A recent 1:24,000 scale mapping project within the northern Beaverhead Mountains along the Idaho-Montana border has resulted in a reinterpretation of both the Mesoproterozoic stratigraphy and the regional structural framework. A 15-km-thick stratigraphic section of the Mesoproterozoic Lemhi subbasin was initially deformed by northeast-southwest shortening into giant northwest-striking, northeast-verging folds, probably during Cretaceous Sevier orogenesis. These initial folds were then dissected by a system of subparallel and anastomosing, oblique-slip reverse, thrust, and normal faults that generally strike northwest, but that exhibit east-west–oriented lineations, suggesting components of strike-slip displacement. Contractional faulting appears to have been followed by Eocene to Miocene extensional faulting, with many normal faults following the preexisting fabrics. Extension opened Tertiary basins along some of these faults, including the Salmon Basin along the southwestern side of the Beaverhead Range. Subparallel faults in the surrounding region appear to have a similar complex history, and all appear to be part of a major northwest-striking Cretaceous fold-and-thrust belt that was later dissected by Tertiary extension. Although the faults of the Beaverhead Mountains are significant and long-lived, they are not terrane-bounding structures separating the Belt and Lemhi sedimentary sequences. Instead, Lemhi strata extend across the range and northward to Missoula, where they grade into correlative Missoula Group strata.

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.

Maps of the distribution of Middle Proterozoic through Cretaceous rocks beneath the Tertiary unconformities in eastern Idaho effectively remove differential uplift associated with Basin and Range tectonism and reveal preextensional structural relief associated with folds and thrusts in the western part of the Idaho-Wyoming-Montana thrust belt. North of the Snake River Plain, the paleogeologic map shows that regionally extensive Middle Proterozoic to Triassic strata and the Cretaceous Idaho batholith were variably uplifted and exposed prior to formation of the Eocene Challis volcanic field. In the Beaverhead Mountains, Ordovician and Triassic strata are juxtaposed along the Hawley Creek thrust. In the northern Lemhi and Lost River ranges, west- and south-dipping homoclines beneath the Eocene unconformity are interpreted to reflect folding above frontal and lateral ramps in the footwall of the Hawley Creek thrust system. In the southern Lemhi and Lost River ranges, the White Knob Mountains, and eastern Pioneer Mountains, Mississippian to Permian strata underlie most of the unconformity. Stratigraphic offset is evident along the exposed Pioneer and Copper Basin thrust faults, but along the exposed Glide Mountain thrust as well as the concealed White Knob, Grouse, and Lost River-Arco Hills thrusts uplift was insufficient to juxtapose rocks of different systems. To the west, the Idaho batholith and Pennsylvanian-Permian rocks are juxtaposed beneath the Eocene unconformity. Several kilometers of overburden covered both the batholith and Permian rocks in Cretaceous time and were regionally eroded prior to extrusion of Eocene volcanic rocks. South of the Snake River Plain and west of the exposed traces of the Paris and Putnam thrust faults, Late Proterozoic to Permian strata unconformably underlie Miocene sedimentary and volcanic rocks. The Paris thrust separates Late Proterozoic through Mississippian strata in its hanging wall from Pennsylvanian through Triassic rocks in its footwall, and the Putnam thrust separates Late Proterozoic to Ordovician strata from Pennsylvanian to Triassic strata. Beneath the unconformity, stratigraphic displacement appears to diminish northwestward along the Paris thrust and southeastward along the Putnam thrust, supporting the interpretation that displacement is progressively transferred from one thrust to the other. Lower Paleozoic strata generally underlie the unconformity between the emergent thrust system and the Arbon and Malad valleys. West of these valleys, Pennsylvanian-Permian strata everywhere underlie the unconformity. A simple flat-ramp-flat-thrust fault geometry of the Paris-Putnam thrust system may explain the outcrop pattern, with hanging-wall and footwall flats beneath the regions of low structural relief, separated by a footwall ramp located beneath the modern Malad and Arbon valleys. Paleogeologic maps show that the Paris-Putnam thrust sheet was probably not an important source of quartzose clasts in the Cretaceous-Tertiary Harebell and Pinyon formations of northwest Wyoming.