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Giant folds and complex faults in Mesoproterozoic Lemhi strata of the Belt Supergroup, northern Beaverhead Mountains, Montana and Idaho

By
Jeffrey D. Lonn
Jeffrey D. Lonn
Montana Bureau of Mines and Geology, Montana Tech, Butte, Montana 59701, USA
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Russell F. Burmester
Russell F. Burmester
Idaho Geological Survey, University of Idaho, Moscow, Idaho 83844, USA, and Geology Department, Western Washington University, Bellingham, Washington 98225, USA
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Reed S. Lewis
Reed S. Lewis
Idaho Geological Survey, University of Idaho, Moscow, Idaho 83844, USA
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Mark D. McFaddan
Mark D. McFaddan
Idaho Geological Survey, University of Idaho, Moscow, Idaho 83844, USA, and Geology Department, North Idaho College, Coeur d'Alene, Idaho 83814, USA
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Published:
September 01, 2016

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.

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GSA Special Papers

Belt Basin: Window to Mesoproterozoic Earth

John S. MacLean
John S. MacLean
Department of Physical Science, Southern Utah University, 351 W. University Boulevard, Cedar City, Utah 84720, USA
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James W. Sears
James W. Sears
Department of Geosciences, University of Montana, 32 Campus Drive #1296, Missoula, Montana 59812-1296, USA
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Geological Society of America
Volume
522
ISBN print:
9780813725222
Publication date:
September 01, 2016

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