The regional-scale salients and recesses in the Appalachian orogen are well accepted as being a product of tectonic inheritance. Smaller map-view curves are present in the slate belt of the Taconic allochthon, which lies between the New York recess and Quebec salient, and we investigate the possibility that these curves are also related to the geometry of the preexisting Iapetan rift margin. The orientation of the slaty cleavage (S 2 ) and mineral/stretching lineation (L 2 ) and the geometry of syntectonic fibers were used to identify along-strike differences in the nature of the main stage of deformation (D 2 ) in the slate belt. Where the axial traces of F 2 folds lie parallel to the overall trend of the Taconic allochthon, the strata are characterized by an approximately downdip L 2 , plane strain, monoclinic strain symmetry, and top-to-west-northwest noncoaxial flow. The strata also underwent noncoaxial flow where the axial traces of F 2 folds are oblique to the overall trend of the Taconic allochthon. However, L 2 rakes moderately from the south on S 2 , and syntectonic fibers record evidence for flattening strain, triclinic strain symmetry, top-to-northwest shear in the XZ plane, and sinistral shear in the XY plane. We infer that the deformation zone in which D 2 structures formed changed orientation along strike such that the region of obliquely oriented F 2 folds underwent transpression, and we suggest that the transpressional deformation was a result of reactivation of a northwest-striking Iapetan transform fault as an oblique ramp during the Taconic orogeny.

The Galice Formation is characterized by slaty cleavage, overturned tight-to-isoclinal folds having variable hingeline orientations, and a south–southeast-trending stretching lineation formed during the Nevadan orogeny. Calc-alkaline dikes and sills (151–146 Ma) that intruded the Galice Formation and its basement (Josephine ophiolite) are regionally metamorphosed, and some are deformed; however, some plutons of this age also overprint slaty cleavage, suggesting syntectonic intrusion. Amoeboid margins on some sills suggest intrusion began prior to lithification of the Galice Formation. Some dikes are intruded into pre-existing small thrust faults that predate the slaty cleavage. Dikes show a wide range of orientations, and poles to dikes are consistently oriented at a high angle to poles to extension veins and to the stretching lineation in the Galice Formation. Poles to dikes define two quadrants on an equal-area, lower-hemisphere projection separated by planes oriented at right angles. These planes are analogous to nodal planes of a fault-plane solution, and thus allow determination of P- and T- axes. Restoration of structures to their original (Nevadan) orientation results in the P - and T -axes, stretching lineations, poles to extension veins, poles to small syn-cleavage faults, and poles to cleavage all being essentially coplanar with the “movement plane” that strikes to the northwest and dips steeply. The “fault-plane solution” derived from dike orientations indicates northwest-southeast contraction, consistent with slip directions for most small faults having slickenfibers. A wide range of fold hingeline orientations and slip directions on small pre-cleavage faults, however, may record early west-directed shortening.

Three stratally disrupted chaotic units crop out within a horst in west-central Maine. These mélanges and olistostromes are below, within, and above a 2.4-km-thick sequence of ophiolitic metabasalt. Each is characterized by a predominantly pelitic matrix. Textures of the matrix range from slate through phyllite to very fine grained schist. The chaotic units are differentiated by the structures within the matrix, and the lithology and structures of the inclusions. The matrix of the Loon Stream mélange is characterized by slaty cleavage. The inclusions are metasandstones and metarhyolites. The metasandstone inclusions exhibit layer-parallel extension and layer-normal shears. This mélange is interpreted to have formed either through (1) slope failure related to volcanic activity associated with the overlying metabasalts, or (2) as a thrust zone associated with the transport and emplacement of the previously erupted metabasalt pile. The first implies that this mélange is an olistostrome, while the second implies a broken formation. Chaotic units within the Caucomgomoc Lake Formation are olistostromes. These are thin units (3 to 15 m) enclosed within a stratiform sequence of grain flow and turbidite origin. The matrix is slaty. The inclusions are metasandstone, metashale, and rarely, metabasalt. The metasandstone beds exhibit features associated with sedimentary boudinage, including necking, distension, and rotation. The olistostromes are laterally restricted and are interpreted to have been deposited within a channel, such as a canyon, at or near the base of a slope. Chaotic rocks of the Hurd Mountain Formation warrant interpretation as a tectonically formed mélange that originated through the combined processes of offscraping and underplating. The mélange matrix exhibits an extensive shear cleavage, with widespread cataclasis of the matrix and inclusions. A wide range of sizes, structural features, and mineralogies is displayed in the inclusions. Metabasalt inclusions derived from the underlying Caucomgomoc Lake Formation were combined within the mélange through tectonic plucking during underplating. Later features related to uplift within an accretionary prism include: chevron folds, crenulation cleavage, late-stage syntectonic igneous intrusion, and imbricate thrust emplacement of metabasalt slabs. The preferred interpreted sequence of formation is as follows. (1) Channelized olistostromes were deposted within the Caucomgomoc Lake Formation. (2) Subduction, once initiated, offscraped and underplated newly deposited trench and trench-slope sediments, as well as detaching, metamorphosing, and tectonically plucking metabasalt rafts and inclusions. Imbricate thrusting of metabasalt rafts culminated in the development of the Hurd Mountain mélange. (3) Finally the tectonically assembled mélange (Hurd Mountain) and ophiolite (Caucomgomoc Lake Formation) were transported, producing the mélange of the Loon Stream Formation. These sequences are interpreted to be related to the development of a Late Cambrian(?) through Early Ordovician forearc. This zone is now recognized as the suture between the Boundary Mountains and Traveler (Gander) terranes.

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 S 1 >> S 2 > 1 > S 3 , with S 1 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 (S 2 >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.

Strains thought to be associated with movement of a thrust sheet over a ramp are recorded by cleavages and syntectonic fibers within a thrust slice of the Hamburg sequence north of Reading, Pennsylvania. The three cleavages and bedding are coaxial with the regional east-northeast strike, and three related sets of fibers are found in pressure shadows on microscopic, near-spherical pyrite framboids. Viewed east-northeast at XZ sections, the oldest fiber set in the pressure shadows begins with a short segment at ∼40° to the slaty cleavage (S 1 ), but smoothly reorients clockwise to parallelism with the gently dipping S 1 . A crenulation cleavage (S 2 ) crudely fans a broad antiform in S 0 and S 1 , but consistent north-northwest-side-up offsets across cleavage planes throughout the fold, and localized development of an extremely stylolitic character to S 2 in the core of the fold, suggest that this cleavage preceded folding. Fibers of the first set curve sharply but continuously into fibers of the second set, which are parallel to S 2 . A third cleavage (S 3 ) crenulates S 2 and exhibits consistent top-toward-south-southeast offsets. Short fibers parallel to S 3 are found on framboids in zones between S 2 lamellae, but within the S 2 lamellae, long fibers in cylindrical pressure shadows together with other features indicate a volume gain, subhorizontal extension during F 3 development. The progression through these three deformation phases fits well with the strains expected during the movement of a thrust sheet: (1) on a flat; (2) through a closing bend onto a ramp; and (3) through an opening bend onto a higher flat.