Mapping on the west side of the Raft River-Grouse Creek core complex has shown that Upper Permian, Triassic, and Middle to Late Miocene rocks compose an extensive allochthonous sheet that was displaced laterally as well as down-section onto the complex. The sheet is underlain locally by two thin allochthonous sheets of Permian rocks that were displaced at about the same time. That all these sheets moved either northeastward or southwestward is suggested by the trends of fold hinge-lines and by the strike of strike-slip faults confined to the sheets. The Miocene sequence is at least 2000 m thick and consists of poorly sorted fluvial conglomerate intercalated with numerous thick beds of silicic vitric tuff and smaller amounts of monolithologic breccia, lacustrine limestone, basalt, and intrusive as well as extrusive rhyolite. K-Ar dates on a basalt flow and a rhyolite plug show that the Triassic and part of the Permian rocks were infaulted with the Miocene rocks between 14.4 and 11.7 m.y. ago, and that the composite sheet was displaced later than 11.7 m.y. ago.

Solid-state flow during metamorphism was studied in a 300-km 2 exposure where Elba Quartzite forms the upper part of an autochthon and is overlain by two major allochthonous sheets. The quartzite is very locally thrown into north-verging recumbent folds, the largest having a wavelength of 1 km. Fold axes and elongate metamorphic grains trend approximately east, and foliation is subhorizontal. A vertical sequence of samples was collected from the large fold, and a second sequence was collected from unfolded quartzite 4 km away. Axes of strain ellipsoids measured in five quartzites from the unfolded sequence are close to 0.5:1.1:1.7 (assuming an original sphere of rádius one). Five samples from the fold have ellipsoid axes ranging from 0.4:1.1:2.5 at the top of the sequence to 0.2:1.4:4 near the base. Most quartz grains deformed plastically without recrystallizing and developed strong c -axis fabrics. The degree of orientation of both quartz c axes and muscovite plates increased with increasing strain. Most of the quartz fabrics have orthorhombic symmetry and cross-girdle patterns like fabrics produced experimentally by J. Tullis and computer-simulated by G. S. Lister and coworkers, for quartzites extended in plane strain. The results thus indicate a gradual east-west extension and consequent flattening by the force of gravity. Perhaps the fold formed concurrently where material flowing laterally under a broad dome was arrested locally and thus forced to buckle. Dating of nearby granite bodies indicates that the deformation began in Eocene time and continued until Miocene time.