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Obduction of lower Paleozoic North American continental margin rocks during the Taconic orogeny led to the formation of extensive northern Appalachian mélange terranes. These include the mélanges associated with the Hamburg klippe in Pennsylvania, the classic Taconic mélanges of eastern New York, the foreland mélanges of Quebec, and the Companion and related mélanges of western Newfoundland. These mélanges are generally unmetamorphosed, although they do contain rare blocks of low- and medium-grade metamorphic rock. They are typified by an assemblage of flyschlike graywacke, siltstone, and argillite blocks within a phacoidally cleaved, fine-grained matrix. The dominant meso-scale deformation mechanisms are progressive shear fracturing and block rotation, accommodated at the grain scale by microshearing, grain-boundary sliding, extension fracturing and limited diffusive mass transfer. These processes occurred within a deformational environment that is inferred to have undergone both high strain rates and high degrees of noncoaxiality. Measured strain states are generally of the form S1 >> S2 > 1 > S3, with S1 trending parallel to the regionally deduced transport direction in areas that have escaped post-mélange formation deformations. The minimum deformation represented by phacoidal fabrics appears to involve about 100 percent layer-parallel extension and shear strains (γ max) greater than approximately three. Some extension parallel to regional strike (S2 >1) is noted in most mélange samples. This is accommodated at least in part by zones with prolate ellipsoid strain states in which strike-parallel shortening occurs, and probably also by larger scale structures such as lateral ramps. Tectonic dewatering may have accompanied the formation of some of the mélange units within this regional association. However, this can be shown to have been a noncritical factor in the general mélange deformational history, as both poorly consolidated and well-indurated (completely dewatered) flysch sequences were transformed into mélange at different points and at different times within the orogen, producing very similar end products. Fabric relationships within the Taconic orogen of eastern New York demonstrate that significant westward transport occurred after the development of the regional slaty cleavage, whereas in western Newfoundland the regional slaty cleavage postdates emplacement of the allochthonous terranes in roughly their present positions.

Comparison of mélange fabrics from the northern Appalachians and those of other orogenic belts suggests that a simple, consistent structural classification of rock types can be defined that differentiates among undeformed units, deformed but undisrupted units, olistostromes, and mélanges on the basis solely of foliation development and degree of stratal disruption. These are both readily recognizable field characteristics that are nongenetic yet process-oriented. In this classification, a mélange then becomes simply blocks within a phacoidally cleaved matrix.

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