three decades; however, identification of these new structures may resolve many past discrepancies.