Oligocene and Miocene metamorphism, folding, and low-angle faulting in northwestern Utah
An area of 3,000 km 2 in and around the Grouse Creek Mountains and the Raft River Mountains exposes Precambrian, Paleozoic, and Triassic sedimentary rocks that were folded several times and displaced tens of kilometres on low-angle faults. Overturned folds and local imbrications indicate transport westward and northward during two episodes of metamorphic deformation and transport eastward after metamorphism. Metamorphic grade increases downward in the allochthonous sheets and autochthon and increases westward in the autochthon. Mineral grains are flattened into the horizontal plane, and shear strains increase upward, suggesting that the deformations were caused by gravity acting on a broadly heated dome. Rb-Sr dating of granitic plutons affected by the deformations indicates that (1) the area is underlain by adamellite, about 2.5 b.y. old, in which deformation decreased progressively downward; (2) the first metamorphic deformation probably ended before 38.2 ± 2.0 m.y. ago; and (3) the second metamorphic deformation was still underway 24.9 ± 0.6 m.y. ago. High-grade allochthonous rocks that lie on low-grade parts of the autochthon indicate as much as 30 km of eastward transport after metamorphism. Parts of the dome sagged to form broad basins 12 m.y. ago, and the coarse sediments and tuffs that accumulated in them were overrun by allochthonous sheets measuring at least 11 by 19 km. Two Rb-Sr mineral isochrons and several fission-track ages indicate that some parts of the area cooled below 400 °C only 10 m.y. ago.