Skip to Main Content
Book Chapter

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

By
Tekla A. Harms
Tekla A. Harms
1
Department of Geology, Amherst College, Amherst, Massachusetts 01002, USA
Search for other works by this author on:
John B. Brady
John B. Brady
2
Department of Geology, Smith College, Northampton, Massachusetts 01063, USA
Search for other works by this author on:
H. Robert Burger
H. Robert Burger
2
Department of Geology, Smith College, Northampton, Massachusetts 01063, USA
Search for other works by this author on:
John T. Cheney
John T. Cheney
3
Department of Geology, Amherst College, Amherst, Massachusetts 01002, USA
Search for other works by this author on:
Published:
January 01, 2004

Integrated studies by Keck Geology Consortium participants have generated many new insights into the Precambrian geology of the Tobacco Root Mountains. We have clarified 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) mafic 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 mafic 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 flanked to the southeast by a foreland of discrete ductile shear zones cutting older basement, and to the northwest by arc-related meta-plutonic 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.

You do not currently have access to this article.

Figures & Tables

Contents

GSA Special Papers

Precambrian Geology of the Tobacco Root Mountains, Montana

John B. Brady
John B. Brady
Search for other works by this author on:
H. Robert Burger
H. Robert Burger
Search for other works by this author on:
John T. Cheney
John T. Cheney
Search for other works by this author on:
Tekla A. Harms
Tekla A. Harms
Search for other works by this author on:
Geological Society of America
Volume
377
ISBN print:
9780813723778
Publication date:
January 01, 2004

GeoRef

References

Related

Citing Books via

Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal