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Polyphase collapse of the Cordilleran hinterland: The Anaconda metamorphic core complex of western Montana—The Snoke symposium field trip

By
Thomas J. Kalakay
Thomas J. Kalakay
Department of Geology, Rocky Mountain College, 1511 Poly, Billings, Montana 59102, USA
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David A. Foster
David A. Foster
Department of Geological Sciences, University of Florida, Gainesville, Florida 32611, USA
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Jeffrey D. Lonn
Jeffrey D. Lonn
Montana Bureau of Mines and Geology, 1300 West Park Street, Butte, Montana 59701, USA
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Published:
January 01, 2014

Abstract

The Anaconda and Bitterroot metamorphic core complexes are located in western Montana, along the eastern edge of the Cordilleran hinterland. This multi-tiered extensional terrain contains exceptional exposures that collectively exhibit a crustal cross section through orogenic continental crust (i.e., middle through upper crust). The core complex footwall rocks consist of Late Cretaceous arc-related plutons and Eocene granitic plutons intruded into deformed and metamorphosed Midproterozoic Belt Supergroup and Paleozoic to Cretaceous shelf-platform strata. Late Cretaceous shear zones and folds dominate footwall structure, representing significant thinning of the stratigraphic section. Eocene detachments, mylonites, and plutonic suites distinctly overprint the Late Cretaceous structures. A stark example of this Eocene overprint is the Anaconda detachment, which resulted in eastward translation of the Late Cretaceous, arc-related Boulder batholith. This field trip will cover a transect through the Anaconda core complex from the Philipsburg valley to Butte, Montana. Field trip participants will examine key locations that clarify the distinction between the timing and structural style of Late Cretaceous crustal thickening and/or collapse features versus those related to Eocene core complex development.

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Contents

GSA Field Guide

Exploring the Northern Rocky Mountains

Colin A. Shaw
Colin A. Shaw
Department of Earth Sciences Montana State University Bozeman, Montana 59717, USA
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Basil Tikoff
Basil Tikoff
Department of Geoscience University of Wisconsin 1215 W. Dayton Street Madison, Wisconsin 53706, USA
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Geological Society of America
Volume
37
ISBN electronic:
9780813756370
Publication date:
January 01, 2014

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