The thrust belt in Southwest Montana and east-central Idaho

Ruppel, E.T.; Lopez, David A.

doi:10.3133/pp1278

Cited by 34 publications

(75 citation statements)

References 14 publications

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“…The Argenta mining district is in a north-trending segment of the Cordilleran thrust belt that was referred to by Ruppel and Lopez (1984) as the "frontal fold and thrust zone," a structural belt characterized by parallel anticlines, synclines, and minor thrust faults. The belt is transitional between the Rocky Mountain foreland to the east and the region of major thrust faults to the west.…”

Section: Geologic Setting Of the Argenta Areamentioning

confidence: 99%

“…The district is about 6 km east of the Kelley thrust ( fig. 2), a segment of the Grasshopper decollement, which is one of the major thrust faults of southwestern Montana (Ruppel and Lopez, 1984). The upper plate of this thrust, now called the Grasshopper plate (figs.…”

Section: Geologic Setting Of the Argenta Areamentioning

confidence: 99%

“…The upper plate of this thrust, now called the Grasshopper plate (figs. 2 and 3), consists of at least 7 km of upper Belt (Missoula Group) strata (Ruppel and Lopez, 1984) and local thin remnants of overlying lower Paleozoic rocks.…”

Section: Geologic Setting Of the Argenta Areamentioning

confidence: 99%

“…Inferences that the Belt Basin extended far to the south of their present remnant were based solely on tenuous facies relations and thickness trends (Reynolds, 1984). On the Grasshopper thrust plate, Missoula Group (upper part of the Belt Supergroup) rocks are widespread and more than 7 km thick (Ruppel and Lopez, 1984), but, east of the Grasshopper decollement, outcropping Belt strata, which are also Missoula Group, are restricted to a small area in the core of an anticline near Argenta, Mont. ( fig.…”

Section: Introductionmentioning

confidence: 99%

“…1). After Ruppel and Lopez (1984) correlated the outcropping rocks near Argenta with Garnet Range Formation, the youngest widespread Belt unit, the rocks could reasonably be assumed to represent only the uppermost part of a very thick Belt sequence, such as that on the Grasshopper plate, or they could be interpreted as part of a thin thrust-fault sliver (Brandon, 1984).…”

Section: Introductionmentioning

confidence: 99%

See 4 more Smart Citations

Cambrian(?), middle Proterozoic, and Archean rocks penetrated in a borehole near Argenta, Beaverhead County, Montana, and some paleogeographic and structural implications

Pearson¹

1996

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Section: Geologic Setting Of the Argenta Areamentioning

confidence: 99%

Section: Geologic Setting Of the Argenta Areamentioning

confidence: 99%

Section: Geologic Setting Of the Argenta Areamentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

Cambrian(?), middle Proterozoic, and Archean rocks penetrated in a borehole near Argenta, Beaverhead County, Montana, and some paleogeographic and structural implications

Pearson¹

1996

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Syncompressional Emplacement of the Boulder and Tobacco Root Batholiths (Montana‐USA) by Pull‐apart along Old Fault Zones

Schmidt

Smedes²,

MichaelO'Neill

1990

Geological Journal

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The 15 major plutons comprising the 6000 km' Late Cretaceous Boulder batholith were emplaced at mid-to upper-crustal levels (<20 km) during active thrusting in the Montana thrust belt. The Tobacco Root batholith, the largest satellite pluton of the Boulder batholith ( -300 km') was emplaced into uplifted Archean basement rocks immediately southeast of the frontal margin of the thrust belt.Emplacement of the Boulder batholith and its satellite plutons was controlled principally by three long-lived and deeply penetrating regional fault sets. Two of these sets (a NE-trending set and an E-trending set) appear to have controlled the emplacement of the main mass of the batholith (Butte pluton). A third (NW-trending) fault set controlled the emplacement of the Tobacco Root batholith and many of the smaller plutons of the Boulder batholith. The intersecting fault sets give the Butte pluton a distinctly rhomboid shape, elongate in a northeasterly direction. Geophysical data suggest that the pluton may have a flat floor at the level cf the basal decollement of the thrust belt ( -17 km).One mode of pluton emplacement consistent with a rhomboid pattern and with B compressional tectonic setting is the filling of the pull-apart regions of shear zones. We propose that the main mass of the Boulder batholith was emplaced along a pull-apart within a segment of a thin-skinned thrust sheet during ENE translation in the thrust belt. Sinistral slip on the E-trending faults at the north and south margins allowed the rhomboid-shaped cavity to form within the thrust sheet above the basal dkcollement and to progressively fill with magma below a roof of cogenetic volcanic rocks.Late Cretaceous movement on the NW-trending fault set was oblique with roughly equal components of sinistral slip and reverse slip. The main mass of the Tobacco Root batholith is elongate in a northwesterly direction between two of these faults and has a crudely rhomboid map pattern. We suggest that it, too, was emplaced by filling a sinistral pull-apart between these faults.

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Late cretaceous and tertiary history and the dynamic crushing of cobbles, black butte area, southwestern Montana

Gutmann

Pushkar

McKenna

1989

Engineering Geology

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The thrust belt in Southwest Montana and east-central Idaho

Cited by 34 publications

References 14 publications

Cambrian(?), middle Proterozoic, and Archean rocks penetrated in a borehole near Argenta, Beaverhead County, Montana, and some paleogeographic and structural implications

Cambrian(?), middle Proterozoic, and Archean rocks penetrated in a borehole near Argenta, Beaverhead County, Montana, and some paleogeographic and structural implications

Syncompressional Emplacement of the Boulder and Tobacco Root Batholiths (Montana‐USA) by Pull‐apart along Old Fault Zones

Late cretaceous and tertiary history and the dynamic crushing of cobbles, black butte area, southwestern Montana

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