Cordilleran thrust belt and faulted foreland in the Beaverhead Mountains, Idaho and Montana

Abstract

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 westdipping 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 inthe area of the shelf west of the Montana craton, and northeast-trending, northwestdipping, 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 Yellowjacket 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 237 on June 5, ...

Chapter 10: Structural features and emplacement of surficial gravity-slide sheets, northern Idaho-Wyoming thrust belt

“…Gravity-slide blocks associated with Tertiary extensional faulting occur throughout the Basin and Range Province of Nevada, Utah, and Idaho (Longwell, 1951;Drewes, 1959Drewes, ,1963Drewes, , 1967Cook, 1960;Mackin, 1960;Ypung, 1960;Kurie, 1966;Seager, 1970;Moores, 1968;Shackelford, 1975;Krieger, 1977;Bally and others, 1981;Todd, 1983;Faugere and others, 1986;Sable and Anderson, 1986;1987, p. 444;Boyer & Allison, 1987) and adjacent portions of the Idaho-Utah-Wyoming thrust belt (Beutner, 1972;Hait and others, 1977;Moore and others, 1984;Oriel and Moore, 1985;Moore and others, 1987;Hait, 1987;Skipp, 1988;Janecke, 1989;Anders, 1990;McCalpin and others, 1990). As gravity-slide surfaces are difficult to distinguish from thrusts or extensional faults and may have been misinterpreted as such (Boyer and Allison, 1987), gravity-slide surfaces are probably more extensive than has been previously recognized.…”

Synorogenic crustal fluid infiltration in the Idaho‐Montana Thrust Belt

“…1). These ranges, separated by extensional half-graben basins active in the Tertiary to Recent [Janecke, 1992;Anders et al, 1993], are dominated by imbricate thrust sheets containing variably sized, often dishar-' monic, fault-related folds (e.g., Skipp [1988]; Janecke and Wilson [1992]). For westernmost field areas, regional stratigraphic reconstructions, balanced and restored cross sections of the Idaho-Wyoming-Montana thrust belt south of the Snake River Plain, geobarometry around the Idaho batholith, and geothermometry and deformation mechanism partitioning for the Lost River Range, all suggest that compressional deformation of the outer-shelf Late Paleozoic miogeocline occurred-10 km beneath the synorogenic surface (Allmendinger [1990] …”

Pseudotachylites of the Beaverhead impact structure: Geochemicaly geochronological, petrographic, and field investigations

“…First, Cretaceous thrusting dismantled the original crater and transported the fragment now exposed some distance to the east (Skipp, 1988;Ruppel, 1978 (Magloughlin and Spray, 1992). These fault surfaces are commonly oriented in relation to major structural features in the area (e.g., Magloughlin, 1989).…”