Bitterroot and Anaconda Core Complexes: Cretaceous Ductile Flow and Eocene Detachment Faulting in the Northern U.S. Rocky Mountains Defined by Ar/Ar Thermochronology

Foster, David1

1Department of Geological Sciences, University of Florida, Gainesville, United States

The Bitterroot and Anaconda metamorphic core complexes of western Montana and central Idaho, U.S.A. were exhumed by Eocene extensional detachment faulting between about 53 and 38 Ma.  The rocks in the lower plates of these core complexes include highly sheared Proterozoic to Palaeozoic metasediments, Cretaceous granitoids, and Eocene felsic plutonic rocks. Early exhumation of the core rocks occurred after crust of in the Montana hinterland was dramatically thickened (forming the “Montanaplano”) between about 130 and 80 Ma. By Late Cretaceous time the middle crust of the orogenic pile was plastically deforming, resulting in large-scale nappes and shear zones. Voluminous intermediate to felsic igneous rocks intruded the shear zones and formed sheet-like plutons at thrust ramps.  Plastic flow led to ductile thinning of the middle crust coincident with out-wedging in the Montana fold and thrust belt. Rocks exposed in these two core complexes experienced about 5-10 km of exhumation in the Late Cretaceous on the basis of metamorphic assemblages, reconstruction of sections, and thermochronology of upper plate rocks. Eocene extension began in the Northern Rockies about 53 Ma and resulted in linked extensional detachments and magmatism in both core complexes. The Bitterroot complex underwent as much as 15 km of additional exhumation along a ductile to brittle detachment system that initiated with amphibolite facies mylonite. The Anaconda complex underwent about 10-12 km in the deepest part and records green-schist facies mylonite overprinted by transitional brittle-ductile fabrics, and brittle deformation.  Ar/Ar mica cooling ages from both detachments decrease from west to east and constrain the rates of fault slip and unroofing. Exhumation of the middle crustal rocks was at least a two-stage process with significant Cretaceous-Paleogene thinning by ductile flow, followed by unroofing beneath the east-rooted detachment systems. Thermochronology data indicate that both detachment systems developed as composite structures with shallower crustal levels and less exhumation in the west rooting to about 10-km deeper to the east. The hanging wall of the Anaconda complex includes the less-deformed Cretaceous Boulder batholith and related volcanic rocks, while hanging wall rocks of the Bitterroot complex, in the Sapphire Range, record Cretaceous exhumation and more limited Eocene unroofing.


Professor David A. Foster is Chair of the Department of Geological Sciences at the University of Florida.  His research is focussed on applications of geochronology and thermochronology to Precambrian to Recent tectonics.

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