Combined macroscopic to microscopic structural analyses, detailed geologic mapping, and thermochronology were used to study the structural evolution of midcrustal rocks of the Sevier orogenic belt hinterland in northwestern Utah. These results, when combined with previous studies, provide new insight into the structural history of this region, and suggest alternating tectonic contraction and extension during Mesozoic to early Cenozoic time.

Two allochthons form the upper plate of the Miocene Raft River detachment fault in the eastern Raft River Mountains. The lower allochthon comprises Neoproterozoic, Ordovician, and Pennsylvanian(?) strata, and is bounded below by the Raft River detachment fault and above by the middle detachment fault. Strata within the lower allochthon were dramatically attenuated by two episodes of ductile deformation. The first deformation (D1) took place at metamorphic temperatures of ≈500°C, and resulted in penetrative fabrics throughout these rocks that record combined flattening and top-to-northeast shearing strains. The second deformation (D2) resulted in significant stratigraphic attenuation along discrete top-to-the-west shear zones that are generally parallel to lithologic contacts. Separates of synkinematic muscovite from the penetrative fabric yield 40Ar/39Ar cooling ages that indicate that D1 deformation occurred prior to cooling ca. 90 Ma. Both fabrics were subsequently folded about (D3) kilometer-scale recumbent folds.

The areally extensive middle allochthon, composed chiefly of Pennsylvanian and Permian rocks metamorphosed in the greenschist facies, was emplaced (D4) along the low-angle middle detachment fault. This fault cuts across various structural levels of the recumbently folded lower allochthon in its footwall, and juxtaposes greenschist facies over amphibolite-facies metamorphic rocks. The lower and middle allochthon were subsequently deformed (D5) into open folds with north-trending axes. Neogene extension (D6) produced an ≈200-m-thick top-to-east ductile shear zone in Precambrian rocks, and formed the younger Raft River detachment fault, which forms the present upper contact of the ductile shear zone.

Northeast-vergent D1 fabrics probably record shortening deformation, on the basis of fabric correlations with the Grouse Creek and Albion mountains, deformation kinematics, and synkinematic prograde metamorphism. D2 attenuation faults have been interpreted to record crustal extension of Late Cretaceous age. If D3 recumbent folds developed during extension, then the deformation sequence records early shortening followed by protracted extensional unroofing with variable structural styles. The favored alternative is that D3 recumbent folds developed during shortening; in this case, D1 through D4 record episodic alternations of contraction and extension. Such alternations are consistent with observations from analog and theoretical models of contractional mountain belts and suggest that the hinterland of the Sevier orogenic belt underwent dynamic adjustments in crustal thickness and deformation kinematics in response to changes in the boundary conditions of the orogenic belt.

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