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

The BYU-Idaho Advanced Field Methods course has conducted mapping exercises in the southern Beaverhead Mountain Range for the past several years. Mapping in the area has been done by other workers as well, mostly on a reconnaissance scale, with little agreement between the various interpretations. Current mapping has identified a number of previously undescribed structures along the western flank of the range, and research this past summer focused on refining the details of these structures with the ultimate goal of reconciling differences between past and current mapping. Dominant compressional features include thrust faults, one of which appears to form the lateral margin of a minor thrust sheet and to be bound by a tear fault, high-angle reverse faults at the core of fault propagation folds, large-scale recumbent folds, and a major fault-bend fold.

The southern Beaverhead range consists of a thick succession of Middle Proterozoic through Permian strata dominated by Mississippian and Pennsylvanian carbonates, many of which are highly similar in appearance. These strata have been significantly deformed by Sevier thrusting and later Basin-and-Range extension, as well as passage of the area over the Yellowstone Hot Spot. This has resulted in a highly complex setting both stratigraphically and structurally and has led to major variation in map interpretations; however, identification of these new structures may resolve many of the past discrepancies.