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Kinematics and timing of exhumation of metamorphic core complexes along the Lewis and Clark fault zone, northern Rocky Mountains, USA

By
David A. Foster
David A. Foster
Department of Geological Sciences, PO Box 112120, University of Florida, Gainesville, Florida 32611, USA
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P. Ted Doughty
P. Ted Doughty
Department of Geology, Eastern Washington University, Cheney, Washington 99004, USA
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Thomas J. Kalakay
Thomas J. Kalakay
Earth and Environmental Sciences, Rocky Mountain College, Billings, Montana 59102, USA
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C. Mark Fanning
C. Mark Fanning
Research School of Earth Sciences, Australian National University, Canberra ACT 0200, Australia
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Samuel Coyner
Samuel Coyner
Department of Geological Sciences, PO Box 112120, University of Florida, Gainesville, Florida 32611, USA
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Warren C. Grice
Warren C. Grice
Department of Geological Sciences, PO Box 112120, University of Florida, Gainesville, Florida 32611, USA
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James Vogl
James Vogl
Department of Geological Sciences, PO Box 112120, University of Florida, Gainesville, Florida 32611, USA
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Published:
January 01, 2007

The Priest River, Clearwater, Bitterroot, and Anaconda metamorphic core complexes of the northern Rocky Mountains were exhumed in Eocene time by crustal extension, which was linked via dextral displacement on the Lewis and Clark fault zone. Detailed geochronology and thermochronology (U-Pb, 40Ar/39Ar, and fission-track) from the Bitterroot complex indicates that extension started at 53 ± 1 Ma and continued until after 40 Ma. New U-Pb zircon and 40Ar/39Ar data from the Anaconda complex and published geochronology from the Priest River complex indicate a similar timing for the onset of major extension and exhumation. 40Ar/39Ar data from the Clearwater complex, which formed within a relay between strike-slip splays of the Lewis and Clark fault zone, are consistent with exhumation during the same time span. The Lewis and Clark fault zone separates ENE-directed extension in the Priest River complex from ESE-directed extension in the Bitterroot and Anaconda complexes. Large-scale extension was transferred eastward on the south side of this fault zone, where stretching lineations in core complex mylonites are oriented ∼104°–110° and coincide with the general trend of the transcurrent faults. Extension and exhumation of middle crustal rocks along the Lewis and Clark fault zone was concentrated in areas that also experienced voluminous Eocene midcrustal magmatism. Extension was probably initiated by a change in plate boundary conditions combined with the rapid influx of heat from the asthenosphere as a slab window opened beneath the western Cordillera, which led to collapse of the Cordilleran orogenic wedge and widespread early Eocene magmatism.

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Contents

GSA Special Papers

Exhumation Associated with Continental Strike-Slip Fault Systems

Sarah M. Roeske
Sarah M. Roeske
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Alison B. Till
Alison B. Till
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David A. Foster
David A. Foster
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James C. Sample
James C. Sample
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Geological Society of America
Volume
434
ISBN print:
9780813724348
Publication date:
January 01, 2007

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