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.

Remnants of a Late Triassic continental margin and ocean basin are scattered across central and southern Alaska. Little is known about the fundamental nature of the margin because most remnants have not been studied in detail and a protracted period of terrane accretion and margin-parallel translation has disrupted original stratigraphic and structural relationships. Three new conodont collections were recovered from a sequence of Upper Triassic calcareous sedimentary rocks in the central Alaska Range. One of the three localities is north of the Denali fault system in an area previously thought to be underlain by an uninterrupted sequence of metamorphic rocks of the parautochthonous Yukon-Tanana terrane. Structural relations in the immediate vicinity of this conodont locality indicate that mid-Cretaceous(?) thrust faulting imbricated Paleozoic metaigneous rocks with the Triassic sedimentary rocks. This may reflect a closer pre-Cretaceous relationship between the Yukon-Tanana terrane and Late Triassic shelf and slope deposits than previously appreciated. Reexamination of existing conodont collections from the central Alaska Range indicates that Upper Triassic marine slope and basin rocks range in age from at least as old as the late Carnian to the early middle Norian. The conodont assemblages typical of these rocks are generally cosmopolitan and do not define a distinct paleogeographic faunal realm. One collection, however, contains Epigondolella multidentata sensu Orchard 1991c , which appears to be restricted to western North American autochthonous rocks. Although paleogeographic relations cannot be determined with specificity, the present distribution of biofaces within the Upper Triassic sequence could not have been the result of simple accordion-style collapse of the Late Triassic margin.