Character and origin of Precambrian fabrics and structures in the Tobacco Root Mountains, Montana

Harms, Tekla A.; Burger, H. Robert; Blednick, Daniel G.; Cooper, Jacob M.; King, Jack T.; Owen, D. R. J.; Lowell, J. David; Sincock, M. Jennifer; Kranenburg, Steven R.; Pufall, A.; Picornell, Carlos M.

doi:10.1130/0-8137-2377-9.203

Cited by 6 publications

(22 citation statements)

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“…Burger (1969) mapped a fault at the base of the Spuhler Peak Metamorphic Suite because of apparent discordance in structure and metamorphic grade between it and the quartzofeldspathic gneiss suite and because a high-strain zone is coincident with the contact. Our work does not substantiate Burger's original criteria: the grade of Big Sky metamorphism is consistent across all the metamorphic suites of the Tobacco Root Mountains (Cheney et al, 2004a, this volume, Chapter 6); the discordant geometry of Indian Creek Metamorphic Suite folds along the contact may be only an apparent truncation; and high-strain zones are present both along and away from the Spuhler Peak Metamorphic Suite contact (Harms et al, 2004, this volume, Chapter 10). Nevertheless, the Big Sky orogeny is a likely context for tectonic juxtaposition of the Spuhler Peak Metamorphic Suite and the quartzofeldspathic gneiss suite because it was accompanied by substantial simple shear strain across the Tobacco Root Mountains (Harms et al, 2004, this volume, Chapter 10).…”

Section: Origin Of the Spuhler Peak Metamorphic Suitementioning

confidence: 76%

“…Originally there probably were more crosscutting dikes than are seen today, as many have been transposed to layer-parallel geometries by Big Sky deformation (Harms et al, 2004, this volume, Chapter 10).…”

Section: Origin Of the Metamorphosed Mafi C Dikes And Sillsmafi C Magmentioning

confidence: 96%

“…MMDS bodies cut well-developed and folded gneissic fabrics in both the Indian Creek and Pony-Middle Mountain Metamorphic Suites (Brady et al, 2004a, this volume, Chapter 5;Harms et al, 2004, this volume, Chapter 10)-metamorphic fabrics that must, consequently, be older than the 2.06 Ga intrusive age of the MMDS suite , this volume, Chapter 9) and have been of high enough grade to survive Big Sky metamorphism at 1.78 Ga. Because it requires the fewest separate metamorphic events, we propose that the 2.45 Ga monazite grew during the development and deformation of the pre-MMDS gneissic banding. This correlation, however, is not strictly required by the data in hand.…”

Section: Cryptic Tectonothermal Event At 245 Gamentioning

confidence: 97%

“…Low-discordance, low-Th/ U, metamorphic zircon recovered from leucosome gives an age of 1.77 Ga (U-Pb SHRIMP dating) for this stage of the orogeny , this volume, Chapter 9). The localization of this leucosome in dilatant zones associated with boudins and folds, the latter of which, in particular, characterize mesoscopic deformation in the Tobacco Root Mountains, links the time of deformation to the maximum temperature and pressure phase (M1) of the Big Sky orogeny (Harms et al, 2004, this volume, Chapter 10; Mueller et al, 2004, this volume, Chapter 9). Mesoscopic folds are part of a pervasive structural pattern of north-dipping fabric, north-plunging lineation, and a map-scale sheath fold of the Spuhler Peak Metamorphic Suite, fl anked by the Indian Creek and Pony-Middle Mountain Metamorphic Suites, which we interpret to be the result of intense, regional, simple shear (Harms et al, 2004, this volume, Chapter 10) that accompanied crustal thickening and M1 metamorphism.…”

Section: The Big Sky Orogenymentioning

confidence: 98%

“…The localization of this leucosome in dilatant zones associated with boudins and folds, the latter of which, in particular, characterize mesoscopic deformation in the Tobacco Root Mountains, links the time of deformation to the maximum temperature and pressure phase (M1) of the Big Sky orogeny (Harms et al, 2004, this volume, Chapter 10; Mueller et al, 2004, this volume, Chapter 9). Mesoscopic folds are part of a pervasive structural pattern of north-dipping fabric, north-plunging lineation, and a map-scale sheath fold of the Spuhler Peak Metamorphic Suite, fl anked by the Indian Creek and Pony-Middle Mountain Metamorphic Suites, which we interpret to be the result of intense, regional, simple shear (Harms et al, 2004, this volume, Chapter 10) that accompanied crustal thickening and M1 metamorphism. The higher-pressure-higher-temperature (M1) arc of the Big Sky metamorphic path was followed by differential reequilibration at lower temperatures and pressures (M2) via nearly isobaric cooling (from ~750 to 700 °C at ~0.7-0.8 GPa), leading to isothermal decompression (from ~0.7 to <0.6 GPa at ~700 °C) (Cheney et al, 2004a, this volume, Chapter 6).…”

Section: The Big Sky Orogenymentioning

confidence: 98%

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Advances in the geology of the Tobacco Root Mountains, Montana, and their implications for the history of the northern Wyoming province

Harms

Brady²,

Burger³

et al. 2004

Precambrian Geology of the Tobacco Root Mountains, Montana

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Integrated studies by Keck Geology Consortium participants have generated many new insights into the Precambrian geology of the Tobacco Root Mountains.We have clarifi ed the tectonic setting and origin of two suites of metamorphic rocks: (1) a quartzofeldspathic gneiss complex with associated metasupracrustal rocks (the combined Indian Creek and Pony-Middle Mountain Metamorphic Suites) that originated in a continental arc setting between 3.35 and 3.2 Ga with subsequent sedimentation and (2) mafi c metavolcanic rocks with intercalated metasedimentary rocks (the Spuhler Peak Metamorphic Suite) from a suprasubduction zone ophiolite or backarc basin possibly of Proterozoic age. A poorly preserved metamorphic event at 2.45 Ga affected the former but not the latter, as did the intrusion of rift-related mafi c dikes and sills at 2.06 Ga. Both suites were amalgamated, metamorphosed to at least upper amphibolite facies, subjected to simple shear strain and folded into map-and outcrop-scale sheath folds, and tectonically unroofed during the period 1.78 to 1.71 Ga. We name this event the Big Sky orogeny.The Proterozoic geology of the Tobacco Root Mountains can be integrated with coeval features of the geology of the northern Wyoming province to outline a northeast-trending, southeast-vergent belt as the Big Sky orogen. The Big Sky orogen consists of a metamorphic hinterland fl anked to the southeast by a foreland of discrete ductile shear zones cutting older basement, and to the northwest by arc-related metaplutonic bodies and the trace of a fossil subduction zone in the upper mantle. Archean blocks to the north of the Big Sky orogen may have been accreted as allochthonous terranes during collision and convergence.The remarkable synchroneity of collision along the Big Sky orogen with tectonism in the Trans-Hudson orogen along the eastern margin of the Wyoming province and in the Cheyenne belt to the south of the province raise profound but unanswered questions about the process by which the Wyoming province was added to the rest of the ancestral North American craton.

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Section: Origin Of the Spuhler Peak Metamorphic Suitementioning

confidence: 76%

Section: Origin Of the Metamorphosed Mafi C Dikes And Sillsmafi C Magmentioning

confidence: 96%

Section: Cryptic Tectonothermal Event At 245 Gamentioning

confidence: 97%

Section: The Big Sky Orogenymentioning

confidence: 98%

Section: The Big Sky Orogenymentioning

confidence: 98%

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Advances in the geology of the Tobacco Root Mountains, Montana, and their implications for the history of the northern Wyoming province

Harms

Brady²,

Burger³

et al. 2004

Precambrian Geology of the Tobacco Root Mountains, Montana

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Orogens of Big Sky Country: Reconstructing the Deep‐Time Tectonothermal History of the Beartooth Mountains, Montana and Wyoming, USA

et al. 2023

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Thank you, Miranda Grover, for being the most supportive person in the world. I would have certainly lost weight and likely sanity without your nurturing love, understanding, and latenight leftover deliveries. I am grateful to my friend Caden Howlett-every discussion with you leaves me with a renewed enthusiasm for the mysteries of the natural world. Thank you to Stuart Parker, Mariah Romero, Sophie Black, and other student researchers for serving as sources of geologic inspiration. I am indebted to my undergraduate and graduate professors for sharing their knowledge and excitement with me. I am grateful to Dr. Colleen Elliot, Katie McDonald, and others at the MBMG for sharing their knowledge of Montana geology. I am also thankful to Chris Kussmaul, Dom D'Amato, Chris Baird, and others who assisted me with field work. I express immense love and appreciation for my mother and father-Amby and Kyle Ronemus-to whom I owe the entirety of my curiosity. Thank you for your support, encouragement, and understanding unceasing since the day I was born. Finally, this project would not have been possible without financial support from Montana State University, the MSU Department of Earth Sciences and the Tobacco Root Geological Society.

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Paleoproterozoic Metamorphism in the Northern Wyoming Province: Implications for the Assembly of Laurentia

Mueller¹,

Burger²,

Wooden³

et al. 2005

The Journal of Geology

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U-Pb ages measured on zircons from the Tobacco Root Mountains and monazite from the Highland Mountains indicate that the northwestern Wyoming province experienced an episode of high-grade metamorphism at ∼1.77 Ga. Leucosome emplaced in Archean gneisses from the Tobacco Root Mountains contains a distinctive population of zircons with an age of 1.77 Ga but also contains zircons to ∼3.5 Ga; it is interpreted to have been derived primarily by anatexis of nearby Archean schist. A granulite facies mafic dike that cuts across Archean gneissic banding in the Tobacco Root Mountains contains two distinct populations of zircons. A group of small (!50 mm) nonprismatic grains is interpreted to be metamorphic and yields an age of 1.76 Ga; a group of slightly larger prismatic grains yields an age of 2.06 Ga, which is interpreted to be the time of crystallization of the dike. Monazite from a leucogranite from the Highland Mountains yields a well-defined age of 1.77 Ga, which is interpreted as the time of partial melting and emplacement of the leucogranite. These results suggest that the northwestern Wyoming province, which largely lies within the western part of the Great Falls tectonic zone, experienced a metamorphic maximum at 1.77 Ga. This age is ∼100 m.yr. younger than the proposed time of Wyoming-Hearne collision in the central Great Falls tectonic zone (1.86 Ga) and suggests that the northwestern Wyoming province may have been involved in a separate, younger collisional event at ∼1.77 Ga. An event at this time is essentially coeval with collisions proposed for the eastern and southeastern margins of the province and suggests a multiepisodic model for the incorporation of the Wyoming craton into Laurentia.

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Character and origin of Precambrian fabrics and structures in the Tobacco Root Mountains, Montana

Cited by 6 publications

References 0 publications

Advances in the geology of the Tobacco Root Mountains, Montana, and their implications for the history of the northern Wyoming province

Advances in the geology of the Tobacco Root Mountains, Montana, and their implications for the history of the northern Wyoming province

Orogens of Big Sky Country: Reconstructing the Deep‐Time Tectonothermal History of the Beartooth Mountains, Montana and Wyoming, USA

Paleoproterozoic Metamorphism in the Northern Wyoming Province: Implications for the Assembly of Laurentia

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