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Unkinking the Lewis and Clark tectonic zone, Belt Basin, Idaho and Montana

Brian G. White
Brian G. White
National Institute for Occupational Safety and Health (NIOSH) Spokane Research Laboratory, 101 E. Theresa Drive, Coeur d'Alene, Idaho 83814, USA
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September 01, 2016

A succession of separate tectonic events affected Mesoproterozoic Belt Supergroup strata of NW Montana, just as in the better-displayed Coeur d'Alene Mining District of Idaho. A series of these established a tectonic zone historically known as the Lewis and Clark line, here re-identified as the Lewis and Clark tectonic zone, an apparent product of periodic reactivation of fundamental basement structures and physical constraint of a growth fault on developing folds. Six events identify a partial tectonic history of the west-central Belt Basin. The oldest produced growth faults concentrated along at least two structural lineaments, one of which, the Jocko line, substantially controlled the distribution of subsequent structures; the other, the north-trending Noxon line, is implicated in creation of metal-enriched source rock for Coeur d'Alene veins and provides a marker for right-lateral faulting within the Lewis and Clark tectonic zone. Subsequent deformation produced (1) west-northwest–trending folds, mostly confined to the Lewis and Clark tectonic zone and terminating northward against the Jocko line as the likely result of their having been compressed against this pre–Belt Basin structure; (2) north-trending regional folds, which extend southward from Canada and cross the ultimate Lewis and Clark tectonic zone; (3) foliated shear zones in the Lewis and Clark tectonic zone and associated Coeur d'Alene veins and reverse faults; (4) right-lateral, transcurrent faults, so identified by offsets of the Noxon line, north-trending regional folds, and the Montana overthrust belt and its associated foredeep basin; and, last, (5) Lewis and Clark tectonic zone normal faults and associated kink folds, which extensively reached gigantic, “megakink” proportions. These megakinks locally disrupted all prior structures, greatly confusing local structure; these need to be “unkinked,” so that structures resulting from prior tectonism may be fully recognized and correctly interpreted. Many faults of the Lewis and Clark tectonic zone trend southeasterly in its easterly part, tracking pre–Belt Basin structures separate from those associated with the Jocko line.

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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
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Publication date:
September 01, 2016



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