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.