The Taconic rock sequence extends from near Sudbury, Vermont, to near Poughkeepsie, New York, a length of about 150 miles; and from just west of the Green Mountain Range and Berkshire Highlands to the valleys of the Hudson River and Lake Champlain, a width of about 20 miles. The Taconic rocks are now in the axial region of the Middlebury synclinorium and its southward extension. The Taconic sequence consists of about 2000 feet of slate, with subsidiary gray-wacke, quartzite, and limestone; many of the rock units are turbidites. Fossils from the rocks include forms of Early, Middle, and Late Cambrian and Early and Middle Ordovician age. Evidence of stratigraphic tops derived from the fossils and from primary sedimentary features agrees with structural data and demonstrates that the Taconic rocks are geometrically the highest strata within the Middlebury synclinorium. The underlying rocks of the synclinorium (the synclinorium sequence) are right side up; they also range in age from Early Cambrian to Middle Ordovician but belong to a different sedimentary lithofacies (dominantly carbonate and orthoquartzite). The relation between the Taconic sequence and the synclinorium sequence, therefore, is a baffling problem. Lithostratigraphically, the Taconic sequence falls into three groups: (1) the pre-Normanskill “low Taconic” sequence, occurring in the area between the main Taconic Range and the Hudson River, as far south as Rhinebeck, New York; (2) the Normanskill Shale in the same area, as well as in the area south of Rhinebeck at least as far as Poughkeepsie, New York, and also west of the Hudson River; and (3) the “high Taconic” sequence, occupying the main Taconic Range from Dorset Mountain, Vermont, south to Indian Mountain in Sharon, Connecticut, as well as Mount Greylock in Massachusetts. Rocks of (1) and (2) are fossiliferous, but to date no fossil has been found in (3). Rocks of (1) and that part of (2) areally coextensive with (1) thus are of known age but uncertain three-dimensional geometric configuration, whereas rocks of (3) are of known configuration (in the centers of open synclinoria) but unknown age. Only that part of (2) beyond the areal confines of (1) is both of known age and known configuration; these rocks are in sedimentary contact above the older rocks of the synclinorium sequence and are autochthonous. At the north end of the Taconic sequence in western Vermont, rocks of group (1) are beyond reasonable doubt allochthonous. Because of the southward geometric continuity of the structural elements, all the Taconic rocks of group (1), and that part of group (2) areally coextensive with it, are interpreted as allochthonous. The structure of group (3), the high Taconic sequence, is inferred by topography and by detailed lithostratigraphic matching with rocks of the east Vermont sequence; on this basis, as well as on the basis of the broad lithic similarity with rocks of group (1), rocks of group (3) are concluded to be also allochthonous. A discontinuous polymict conglomerate underlies and surrounds the allochthon on all sides and is interpreted here as a record that dates the imminent arrival of the allochthon at each locality. The conglomerate contains unsorted blocks of rocks of both the Taconic sequence and the synclinorium sequence; the matrix is the autochthonous upper Normanskill Shale or its equivalent. Fossils from the matrix shale date the event as Trenton, probably Sherman Fall in age. The geologic history of the area is reconstructed as follows: The pre-Normanskill Taconic rocks were deposited in the area of the present Precambrian massifs of the Green Mountains–Berkshire Highlands belt between the clastic, eugeosynclinal east Vermont sequence to the east and the miogeosynclinal synclinorium sequence to the west; they constitute the transitional facies between these two belts. Conditions were relatively stable until early Middle Ordovician time, when the Green Mountain–Berkshire Highlands area began to rise and the area of the present Middlebury synclinorium began to subside. Subsidence took place largely by a series of high-angle longitudinal faults that, as a whole, step down to the west. Argillaceous sediments (the Normanskill Shale) began to inundate the former miogeosynclinal area; because the conditions of sedimentation had become similar, the sediments resembled, in facies, the synchronous Taconic rocks that were being deposited to the east. Continued rise of the Green Mountains–Berkshire Highlands area led in middle Trenton time to the décollement of the Cambrian and Ordovician sediments into the area of the present Middlebury synclinorium in a series of giant submarine slides. Sedimentation continued at the receiving site throughout the event; sedimentation may also have persisted on the moving slides. The record is found today in the turbidite-laden shale and graywacke in the upper part of the Normanskill Shale of both the allochthon and the autochthon. Restoration of the allochthonous rocks to the original site of deposition leads to correlations between rocks of the Taconic sequence and of the largely autochthonous east Vermont sequence. The lithic correlation can be carried to the level of individual formations and is confirmed by a few known ages in the east Vermont sequence. Several lines of reasoning lead to a plausible correlation of part of the Cavendish Formation of southeastern Vermont with the oldest part of the Taconic sequence. This correlation leads further to the conclusion that in this area the contacts between the Green Mountain massif and the Cavendish Formation and between the Cavendish and the overlying east Vermont sequence must both be thrust faults of large displacements. This conclusion is in fact inevitable because one of the Taconic thrust slices that extends without interruption between the latitudes corresponding to the gap in the Precambrian massifs has been shown by local structural evidence to be allochthonous; an outside original depositional site must be found for it. The present Taconic allochthon is coextensive with an area of marked negative Bouguer gravity anomaly; the Green Mountains–Berkshire massifs constitute a belt of positive anomaly. It is here proposed that these anomalies resulted from a deep-seated transfer of material; subcrustal addition of material caused the rise of the Green Mountains–Berkshire Highlands area, and the concurrent subtraction of material caused subsidence in the Middlebury synclinorium area through a series of faults which were the near-surface expression of an episode of crustal collapse. If this interpretation is correct, then the regional gravity anomaly represents an uncompensated feature that has persisted since Middle Ordovician time.

The New England Appalachians contain two north-south–trending sets of gneiss domes. The western belt, which includes the Chester dome, contains 13 domes that expose either 1 Ga Laurentian basement rocks or ca. 475 Ma rocks of the Shelburne Falls arc. The eastern belt contains 21 gneiss domes cored by either 600 Ma crust of possible Gondwanan affinity or ca. 450 Ma rocks of the Bronson Hill arc. Domes in both belts are surrounded by Silurian and Early Devonian metasedimentary rocks, which were deposited in two north-south–trending basins before the Acadian orogeny. The Chester dome in southeastern Vermont, the main focus of this study, is an intensively studied, classic example of a mantled gneiss dome. Lower Paleozoic units around the Chester dome are dramatically thinner than they are elsewhere in southern Vermont, and are locally absent. A strong spatial correlation between the highly attenuated mantling units and highly strained, mylonitic rocks suggests the presence of a ductile, normal-sense shear zone. Garnet-bearing rocks in the core of the dome record metamorphism during decompression of 2–3 kbar, whereas rocks above the high-strain zone were metamorphosed during nearly isobaric conditions. Strain markers and kinematic indicators suggest that extension occurred during northward extrusion of lower- to middle-crustal wedges of Proterozoic and Ordovician quartz-feldspar–rich gneisses below and up into a thick tectonic cover of Silurian mica-rich metasediments that had been transported westward in large-scale nappes. If the ductile, normal-sense shear zone was responsible for synmetamorphic decompression, as we propose, extrusion occurred at ca. 380 Ma.

The eugeosynclinal rocks of western Massachusetts and western Connecticut are devoid of fossils and, except for the Goshen Formation and parts of the Straits Schist, contain relatively few reliable primary tops data. Local stratigraphic sequences have been developed, but tracing these sequences over large areas has been complicated by stratigraphic facies changes, complex structures, and conflicting geologic interpretations. Inasmuch as the relative age of any one unit within a sequence rarely can be determined reliably by internal information, we have chosen to rely heavily on lithic correlations of the stratigraphic sequences in western Connecticut and southernmost Massachusetts with the better established sequences of northern Massachusetts and southeastern Vermont. As discussants of the papers in this volume by Schnabel, Gates and Martin, and L. M. Hall on western Massachusetts and Connecticut, we wish to briefly outline a f ew points on which we either strongly disagree or believe that the evidence is such that other interpretations should be considered. For purposes of this discussion, we have outlined on Figure 1 the study area in Massachusetts, as well as Schnabel's, Gates and Martin's, and L. M. Hall's study areas, and the Collinsville area. It should also be noted that the interpretations presented here, which appear in considerably greater detail elsewhere (Hatch and Stanley, 1974), differ markedly from those given earlier by Hatch and others (1968). This change in interpretation results from completion of detailed mapping in all of the pre-Silurian rocks and most of the Silurian and Devonian rocks in the western...

Observed maximum rotation (Ω e ) of schistosity and compositional banding (fiducial surface) encapsulated in and passing through the center of a garnet relative to the same surface away from the garnet (fiducial plane = S ) commonly results from a sequence of velocity gradients ([∂ u̇ i / ∂x j ] or [ u̇ i , j ]) that have affected the surrounding rock. The circumstances under which Ω e represents the cumulative rotational motion, ∫ ω̇ dt = Ω, resulting from the antisymmetric part of [ u̇ i , j ] are described, assuming the garnet to have the idealized shape of a rigid sphere. For the example of simple shear of a mass containing an isolated garnet, theory and experiment show that Ω = Ω e = | ½ γ| , where γ = the “amount of the shear,” if S parallels the shearing planes, Ω is also equal to Ω e if two of the principal axes of the symmetric part of [ u̇ i , j ] remained in S throughout the deformation and S did not rotate relative to the same geographic frame of reference in which Ω takes place. Interpretation of Ω in terms of Ω e is complicated where there has been rotation of S , Ω s , because of the effect on Ω s of both symmetric and antisymmetric parts of [ u̇ i , j ]. Ω is affected only by the antisymmetric part. Thus Ω e does not uniquely measure Ω. The primary utility of Ω e lies in the testing of hypothesized models. The geometric arrangement of a fiducial surface passing through the center of a “rotated garnet” (≡ garnet showing nonzero Ω e ), the central surface, is readily described using a set of nested rings sharing an axis containing a diameter of each (suggestion of J. B. Thompson, Jr.). For rotation about a single axis in the fiducial plane during growth, the included and immediately peripheral fiducial surface has symmetry C 2 h (= 2/ m) and consists of a smooth surface having two synclastic regions of opposite curvatures tangent to one another at the center of nucleation, all surrounded by an anticlastic region that extends outward to the bounding fiducial plane. Included surfaces that do not pass through the center of nucleation (noncentral surfaces) are similar except for deterioration of symmetry to C s (= m ). The unique twofold axis of symmetry within the fiducial surface is a line of striction and is the observed rotational axis. It is the only straight line within any of the included surfaces. Growth and rotation about a second axis not parallel to the first is recognizable and causes the symmetry of the included central surface to become S 2 (= 1̄). Methods of determination of axial orientations and amounts of rotation (Ω e ) for both stages of doubly rotated garnets are applied to examples from southeastern Vermont. These methods take advantage of the features of symmetry and differential geometry of the included surfaces. Presence of a plane of symmetry for the set of surfaces included in a garnet indicates that the velocity gradient, referred to Cartesian axes having a principal axis normal to the fiducial plane, shared the same plane of symmetry. A common type of flow in stratified rocks probably consists of superposition of (1) dilatational motion along three mutually perpendicular principal axes and (2) simple shearing motion having the axis of shear parallel to one of these axes and shearing planes parallel to the axis of shear and one of the other principal axes of (1). The shearing planes of (2), which are not the slip surfaces of the superposed deformation, commonly parallel the planes of stratification. If Ω s is due to an additional superposed rigid-body rotational motion on a velocity gradient of the above type, then Ω e can be considered to equal an internal “physical” rotation Ω i for that part of the deformation after nucleation of the garnet. In that case the relationship of Ω s and Ω e to Ω for the same interval would not be simple, except where Ω s took place about the axis of shear of (2). In the latter case Ω = Ω s + Ω i , with due allowance for association of sign of quantities with sense of rotation. Other uses of rotated garnets relate to determination of fold kinematics, shear stress, deformational heating, angular velocity, strain rates, paragenetic sequence and correlation, nature of reactions in the rock, location of their crystallization nuclei, variations of flow laws with time, repeated episodes of tectono-metamorphism, fabric sequence, and variations in growth rates of garnets.

The northwestern New England and adjacent Quebec region is an area of 30,000 square miles on the northwest side of the northeastern Appalachian Mountains belt, and extends into the adjacent Hudson, Champlain, and St. Lawrence Valleys to the northwest. It is athwart a major change in trend of this belt from northerly to northeasterly. The synthesis is a rationale of the tectonic relations of this region, discussed in chronological order. The Precambrian basement, exposed at the core of a Paleozoic anticlinorium in the mountain belt, is made up of complexly deformed diaphthoritic miogeosynclinal rocks intruded by granitic plutons and pegmatite dikes. The exposed rocks are part of a northeast-trending Precambrian mobile shelf at least 300 miles wide that includes a wide belt to the northwest in the North American craton. The lower (Cambrian and Ordovician) and middle (Silurian and Devonian) Paleozoic orthogeosyncline, which coincides mainly with the Appalachian belt, includes a broad eugeosynclinal zone, a miogeosynclinal zone to the northwest and probably also to the southeast of the eugeosynclinal zone, several geanticlines, and a quasi-cratonic belt; it thus contrasts with the broadly miogeosynclinal Precambrian rocks of the basement. The eugeosynclinal deposits, whose maximum thickness is more than 50,000 feet, average at least three times the thickness of those in the miogeosynclinal zone. Pelitic and semipelitic rocks dominate the upper deposits and lap over geanticlines, a quasi-cratonic belt, and the margin of the craton. The sources of sediments include cratonal areas, geanticlines that formed tectonic islands, and volcanic islands. The transition between the miogeosynclinal and eugeosynclinal zones is one of sedimentary facies and thickness change, and of stratigraphic convergence and unconformity. In the lowest Paleozoic rocks the transition is west and north-west, respectively, of the Green and Sutton Mountains. The miogeosynclinal zone is missing in Quebec northwest of the northern Sutton Mountains and the eugeosynclinal zone extends to the northwestern margin of the orthogeosyncline. The belt of transition, however, moved southeast in younger rocks, so that in the middle Paleozoic rocks it lies between the Green, Sutton, and Notre Dame Mountains and the Connecticut and St. Johns Rivers. Unconformities indicate stillstand in the miogeosynclinal zone, and general uplift of the northwestern part of the orthogeosyncline, followed by subaerial denudation of the geanticlines in both the eugeosynclinal and the miogeosynclinal zones and by repeated geosynclinal folding. The unconformities are within the lower Paleozoic (especially beneath the Middle Ordovician), are the most extensive between the lower and middle Paleozoic, and are within the middle Paleozoic (especially beneath the Lower Devonian). Geanticlines, two of which coincide with gravity highs, are recognized by unconformable overlap and convergence of bedded rock units toward their axes. The lower Paleozoic Vermont-Quebec geanticline is northwest of the Green and Sutton Mountains in northwestern Vermont and neighboring parts of Quebec, but to the south and northeast it swings more into line with the mountains. It coincides with the lower Paleozoic belt of northwest-southeast transition from the miogeosynclinal to the eugeosynclinal zone, except near the north end of the Sutton Mountains where it trends into the eugeosynclinal zone. The lower Paleozoic Stoke Mountain geanticline coincides with the Stoke Mountains in Quebec, contains eugeosynclinal lower Paleozoic rocks, and is a little north-west of the belt of transition southeastward between the middle Paleozoic miogeosynclinal and eugeosynclinal zones. The lower and middle Paleozoic Somerset geanticline, which nearly coincides with the upper Connecticut River valley and the Boundary Mountains between Quebec and Maine, is cored by rocks of the lower Paleozoic eugeosynclinal zone and is truncated by the unconformity beneath middle Paleozoic rocks. The distribution of the preorogenic igneous rocks of the eugeosynclinal zone reflects the southeastward retreat of this zone during the lower and middle Paleozoic. These rocks include mafic to intermediate metavolcanic and hypabyssal bodies, prevailingly of oceanic theoleiitic composition, and mafic and ultramafic plutons. The plutons reach Lower Cambrian to possibly Middle Ordovician stratigraphic levels. The eugeosynclinal zone, and to a lesser extent the miogeosynclinal zone, are sites of regional metamorphism, shown most universally by the foliation that began to form with compaction of the shales. Isograds climb from low stratigraphic levels in the geanticlines to higher stratigraphic levels in the intervening geosynclinal troughs, showing a direct correlation between the thickness of bedded rocks and metamorphic intensity. Zones of highest grade metamorphism coincide with uplifts, but were probably originally deepest in the geosynclines. Undeformed garnet and staurolite-kyanite coincide with domes and arches, and deformed garnet and chloritoid-kyanite zones with anticlines. Some staurolite and sillimanite zones adjoin granitic plutons, but others are not so associated. Retrograde metamorphic effects in the Precambrian basement include replacement of garnet and hornblende by biotite and chlorite, and sillimanite by muscovite; these effects are caused by folding of the dry basement with the wet Paleozoic. In wet Paleozoic rocks, midway between the dry terranes of the basement and the domes and arches, garnet was replaced by chlorite and kyanite was replaced by muscovite as a result of uplift, denudation and cooling. In Quebec, an exogeosyncline containing about 5000 feet of rocks overlies the northwestern part of the orthogeosyncline and the adjoining craton north-west of the Sutton Mountains. This is a secondary geosyncline northwest of the Vermont-Quebec geanticline. It contains Upper Ordovician sandstone, shale, and limestone which overlie shale at the top of the lower Paleozoic miogeo-synclinal zone but which are eroded from the eugeosynclinal zone. The orthogeosyncline swings through the wide bend of the northwesterly bulge of the New England salient in northern New England and adjacent Quebec—all facies zones and tectonic features show a similar salient. It is deepened near the axis of the salient in a transverse trough that contains as much as 80,000 feet of strata. This section thins by stratigraphic convergence to less than 50,000 feet toward the flanks of the salient. The rocks are most varied in the salient, but thick sections of mafic volcanic rocks and carbonaceous pelites are characteristic. Similar (passive and flexural flow) folds confined to the lower and middle Paleozoic bedded rocks and commonly overturned to the northwest toward the craton, are of an early regime of variously oriented folds. Cross folds, chiefly minor folds, trend northwest at right angles to the northeast structural trends. Longitudinal folds, slides, intrastratal intrusions, and syntectonic bodies of ultramafic rock that intruded in the solid state parallel the latter trends. Oblique folds parallel the flanks of the New England salient and swing into continuity with the longitudinal folds northeast and south of the salient and at its axis. The largest major longitudinal folds are thousands of feet above the Precambrian basement. The recumbent middle Paleozoic Skitchewaug nappe, the best known of the major longitudinal folds, is rooted to the southeast in the eugeosynclinal zone. Other recumbent folds exist, but their relations are more controversial. A middle Paleozoic intrastratal diapiric fold has been described west of the Skitchewaug nappe. Major longitudinal folds northwest of the Stoke Mountain geanticline underlie a Middle Ordovician unconformity; others in the same area are truncated by a pre-Silurian unconformity. Longitudinal folds on the Vermont-Quebec geanticline in the vicinity of the international boundary are nearly upright rather than overturned to the northwest. The lower Paleozoic Taconic slide is beneath Cambrian and Lower and Middle Ordovician eugeosynclinal rocks in the Taconic klippe and above autochthonous miogeosynclinal rocks of the same age west of the Vermont-Quebec geanticline and south of the New England salient. The oblique folds face southwest on the south flank of the New England salient in general harmony with the west-facing longitudinal folds, but on the northeast flank of the salient they face southeast. The largest of the oblique folds, like the large longitudinal folds, are thousands of feet above the basement. The early folds and slides were produced by laminar flow and slip and by minimal flexing and thrusting, principally to the northwest. Several episodes of uplift in the eugeosynclinal deposits are probably accountable. The New England salient provided a basement framework that deflected, blocked, or reversed the northwestward movements to form especially the oblique folds and possibly the cross folds. The westward movement of the Taconic slide was probably assisted by maintenance of fluid pore pressure in the root zone near the top of the Vermont-Quebec geanticline by means of westward migration of water expelled from the thick eugeosynclinal deposits to the east during metamorphism. The semiconcordant ultramafic, mafic, and intermediate intrusive rocks in the eugeosynclinal zone are subparallel to the foliation of the bedded rocks and syntectonic with the early longitudinal folds. The ultramafic rocks were emplaced in a solid and cool state, after transport that is interpreted as northwestward movement as the enclosing strata were folded. The less widely distributed gabbro and diorite, which lost their mobility with crystallization from magma, participated less actively in the folding. Concordant calc-alkalic plutons, also emplaced in eugeosynclinal rocks, are synkinematic magmatic features that are less commonly parallel to foliation than are the semiconcordant intrusive rocks and are truncated upward by unconformities at successively higher levels in the direction of southeastward offlap of the eugeosynclinal zone. Regional foliation, subparallel to both the axial surfaces and limbs and the axial-plane cleavage of the early folds, approaches parallelism with the bedding in most places inasmuch as early minor folds are sparse. Thus restored, the foliation conforms to the geosynclines and geanticlines, masking the Taconic slide. Sericitic mica and fine-grained chlorite, the principal foliate minerals, are features of low-grade regional metamorphism that progressed upward as the eugeosynclinal deposits accumulated, as shown by successive unconformities that mark sharp upward decreases in the foliate condition of the bedded rocks. A longitudinal tract of middle Paleozoic domes and arches, characterized by drag folds that face downdip and some of which are cored by Precambrian basement rocks, trends northeastward across the Vermont-Quebec geanticline in southern Vermont. Largest in this tract is the Strafford-Willoughby arch which extends about equal distances northeast and southwest of the axis of the New England salient. The reverse drag folds are in the regional foliation, which near the crest of the domes and arches is obliterated by a new foliation that parallels the axial surfaces of the drag folds. The reverse drags indicate that the domes and arches were raised by vertical upward pressure, probably of buoyant rock beneath. Grossly parallel (concentric) or flexural folds that trend northeast with the Appalachian structural trends, and the largest of which include the Precambrian basement, are of a late, middle Paleozoic regime. Smaller and variously oriented steeply plunging folds of this regime are above the basement. The principal form surfaces of these folds are the regional foliation in the eugeosynclinal zone and the bedding in the miogeosynclinal zone. Thrust faults, also of this regime, parallel the trend of the major folds. Axial-plane cleavage varies from fracture cleavage through crenulation cleavage to slip cleavage and slip-cleavage schistosity. The parallel fold style gives way to similar (passive-slip and flow) folds in parts of the eugeosynclinal zone. In these parts the form surfaces are offset on the axial-plane cleavage in directions both the same and the opposite of that of flexural drag folds, and the offsets opposite in sense predominate, accentuating the amplitude of the folds. Mineral lineations, less commonly slickensides, and some minor folds plunge downdip on the bedding and bedding foliation near thrust faults and on steep homoclinal limbs of major folds. The late folds form anticlinoria that rudely coincide with the previously formed geanticlines, and synclinoria that coincide with the intervening and adjoining geosynclinal troughs. The axial surfaces of most folds in and south of the New England salient dip steeply southeast and the folds face northwest, but to the north of the axis of the salient the folds are nearly upright. The folds are also nearly upright in eastern Vermont, New Hampshire, and neighboring areas The orientation of the axial surfaces of the folds changes gradually to subparallel with the flanks and tops of the domes and arches as the latter are approached. The folds in the northwestern part of the orthogeosyncline are tipped over to the northwest toward the craton, and the thrust faults in this same belt dip east in the same direction as the axial surfaces of the folds. The late folds of first magnitude are, from northwest to southeast, the Middlebury-Hinesburg-St. Albans synclinorium, the Green Mountain-Sutton Mountain anticlinorium, the Connecticut Valley-Gaspé synclinorium, the Bronson Hill-Boundary Mountain anticlinorium, and the Merrimack synclinorium. The Middlebury-Hinesburg-St. Albans synclinorium is a major foreland fold in lower Paleozoic miogeosynclinal rocks and correlative allochthonous eugeosynclinal rocks of the Taconic klippe. This synclinorium is bordered to the west and east by thrust faults, which are most extensive on the south flank of the New England salient. The Green Mountain-Sutton Mountain anticlinorium, containing chiefly lower Paleozoic eugeosynclinal rocks, coincides with the Vermont-Quebec geanticline in the Green Mountains in central Vermont and the Notre Dame Mountains in Quebec, but near the axis of the New England salient it is southeast of the geanticline. The Connecticut Valley-Gaspé synclinorium, which contains middle Paleozoic rocks transitional from the miogeosynclinal to the eugeosynclinal zone, coincides with a geosynclinal trough between the Stoke Mountain and Somerset geanticlines and southeast of the southern part of the Vermont-Quebec geanticline. The configuration of the folds in the synclinorium is determined principally by the domes and arches near the synclinorial axis The Bronson Hill-Boundary Mountain anticlinorium, which contains both lower and middle Paleozoic rocks, coincides in its northern parts with the Somerset geanticline. The Merrimack synclinorium to the southeast, which also contains lower and middle Paleozoic rocks, is a relic of a geosynclinal trough southeast of the Somerset geanticline. The late folds and thrust faults were probably produced by subhorizontal movements as part of outward spread from the rising domes and arches. Rocks moved from the southeast into the New England salient. Steeply plunging minor folds, free from basement control, evolved in response to horizontal adjustments between major folds, and domes and arches in the thick eugeosynclinal section in the salient. Thrust faults evolved in the miogeosynclinal rocks south of the axis of the salient. The resulting counterclockwise movement of the thrust slices and their included folds and the folded rocks to the east of them in the eugeosynclinal zone continued until the present northward trend was achieved. Monoclinal flexures and related kink layers, which dip northwest and parallel to which rock to the northwest was displaced upward and to the southeast, have been recognized in north-central and northwestern Vermont. Joints include systematically oriented undeformed planar sets that dip almost vertically and cross the trend of the longitudinal folds and thrust faults at large angles. They also include less extensive nonsystematic joints that are curved or irregular and that end against the systematic joints. Some conjugate joint sets, the bisectrices of whose acute angles trend at right angles to the axes of the longitudinal folds, are possibly shear joints. Tension produced by bending of folds into the New England salient seems a doubtful cause of the joints, especially in the thick and deeply confined rocks of the eugeosynclinal zone. Discordant and commonly nonfoliate middle Paleozoic calc-alkalic plutons are postkinematic magmatic features randomly emplaced in rocks deformed in both the early and late folds. They are most abundant near the axis of the New England salient where it crosses the eugeosynclinal zone. Superimposed unconformably on the southeastern part of the orthogeosynclinal belt is an epieugeosyncline, containing upper Paleozoic clastic coal-bearing rocks. Before being eroded it probably covered wider areas of the eugeosynclinal zone, especially in the Merrimack synclinorium. Systems of early Mesozoic high-angle faults, made up of nearly parallel longitudinal sets, strike north-northeast south of the axis of the New England salient and northeast north of the salient axis, parallel to the trends of the late longitudinal folds. Faults in the foreland belt of the Champlain-St. Lawrence Valley are downthrown to the southeast of domal structural features, and others in the Connecticut Valley are downthrown mainly to the northwest of similar features. Lower Mesozoic terrestrial clastic and mafic volcanic (and hypabyssal) rocks unconformably overlie the eugeosynclinal zone and the epieugeosynclines (in taphrogeosynclines bounded by the high-angle faults) in southern New England and the Maritime Provinces. Discordant and nonfoliate Mesozoic alkalic plutons are in curvilinear tracts that transect both the orthogeosynclinal belt and the craton. Dike rocks, also alkalic, are associated with the plutons and occur widely in areas between the plutons. The chronology of the region is supported by biostratigraphic, radiometric, and structural data punctuated by unconformities. The Precambrian chronologic record is inherently scanty. Metasedimentary basement rocks exposed in the Green Mountain-Sutton Mountain anticlinorium in Vermont provide late Precambrian radiometric ages and a regional metamorphic overprint dating from about a billion years ago. Comparable metasedimentary rocks in the basement of the Adirondack Mountains were deposited in the late Precambrian. Pegmatites provide radiometric ages about the same as those of the metamorphic overprint, which records erosional unloading and cooling that restarted the potassium-argon systems about 0.4 b.y. before the end of the Precambrian. The earliest Paleozoic rocks, which are assigned to the Cambrian(?), overlie the Precambrian basement unconformably and are overlain conformably by miogeosynclinal strata containing fossils of Early, Middle, and Late Cambrian and Early and Middle Ordovician age. All epochs of the Cambrian and Ordovician are represented in the eugeosynclinal zone, and Late Ordovician fossils are found in the exogeosyncline. Potassium-argon radiometric values corresponding to Middle and Late Ordovician are mostly hybrids, between Cambrian to Early Ordovician metamorphic dates and the dates of widespread middle Paleozoic metamorphic overprints that with yet later overprints, have been revealed by Rb-Sr whole-rock isochron dating. Granitic plutons emplaced in Middle Ordovician rocks have yielded Middle or Late Ordovician Rb-Sr whole-rock isochron ages. Quasi-cratonic middle Paleozoic strata, eroded from the Champlain-St. Lawrence Valley belt, were probably Upper Silurian or higher. Chiefly in the miogeosynclinal zone, or in comparable thin lithofacies, southeast of the Green Mountain-Sutton Mountain anticlinorium, are Early, Middle, and Late Silurian and Early and Middle Devonian fossils. The middle Paleozoic K-Ar values suggest mainly the time of metamorphism and several are probably hybrids of early dates and true Devonian dates. Southeast of the belt of rocks of hybrid ages is a belt that shows true K-Ar dates of Middle (?) Devonian Acadian metamorphism and deformation about 360 m.y. ago; this belt is without later (Appalachian?) metamorphic overprint, contains Early Devonian fossils, and its tightly folded strata are overlain unconformably by gently flexed strata of Middle Devonian age. Discordant calc-alkalic granitic plutons of comparable radiometric age transect some of the late folds. Rb-Sr whole-rock and Pb/alpha determinations to the southeast in the area of the post-Devonian overprint approximate the Acadian metamorphic date. The late Paleozoic chronology in the northwestern New England and Quebec region is limited to a middle Permian metamorphic overprint with a K-Ar age of 250 ± 10 m.y. in the Merrimack synclinorium and environs. Unmetamorphosed felsic volcanic rocks that lie unconformably on the metamorphic rocks are possibly of Permian age. If this age is correct, the unconformity marks the Appalachian orogeny. The Mesozoic chronology is furnished by high-angle faults that bound the Late Triassic taphrogeosynclinal deposits in southern New England, and by alkalic intrusives of various radiometric ages (96 m.y.-to-180 m.y.), that intersect or are transected by the faults. The Cenozoic chronology is recorded by valleys and uplands produced by a continued selective downwasting, by Tertiary residual deposits containing lignite that were let down into valleys formed partly by solution of carbonate rocks, and by various Quaternary features related principally to glaciation. The orthogeosyncline was formed in the earliest Paleozoic or possibly the latest Precambrian. The Vermont-Quebec geanticline started to form during the Cambrian by tectonic stillstand relative to subsiding adjacent geosynclinal troughs; other geanticlines probably first appeared in the Early to Middle Ordovician. As the geosynclinal troughs subsided, especially in the New England salient, volcanics were extruded and sediments that were derived from the craton, from geanticlines, from volcanic accumulations, and from intrageosynclinal uplifts, were deposited mainly in the troughs. Ultramafic, mafic, and intermediate plutonic rocks were first emplaced at the end of the Cambrian or beginning of the Ordovician in the eugeosynclinal zone, and the ultramafics were transported northwestward tectonically as serpentinization continued. Albitic granitic plutons were emplaced in the Early or Middle Ordovician. Regional foliation that had first appeared in the Cambrian as the geosynclinal troughs subsided continued to form. In the Middle Ordovician, stillstand of the Vermont-Quebec and Stoke Mountain geanticlines gave way to general uplift and denudation which included a westward sliding of the Taconic allochthon. During the late Middle and Late Ordovician, the Somerset geanticline appeared, granitic rocks were emplaced, and the other geanticlines continued as sources of sediments deposited in adjacent geosynclinal troughs, including the exogeosyncline. New generations of early folds formed as geosynclinal subsidence and uplift was renewed. The early Paleozoic closed with general uplift and erosion at and northwest of the Somerset geanticline, culminating in the climax of the Taconic disturbance. The northwestern part of the orthogeosyncline stabilized to a quasi-cratonic belt early in the middle Paleozoic. The miogeosynclinal zone overlapped south-eastward on the eugeosynclinal zone and eventually across the Stoke Mountain geanticline in the Late Silurian and Early Devonian. In the Late Silurian, rapid subsidence resumed in a geosynclinal trough southeast of both the Stoke Mountain geanticline and the southern part of the Vermont-Quebec geanticline that contained the belt of lateral transition from the miogeosynclinal to the eugeosynclinal zone. Meanwhile, the Somerset geanticline continued as a source of part of the eugeosynclinal clastics. An intrageosynclinal uplift from which sediments, recumbent folds, and intrastratal diapiric folds moved northwest and possibly southeast, probably formed in the geosynclinal trough southeast of the Somerset geanticline. Mafic to felsic intrusive rocks, especially concordant calc-alkalic plutons, continued to be emplaced and the regional foliation continued to form in the eugeosynclinal zone. Growth of the domes and arches and concomitant evolution of the late longitudinal folds and thrust faults during the Acadian orogeny climaxed the middle Paleozoic. The discordant calc-alkalic plutons were emplaced soon after, and then, 360 m.y. ago, northwest of the Merrimack synclinorium uplift and erosion followed, as did cooling, opening of joints, and restarting of K-Ar systems. In the late Paleozoic the Merrimack synclinorium stood still, or possibly resumed subsidence to form the northwestern extremity of the epieugeosyncline that is preserved in southeastern New England. The epieugeosyncline was folded, faulted, and uplifted in the Appalachian orogeny, and then, with the adjacent Acadian uplift, was deeply eroded in the early Mesozoic. The taphrogeosyncline in southern New England was formed in the early Mesozoic and was followed in the middle Mesozoic by the alkalic intrusives. Thereafter until the present time, the Paleozoic and Mesozoic terranes were selectively weathered and eroded, and streams that possibly survived from the Appalachian orogeny flowed north-westward in northwestern New England and adjacent Quebec.

The lower Paleozoic rocks that extend from northwestern Newfoundland, through the Gaspé Peninsula, the south shore of the St. Lawrence River as far west as Quebec City, the Champlain Valley, western New England, eastern New York, and north-central New Jersey to southeastern Pennsylvania were deformed markedly by the Ordovician Taconic orogeny. This belt is bordered to the north and west by the little-disturbed foreland; in Canada the boundary includes Logan’s Line. To the south and east, the identity of this belt is lost in rocks that have been more severely deformed and metamorphosed by later, principally Acadian, orogeny. Rocks of this identifiable Taconic orogenic belt are here termed the Taconides. During Cambrian and much of Early Ordovician time, sedimentation within the Taconide belt was arranged in parallel zones: to the northwest, on the craton, was a shelf environment of shallow subtidal, intertidal, and supratidal carbonate deposition. Southeast of this shelf across a steep slope and an abrupt facies change, that probably reflects a sharp increase in water depth, was an area of clastic sedimentation; between these two zones was a zone of carbonate-clast slump conglomerates. The zone of clastic sediments, called the transitional zone, in turn passed seaward into one of typical eugeosynclinal sedimentation where the rocks are poor in carbonate, but rich in volcanic components. Another major facies change is preserved within the area of the shelf sequence deposition. Depending on the geographic location, this second facies change is late Early Ordovician to Middle Ordovician in age; it is marked by the regional unconformable overlap of a syntectonic black-mantling shale sequence on the carbonate rocks of the shelf sequence and older units. The shelf and mantling shale sequences are divisible into two tectonic zones: a foreland to the northwest, and a zone of deformed autochthonous rocks to the southeast, in which the intensity of deformation and metamorphism increases to the southeast. Rocks of the transitional sedimentary facies are even more intensely deformed, and over wide areas have been moved bodily northward and westward for long distances. These moved rocks are preserved in three forms: (1) klippe, now surrounded entirely by rocks of the shelf or mantling shale sequence across structural contacts; (2) allochthonous rocks only partly surrounded by rocks of the shelf or mantling shale sequence across contacts; (3) allochthons now eroded to a mere structural stump, but whose former extension and large movement are recorded by distinctive syntectonic sedimentary rocks. To the first category belong the Hare Bay and Humber Arm klippen of Newfoundland, small klippen in the Quebec City area, the Taconic klippe in New England and New York, and small scattered areas of allochthonous sedimentary rocks as well as a large klippe of Precambrian crystalline rocks in New Jersey and Pennsylvania. To the second category belong the rocks immediately southeast of Logan’s Line, from the tip of Gaspé Peninsula to the Vermont-Quebec border. To the third category belong the rocks of the Hinesburg thrust in northern Vermont, where large-scale Ordovician movement at the surface level is recorded in the wildflysch-type sedimentary rocks exposed along the Lake Champlain shore. South and east of the zone of allochthons and of the deformed shelf sequence is a zone of structurally high ground, part of the axial region of the composite Berkshire–Green Mountain–Sutton Mountain–Notre Dame Mountain–Shickshock Range–Indian Head Range–northern Long Range anti-clinoria. Where the structural relief is especially great or where epeirogenic uplift has caused erosion to reach sufficient depth, the zone is marked by Precambrian basement rocks; these Precambrian rocks approximately mark the southeast limit of basement rocks of 1 (±) b.y. (billion years) age and appear to be the edge of a lower Paleozoic craton. Coincident with the zone of structural highs is a zone of Bouguer gravity highs; the coincidence extends from northwestern Newfoundland through the Gulf of St. Lawrence, as far south as the north end of the Berkshire massif. From here south to Long Island Sound, the gravity ridge is displaced east of the structural ridge. It passes through the Coastal Plain deposits and reappears in the area of the Glenarm Series in Maryland. Northwest of the gravity ridge is a coextensive zone of Bouguer gravity troughs. The troughs follow the zone of deformed shelf sequence; where the allochthons occur, the troughs coincide with these features. Lower to Middle Ordovician ultramafic rocks occur near the western boundary of the eugeosynclinal facies, in a narrow belt parallel with and just east of the zone of structural highs. From Gaspé Peninsula southwest, these ultramafic rocks are apparently strictly intrusive. In Newfoundland, however, intrusive ultramafic bodies may be genetically related to an apparently extrusive ultramafic-mafic ophiolite complex preserved in the allochthons. I suggest that the process leading to the locations of the structural, igneous, and gravity features was the interaction of an oceanic segment of the crust with the adjoining craton. The location of this junction of crustal segments originally determined the location of the sedimentary facies junction between the shelf and basin sequences; compressive plunging of the oceanic crust under the craton caused rafting of the lighter cratonal margin, thus accounting for the structural uplift of the outermost (southeasternmost) known belt of 1-b.y.-old Precambrian basement rocks through much of the length of the Taconides. Farther into the craton (west and north), the compressive forces caused a gentle downwarp of the crust, leading to the subsidence of the former shelf area and, therefore, to a bathymetric reversal. The reversal allowed a black mantling shale sequence, whose sediments were derived in large part from the uplifted cratonal margin to the east, to be deposited over the former shelf area; continued uplift of the cratonal margin and subsidence of the former shelf area led eventually to wholesale emplacement of allochthons by gravity sliding of rocks of the transitional facies off the uplifted cratonal margin into the basin that was the former shelf area. Continued compression in the Taconides after the initial submarine gravity sliding led to northwestward thrusting of consolidated rocks, including Precambrian crystalline rocks of the uplifted cratonal margin, in a more deep-seated environment, probably in Late Ordovician or Early Silurian time. Regional metamorphism accompanied this last stage of diastrophism. The underthrusting of oceanic crust, in a process that probably involved the upper mantle as well, was accompanied by intrusion of the ultramafic bodies and the extrusion of siliceous, mafic, and ultramafic rocks on the surface. These igneous rocks are preserved today mainly in the eugeosynclinal sequence formerly deposited on the oceanic crust, but they are found also among the gravity slides and as volcanic ash in the shelf sequence. The addition of a mass of relatively dense oceanic material under the margin of the craton, as well as the concomitant introduction of dense intrusive rocks, resulted in the belt of positive Bouguer gravity anomalies. Where the gravity ridge is southeast of rather than coinciding with the belt of Precambrian rocks in the Taconide zone, the Precambrian rocks have undergone large lateral transport toward the craton. Compared to the Taconic orogeny, the Acadian orogeny in the northern Appalachian region was of wider regional extent, developed larger systems of nappes, led to more intense regional metamorphism, and was accompanied by larger scale plutonism. Despite these facts, however, the Taconic orogeny appears to have defined structural trends in the lower Paleozoic rocks that effectively controlled structural evolution of the northern Appalachian orogen during later Paleozoic orogenies, including the Acadian orogeny.