Late Cretaceous extension in the hinterland of the Sevier thrust belt, northwestern Utah and southern Idaho; with Suppl. Data 90-20

Cover rocks of the Raft River metamorphic core complex, located in the Sevier belt hinterland, preserve a structural and metamorphic history that predates the middle Tertiary extension of the region. In the eastern Raft River Mountains, Cambrian(?)-Permian rocks form two allochthons that occur in the hanging wall of the mid-Miocene Raft River detachment fault. Dramatically attenuated, metamorphosed Cambrian(?)-Pennsylvanian strata of the lower allochthon yield Late Cretaceous muscovite (super 40) Ar/ (super 39) Ar plateau ages (ca. 82-90 Ma) that are interpreted to date cooling during final stages of layer-parallel extension. Devonian-Permian rocks exposed in the Black Pine Mountains underwent east-west extension ( approximately 160%) associated with syntectonic growth of white mica along cleavage. (super 40) Ar/ (super 39) Ar whole-rock spectra of associated slates also indicate a Late Cretaceous metamorphism. Together, field and (super 40) Ar/ (super 39) Ar results suggest that Late Cretaceous extension occurred in the Sevier belt hinterland at the same time as shortening in the eastern foreland and at depth in the hinterland. Sufficient topography must have been present to drive upper-crustal extension in the eastern hinterland.