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.

Fault rocks can be studied by charting how undeformed rocks near a fault transform into mylonitic or cataclastic tectonites, or by examining rock masses at different points along a fault to determine how changes in temperature, pressure, etc. affected the fault’s history. Both approaches have merit in thrust belts because thrust faults form under a range of conditions and may evolve along several different paths. Using the first approach, we distinguish two fault zone types analogous to Means’ (1984) two types of shear zones: Type I fault zones grow in thickness as movement on the fault increases; Type II fault zones initiate as zones of localized deformation, and deformation becomes further localized as displacement increases. Both Type I and Type II fault zones occur in the Appalachian fold-and-thrust belt. The second approach shows that fault rocks from the thrust zone beneath the southern Appalachian Blue Ridge and that beneath the Bay of Islands ophiolite evolved in similar ways, despite differences in rock types and local structural history. Three conclusions emerge from our survey of fault rocks from thrust faults: (1) rocks from both external and internal thrust zones may deform by fracturing or by plastic flow, and may alternate between those modes as local physical conditions change; (2) fault zones with large displacement nearly always weaken with continued displacement; (3) fluid phases are critically important to the softening processes, which accommodate large displacements in both external and internal thrust zones.

Precambrian crystalline basement of the Appalachian Blue Ridge deforms inhomogeneously by developing relatively narrow ductile deformation zones (DDZs). The Paleozoic sedimentary cover develops open to tight folds and penetrative fabrics. A transition between these two styles occurs at the base of the sedimentary cover in the Early Cambrian Chilhowee quartzites of the central Appalachians and in the Late Proterozoic arkosic sandstones of the southern Appalachians. On a mesoscopic scale, the transition zone sediments show tight to isoclinal folds with highly deformed overturned limbs analogous to mesoscopic (1 cm to 10 m wide) DDZs in crystalline basement. Deformation zones in the basement cut across the basement/cover contact and feed into the overturned limbs of tight folds. On a microscopic scale, both arkoses and granitic basement rocks show thin (5 mm) DDZs characterized by grain-size reduction and alteration of feldspars to quartz and mica. The actual style and symmetry of deformation varies with metamorphic grade, proximity to major thrust faults, and amount of tectonic shortening. In the Grandfather Mountain area of the southern Blue Ridge Province, sets of low-dipping DDZs close to major thrust faults approximate a simple shear deformation field. In the central Appalachians of northern Virginia, similar simple shear deformation features are observed close to major thrust faults, but sets of DDZs define a flattening plane perpendicular to tectonic transport direction higher up within the thrust sheets.

Basement constitutes rocks which belong to a previous orogenic cycle which have been reactivated and incorporated into a younger cycle. Basement massifs may be classified according to their relative position in an orogen as external or internal massifs. They may also be categorized according to their role in deformation, as thrust-related, fold-related and composite massifs. All Appalachian external massifs were transported following removal from the overridden edge of the ancient North American continental margin. Most of the internal massifs are also probably transported, but several (Pine Mountain and Sauratown Mountains) may exist as windows exposing parauthochthonous basement beneath the main thrust sheet. The latter reside immediately west of the low (west) to high (east) gravity gradient which probably outlines the old edge of Grenvillian crust. Reactivated crustal material generated during early Paleozoic orogeny plays the same mechanical role in reactivation as basement from the previous Grenville cycle. Basement (Grenville) massifs are distributed throughout the western Blue Ridge from Georgia to Maryland. Additionally, internal massifs are also present (Pine Mountain belt, Tallulah Falls and Toxaway domes, Sauratown Mountains anticlinorium, State Farm Gneiss dome, Baltimore Gneiss domes, and Mine Ridge anticlinorium). Basement internal massifs probably served to localize thrusts by causing them to detach and ramp over and around the massifs. Their antiformal shape may in part be as much related to thrust mechanics as to folding.

Within the southern and central Appalachians, Grenville-age basement rocks are found in major massifs in the Blue Ridge and Sauratown Mountains anticlinoria and in the vicinity of the Grandfather Mountain window. These massifs are, respectively, Pedlar and Lovingston Massifs in the Blue Ridge anticlinorium, Sauras Massif in the Sauratown Mountains anticlinorium, and Watauga, Globe, and Elk River Massifs near the Grandfather Mountain window. In central Virginia the Lovingston Massif is juxtaposed against the Pedlar Massif, and in northwestern North Carolina-southwestern Virginia, the Elk River Massif is thrust over the Globe and Watauga Massifs, all along faults of the Fries fault system, which includes the Rockfish Valley, Fork Ridge, Devil’s Fork, and Linville Falls faults, as well as the Fries fault per se . The Pedlar Massif is a deeper granulite facies country-rock terrane intruded by charnockite plutonic suites. The Lovingston Massif primarily is a shallower granulite/amphibolite facies terrane intruded by biotite dioritoid plutonic suites containing bodies of charnockite. Country rocks of the Watauga Massif were subjected to metamorphic conditions similar to those of the Lovingston Massif, but were intruded by a plutonic suite of biotite dioritoid, biotite granitoid, and granitoid. The Elk River, Globe, and Sauras Massifs all are terranes metamorphosed to amphibolite facies and intruded by granitoid/dioritoid suites containing some porphyritic biotite dioritoid phases. A suite of late Precambrian (post-Grenville) peralkaline granitoid plutons intruded all of the massifs except the Pedlar. These plutons presumably are related to upper Precambrian volcanic rocks that were associated with a rifting environment and that were later metamorphosed and deformed along with overlying sedimentary rocks to form part of the Appalachian orogenic belt.