CRETACEOUS SYNOROGENIC AND EOCENE POST-OROGENIC EXTENSION IN THE SEVIER HINTERLAND, SOUTHWESTERN MONTANA

Recent geologic mapping and geochronology in the Anaconda and Flint Creek Ranges of SW Montana suggest that extensional features developed during regional crustal contraction in the late Cretaceous, followed by extensional tectonism in the Eocene. The area lies within the Sevier thrust belt hinterland, the Eocene Anaconda metamorphic core complex footwall, and the northern Basin and Range province. Exposed rocks are complexly folded greenschist- to amphibolite-grade Mesoproterozoic to Mesozoic metasedimentary rocks intruded by voluminous granitic plutons ranging in age from 80 to 50 Ma. Crustal thickening and intrusion drove metamorphism prior to 75 Ma. Earlier high pressure metamorphism produced kyanite and is overprinted by a high temperature-low pressure metamorphic event. During metamorphism, the nearly flat-lying >12-km-thick sedimentary section was tectonically thinned by an array of bedding-parallel structures such as concordant mylonitic shear zones that cut out stratigraphic section, zones of vertical shortening that thin the units through coaxial plastic flow, and brittle bedding-parallel faults that place younger units over older units. Sparse shear-sense indicators show top-west movement. The age of these structures is constrained by 75 Ma cross-cutting plutons; older intrusions developed strain fabrics parallel to those in the country rocks. Because the bedding-parallel structures omit section throughout the entire 12-km-thick sedimentary sequence, they are attributed to late Cretaceous synorogenic extension in the Sevier hinterland rather than to deformation along thrust faults. Repetition of these structures by thrust faults confirms that a convergent setting persisted until the Paleocene. These rocks were then deformed into spectacular west-verging overturned folds, and refolded into upright NNE-trending folds. Continued intrusion of granitic rocks from 75-62 Ma accompanied the folding. Crustal extension replaced contraction in the Eocene, producing the Anaconda core complex. Footwall rocks have been exhumed from depths of 12-18 km beneath the top-east detachment fault, but late Cretaceous extension may have been responsible for some exhumation. The contrasting Cretaceous structures are best explained by mid-crustal plastic flow processes.