The Idaho-Wyoming segment of the Cordilleran thrust belt is characterized by west-dipping folded thrusts that place older strata over younger, by thrust plates that have lateral continuity and distinctive stratigraphic sequences, and by a gently west-dipping uninvolved basement beneath the thrust plates. Northwestward across the Snake River Plain. frontal thrusts and thrust plates of the Idaho-Montana segment of the Cordilleran belt exhibit the first two characteristics, but differ in that basement rocks locally are involved in the thrusts, indicating that these Idaho-Montana thrust plates overrode a previously faulted foreland. Distribution of basement rocks indicates that the faulted foreland consisted of west-northwest- and east-northeast-trending faults of probable Proterozoic ancestry in the area of the shelf west of the Montana craton, and northeast-trending, northwest-dipping, basement-rooted Cretaceous thrust faults of the southwestern Montana craton to the east. The hanging wall of the Cordilleran Cabin thrust contains Archean(?) rocks in a fragment of the Cabin block, a regional Proterozoic basement uplift cut by the thrust as it propagated northeastward. Hanging walls of structurally lower Cordilleran thrusts contain segments of northeast-trending Cretaceous foreland thrusts and fold structures, such as the Snowcrest Range thrust system and the Little Water syncline. Renewed movement on foreland thrusts subsequently locally folded Cordilleran thrusts. Available paleontological data and radiometric age determinations indicate that major movements on both foreland and Cordilleran thrusts took place in Late Cretaceous time in the Beaverhead Mountains and vicinity. Major Cordilleran thrust plates in the Beaverhead Mountains are, from west to east: the Hawley Creek, Fritz Creek, Cabin, Medicine Lodge, Four Eyes Canyon, and Tendoy. A west-to-east deformational sequence is assumed for all of the plates except part of the Cabin. Diagrammatic cross sections of the southern Beaverhead Mountains suggest that locally the Cabin may have overridden the Medicine Lodge, and is out of sequence. The redefined Cabin thrust plate is thick and more than 200 km in length. It has been thinned secondarily by several younger-over-older Cenozoic normal faults, some of which were mapped previously as thrusts. Archean(?) through Triassic rocks make up the plate in the central and southern Beaverhead Mountains, and Proterozoic Yellow-jacket Formation and Lemhi Group rocks make up the plate in the northern Beaverhead Mountains. There, Proterozoic rocks are thrust over Belt Supergroup strata of the Grasshopper thrust plate and are part of a structural culmination in the position of the Salmon River Arch. A large lateral ramp in the hanging wall of the Cabin thrust marks the northern margin of the 75-km-long transported segment of the Proterozoic Cabin block. Structural and stratigraphic throw diminish to the south near Bannack Pass, but increase north of the ramp near latitude 45°, where the Cabin thrust cuts down section with respect to the hanging wall to include several thousand meters of Proterozoic Yellowjacket Formation. The northern margin of the Cabin block may compose the northern margin of Archean basement beneath the thrust belt in south-central Idaho. Northeastward translation of the Cabin and Medicine Lodge thrust plates is about 40 km in the southern Beaverhead Mountains; thus, Archean(?) crystalline basement rocks of the block originally were at least as far southwest as the present Lemhi Range. These old crystalline rocks constitute a western projection of the southwestern Montana reentrant.

The Blacktail-Snowcrest uplift in Beaverhead County, Montana, is a northeast-trending structure of the Rocky Mountain foreland. Models of a detailed gravity traverse across the steep southeastern limb of the structure require the presence of a major thrust-fault system, only subsidiary parts of which are exposed at the surface. Gravity models, consistent with mapped surface structure, are based on density logs and thicknesses of Paleozoic and Mesozoic stratigraphic intervals penetrated in nearby drillholes. Interpretation of the models requires a thicker sequence of low-density Cretaceous rocks southeast of the uplift than was encountered in the wells, and also suggests that the thrust system beneath the uplift involves rocks of both the sedimentary sequence and the crystalline basement with at least 3,700 m (12,000 ft) of vertical separation. The models require the assumption of unusually low densities for some of the Precambrian and Paleozoic rocks; these densities are interpreted as being related to pervasive fracturing of these rocks during thrust deformation. Gravity and aeromagnetic trends associated with the Snowcrest-Greenhorn fault system extend westward beneath the Cordilleran thrust belt, indicating that the fault system is present there in the subsurface. A reentrant in the Medicine Lodge thrust sheet, the termination of the Four Eyes Canyon thrust sheet, and a change in strike of the Cabin thrust sheet, all in the frontal thrust belt, overlie the inferred westward extension of the Snowcrest-Greenhorn fault system, and probably are caused by the impingement of east-directed Cordilleran thrust faults on the older Rocky Mountain foreland structure. Cordilleran thrust sheets appear not only to overlap but also to crosscut older foreland faults, uplifts, and basins. Where crosscutting occurs, both upper and lower plates may exhibit rapid changes in stratigraphic level along strike, and these changes may be used as field evidence of overlap in the subsurface.