MACROSCOPIC STRUCTURAL ANALYSIS OF THE BEAR CREEK, POKER PEAK, AND BIG ELK ANTICLINES, IDAHO FOLD-THRUST BELT

The Bear Creek, Poker Peak, and Big Elk anticlines are macroscopic structures within the Idaho fold-thrust belt. Structural analysis of the area has been conducted by supplementing existing geologic maps with data from digital orthophotos, and LANDSAT images. Draping of these images over digital elevation models (DEM), has provided a three-dimensional perspective of the area, and enabled detailed mapping of the structures. The mapping has been further refined by reconnaissance field mapping of selected outcrops. A series of five balanced cross sections have been constructed for these structures, and these have been integrated into a three-dimensional structural model for the area. The late Paleozoic to Mesozoic stratigraphy in the area consists of competent units, including carbonates and sandstones, interlayered with ductile siltstones, shales, and thin-bedded carbonates. The structures are interpreted to be detached within the Devonian to Mississippian units. Additional local detachments result in disharmonic folding of the units. The folds consist of relatively planar to curved limbs separated by multiple angular hinges within the more competent units. They vary from almost symmetric to highly asymmetric, and show variable vergence both along and across trend. Furthermore, the folds show considerable disharmony along trend. All of these observations suggest that the folds formed primarily by detachment folding above a weak basal detachment.