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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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