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.

Recent geological mapping, geophysical studies, and deep drilling in the Raft River area, Idaho, have yielded information that is not consistent with fault-block development of the Raft River basin. Paleozoic and lower Mesozoic allochthonous rocks that occur in the surrounding Sublett, Black Pine, Albion, and Raft River Mountains do not occur beneath the Cenozoic basin fill deposits of the Raft River Valley, nor do Cenozoic volcanic rocks that form the adjacent Cotterel and Jim Sage Mountains. Range-front faults have not been identified along the margins of ranges flanking the Raft River Valley. Normal faults found in the Cotterel and Jim Sage Mountains are inferred to be concave-upward extensional structures that involve only Tertiary volcanic rocks and basin-filling sediments. Concave-upward faults within the Raft River basin have been identified in seismic reflection profiles. Fault displacement of the basement rocks beneath the Raft River Valley has not been documented. Structural development of the Raft River basin based on gravity-induced tectonic denudation of nearby metamorphic core complexes is suggested.

The Matlin Mountains, an area of low hills on the northern edge of the Great Salt Lake Desert, expose a rootless sequence of upper Paleozoic, lower Mesozoic, and upper Miocene rocks that have been affected by folds and by older-on-younger low-angle faults. The area lies 5 to 10 km east of the Grouse Creek Mountains, the southernmost range of a metamorphic core complex which extends from south-central Idaho into northwestern Utah. In this core complex, metamorphism that was accompanied by intrusion, younger-on-older low-angle faulting, recumbent folding, and horizontal extension of rock units did not end until Miocene time. The Matlin Mountains expose the coarse Tertiary sedimentary rocks and the displaced sheets that were shed from uplifts that followed metamorphism and intrusion in the core complex. These structurally interlayered sedimentary deposits and rock sheets overlie the zone of transition between metamorphically attenuated Paleozoic rocks in the core complex and unmetamorphosed, undeformed Paleozoic formations of normal thickness less than 15 km to the east. The Matlin Mountains consist of two structurally distinctive terranes: an eastern rooted terrane in which Miocene(?) sedimentary rocks unconformably overlie upper Paleozoic carbonate rocks, and a western displaced terrane that is composed of five brecciated displaced sheets of pre-Tertiary rocks, each resting upon Tertiary sedimentary rocks; the lowest sheet overlies the rooted terrane. All of the Tertiary sedimentary rocks contain abundant silicic volcanic ash probably derived from nearby volcanic centers. The displaced sheets strike north-northeast, are nearly flat-lying, and consist of steeply dipping, locally folded strata. Two major low-angle faults are inferred to divide the four lowest sheets into two superimposed composite plates, each of which has a unique lithologic, metamorphic, and structural character. The fifth and highest displaced sheet was emplaced after both of these plates were in their present position. The Miocene(?) sedimentary rocks and displaced sheets of the structurally lower composite plate probably were derived from an ancestral range in the site of the Grouse Creek Mountains and moved from west to east in late Miocene time. The higher plate may once have been continuous with a complex upper allochthon of late Miocene age that crops out widely on the west side of the Grouse Creek and Raft River Mountains. The source of the upper Miocene sedimentary rocks and the displaced sheets of the higher composite plate lay somewhere west of the Grouse Creek Mountains and the plate apparently travelled eastward at some time after 11.1 to 10.5 m.y. B.P. Ductile structures that formed during, and after, brecciation in the most completely exposed displaced sheet suggest that the sheets moved initially by slow creep under moderate load. In spite of locally intense brecciation, the orientation in the sheets of sedimentary and metamorphic structures that were inherited from the source areas remained remarkably consistent. In the Matlin Mountains, the sheets apparently moved over an eroded Tertiary surface and (or) into a depositional basin. Clastic dikes preserved in the brecciated base of one sheet suggest that fractures were propagated by relatively high fluid pressures derived from underlying sedimentary water, thus allowing the sheet to disintegrate in place (hydrofracturing). Locally, movement of the sheets was facilitated by thin layers of silicic volcanic ash and (or) tuffaceous lacustrine beds. Little is known about the source areas of the displaced sheets, but compositions, textures, and fabrics of the associated upper Miocene sedimentary rocks and the lithologies and geometries of the sheets point to the existence of postmetamorphic, broad domal uplifts whose locations shifted temporally and spatially but generally lay to the west, near the Utah-Nevada state line. The postmetamorphic low-angle faults in the core complex cut down-section to the Precambrian basement as well as carrying displaced sheets laterally for tens of kilometers. The presence of rugged highlands to the west suggests a gravitational sliding mechanism and the close spatial and temporal association of volcanic activity suggests that uplifts were genetically related to subjacent intrusions. The older-on-younger geometry of the Matlin sheets and their local tight folding and reverse faulting indicate that this area represents the distal, eastern toe of a distending complex allochthon to the west. Basin and Range high-angle faulting did not begin in the area until Pliocene time, after postmetamorphic uplifts and low-angle faulting had ceased.