Mesoscopic to macroscopic block-in-matrix structures are widely distributed in the Blue Ridge belt of the southern Appalachian orogen. The belt is subdivided into four tectonostratigraphic terranes: (1) the eastern Upper Proterozoic Toe terrane, consisting of metasedimentary rocks, metabasites (amphibolites), and ultramafic rocks; (2) a western, Middle Proterozoic, cratonic terrane, the Sherwood terrane, consisting of metamorphosed granitoid rocks and structurally overlying Upper Proterozoic to Paleozoic metasedimentary and sedimentary rocks; (3) the enigmatic, intervening Cullowhee terrane, lithologically similar to the Toe terrane, largely of unknown age but yielding a Middle Proterozoic age from one area in the north; and (4) a Middle to Upper Proterozoic terrane, the Grandfather terrane, exposed beneath the Toe and Sherwood terranes, in the Grandfather Mountain window. Block-in-matrix structures occur principally in the Toe and Cullowhee terranes. Block-in-matrix structures are formed by a variety of sedimentary, igneous, diapiric, and tectonic (including metamorphic) processes. Notably, such structures characterize mélanges and migmatites. In the southern Appalachian orogen, inasmuch as the Toe and Cullowhee terrane rocks are metamorphic, where block-in-matrix structures have been recognized in the past, they have generally been assigned a metamorphic-tectonic origin involving tensile or compressive, ductile, penetrative strain. Specifically, they have been considered to be migmatites or the blocks to be boudins of metamorphic rock in a metamorphic matrix. Metamorphosed mélanges are identified primarily on the basis of: (1) block-in-matrix structures (with included exotic lithologies) and (2) paleogeographic position in the orogen. The wide distribution, paleogeographic position, exotic ultramafic rocks, and pre-peak metamorphic fragmentation history of Toe and Cullowhee terrane rock units, which exhibit block-in-matrix structure, suggest that their protoliths could have been mélanges.

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.