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.

Abstract Welcome to the Appalachian landscape! Our field trip begins with a journey across Fall Zone (Fig. 1 ), named for the falls and rapids on streams flowing from the consolidated rocks of the Appalachians onto the unconsolidated sediments of the Coastal Plain. The eastern U.S. urban centers are aligned along the Fall Zone, the upstream limit of navigation. Typically, the rocks west of the Fall Zone are part of the Piedmont province. This province exposes the metamorphic core of the Appalachian Mountains exhumed by both tectonics and erosion. At least four major phases of deformation are preserved in Piedmont rocks, three Paleozoic convergent events that closed Iapetus, followed by Mesozoic extension that opened the Atlantic Ocean. A record of Cretaceous to Quaternary exhumation of the Appalachians is preserved as Coastal Plain sediments. Late Triassic and Jurassic erosion is preserved in the syn-extensional fault basins, such as the Newark basin, or is buried beneath Coastal Plain sediments (Fig. 1 ). The trip proceeds northwest across the Fall Zone and Piedmont and into the Newark basin. Late Triassic and Jurassic fluvial red sandstone, lacustrine gray shale, and black basalt were deposited in this basin. The Newark basin is separated from the Blue Ridge by a down to the east normal fault that locally has contemporary microseismicity. The Blue Ridge represents a great thrust sheet that was emplaced from the southeast during the Alleghenian orogeny (Permian). The summits of the Blue Ridge are commonly broad and accordant. Davis (1889) projected that accordance westward to the summits of the Ridge and Valley to define his highest and oldest peneplain—the Schooley peneplain. North and west of the Blue Ridge is the Great Valley Section of the Ridge and Valley Province (Fig. 1 ). Where we cross the Great Valley at Harrisburg, it is called the Cumberland and Lebanon valleys. This section is underlain by lower Paleozoic carbonate, shale, and slate folded and faulted during the lower Paleozoic Taconic orogeny. The prominent ridge on the west flank of the Great Valley is Blue or Kittatinny Ridge. It is the first ridge of the Ridge and Valley Province; the folded and faulted sedimentary rocks of the Appalachian foreland basin, deformed during the Alleghenian orogeny. Drainage during most of the Paleozoic was to the northwest, bringing detritus into the Appalachian foreland basin. The drainage reversed with the opening of the Atlantic Ocean and southeast-flowing streams established courses transverse to the strike of resistant rocks, like the Silurian Tuscarora Sandstone holding up Blue Mountain. West and north of the Ridge and Valley is the Allegheny Plateau, that part of the Appalachian foreland that was only gently deformed during Alleghenian shortening. Our trip will traverse that part of the plateau called the Pocono Plateau which is underlain by Devonian to Penn-sylvanian sandstone. At the conclusion of our trip, we will reverse our transverse of the Appalachians by traveling from the Pocono Plateau to the Ridge and Valley, to the Great Valley, to the Newark Basin, to the Piedmont, and then to one of the great Fall Zone cities—Philadelphia—via the Lehigh and Schuylkill rivers.

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.

ABSTRACT The Ordovician Bronson Hill arc and Silurian–Devonian Central Maine basin are integral tectonic elements of the northern Appalachian Mountains (USA). However, understanding the evolution of, and the relationship between, these two domains has been challenging due to complex field relationships, overprinting associated with multiple phases of Paleozoic orogenesis, and a paucity of geochronologic dates. To constrain the nature of this boundary, and the tectonic evolution of the northern Appalachians, we present U-Pb zircon dates from 24 samples in the context of detailed mapping in northern New Hampshire and western Maine. Collectively, the new geochronology and mapping results constrain the timing of magmatism, sedimentation, metamorphism, and deformation. The Bronson Hill arc formed on Gondwana-derived basement and experienced prolonged magmatic activity before and after a ca. 460 Ma reversal in subduction polarity following its accretion to Laurentia in the Middle Ordovician Taconic orogeny. Local Silurian deformation between ca. 441 and 434 Ma may have been related to the last stages of the Taconic orogeny or the Late Ordovician to early Silurian Salinic orogeny. Silurian Central Maine basin units are dominated by local, arc-derived zircon grains, suggestive of a convergent margin setting. Devonian Central Maine basin units contain progressively larger proportions of older, outboard, and basement-derived zircon, associated with the onset of the collisional Early Devonian Acadian orogeny at ca. 410 Ma. Both the Early Devonian Acadian and Middle Devonian to early Carboniferous Neoacadian orogenies were associated with protracted amphibolite-facies metamorphism and magmatism, the latter potentially compatible with the hypothesized Acadian altiplano orogenic plateau. The final configuration of the Jefferson dome formed during the Carboniferous via normal faulting, possibly related to diapirism and/or ductile thinning and extrusion. We interpret the boundary between the Bronson Hill arc and the Central Maine basin to be a pre-Acadian normal fault on which dip was later reversed by dome-stage tectonism. This implies that the classic mantled gneiss domes of the Bronson Hill anticlinorium formed relatively late, during or after the Neoacadian orogeny, and that this process may have separated the once-contiguous Central Maine and Connecticut Valley basins.

Abstract In latest Devonian time, the collision between Avalonia, the New York promontory and Carolina terrane under the impact of Gondwana, generated an orogeny that began in New England and migrated southward in time. Once thought to be the fourth tectophase of the Acadian orogeny, this event is now called the Neoacadian orogeny. Active deformational loading during the event initially produced the Sunbury black-shale basin, whereas subsequent relaxational phases produced the Borden-Grainger-Price-Pocono and Pennington–Mauch Chunk clastic wedges, which largely reflect the dextral transpressional docking of the Carolina terrane against the Virginia promontory and points southward. The Sunbury black-shale basin and the infilling clastic wedges are among the thickest and most extensive in the Appalachian foreland basin. This trip will demonstrate differences in basinal black-shale and deltaic infilling of the foreland basin, both in more active, proximal and in more distal, sediment-starved parts of the basin. In particular, we will examine relationships between sedimentation and tectonism in the Early-Middle Mississippian Sunbury/Borden/Grainger/Fort Payne delta/basin system in the western Appalachian Basin during the Neoacadian Orogeny. We will emphasize the interrelated aspects of delta sedimentation, basin starvation, and mud-mound genesis on and near the ancient Borden-Grainger delta front. Temporal constraints are provided by the underlying Devonian-Mississippian black shales and by the widespread Floyds Knob Bed/zone, a dated glauconite/phosphorite interval that occurs across the distal delta/basin complex.

Abstract Until very recently, most geologists were conditioned to seek the effects of three major events–the Taconian, the Acadian, and the Alleghanian–within the Appalachian orogen. Things are not that simple, however, as the importance of older deformations is increasingly being recognized. Although this chapter is concerned primarily with the Taconic orogen (sensu stricto), two older deformational events are considered herein. These events are the Blountian and Penobscottian orogenies. The Penobscottian event has been recognized for some time (Neuman, 1967; Hall, 1969, 1970), but its importance in Appalachian geology has only recently become apparent by work in northern Maine (Osberg, 1983; Boone and others, 1984) and the Potomac Valley of Virginia and Maryland (Drake and Lyttle, 1981; Drake, 1987). In Maine, the Penobscottian can only be dated as pre-late Ibexian (pre-Arenigian), whereas in the Potomac Valley it is thought to be of late Middle Cambrian to early Late Cambrian (Dresbachian) age. Neither syn- nor post-orogenic sediments are recognized that could have resulted from the Penobscottian deformation. On the contrary, the Blountian event is recognized because of its syn-and post-orogenic sediment wedge, but deformational features related to the event have not as yet been recognized in the Blountian hinterland, although isotopic dating within the Blue Ridge is permissive of deformation at this time. The Blountian orogeny has been recognized for many years (Kay, 1942), and Rodgers (1953) has termed it the Blountian phase of the Taconicorogeny. In my opinion, it was a separate tectonic event that was completed prior to the Taconic (sensu stricto), as its uppermost molasse is overlain by distal Taconian syn- and post-orogenic deposits. It goes without saying that the effects of the Penobscottian and Blountian orogenies are difficult to recognize and separate from those of the Taconic orogeny. For this reason, the effects of the earlier events will be discussed with those of the Taconic where they are believed to be present.

The Middle Devonian Acadian orogeny affected the entire Appalachian orogen from Newfoundland to Alabama with varying intensities of deformation and metamorphism. Part of the erosional debris from this uplifted area of tectonism was shed westward into the adjacent Appalachian basin. The basin subsidence and filling varied along its length, but not in concert with the adjacent tectonism. In the Early Devonian, the basin was stable with very little subsidence. Beginning in the Middle Devonian and continuing throughout the Late Devonian, the eastern part of the basin (now Pennsylvania and central New York) underwent rapid subsidence, where the largest volume and coarsest sediments were deposited as the Catskill Delta. To the southwest, grain size and thickness progressively decrease to the thin sequence of black shales in eastern Tennessee and Kentucky. To the north (northern New York, eastern Ontario and southern Quebec), no real evidence exists that a Devonian basin developed—the sediment from the orogenic zone probably passed over the Laurentian shield to the Michigan basin and perhaps elsewhere. The Acadian orogeny did not impinge upon the basin and its contained sediments, in distinct contrast with the Taconian and Alleghanian orogenies, both of which directly affected the basin, albeit in different ways. Reported Devonian structures within the basin are insignificant (growth folds), questionable (radiometric dating of faults), or probably incorrect (folding and angular unconformity). Acadian structures that occur between the basin and the main tectonic belt include open, upright folds and steep faults; cooling of Taconian metamorphic terrane during the Devonian and Early Carboniferous is also indicated for these rocks.

The Acadian orogeny in the North Atlantic region is assessed in this chapter in the light of mid-Paleozoic tectonics; throughout, plate tectonic nomenclature is used, and cycles are avoided. In North America nine regions bearing the imprint of the Acadian orogeny are recognized. In Newfoundland, in the Maritime Provinces of Canada, and in Vermont and New Hampshire a continuous sequence of lithotectonic belts correlates along the orogen. The Bronson Hill belt, although a continuous structure in southern New England, is not recognized as such but splits into two structures northeast of the Maine-New Hampshire border: the Boundary Mountain anticlinorium and the Lobster Mountain anticlinorium. Other lithotectonic belts are partly continuous from Canada into the United States; they include: (1) North-Central Maine belt, (2) Aroostook-Matapedia belt, (3) Miramichi belt, (4) Fredericton-Central Maine belt, (5) Richmond belt, (6) Casco Bay belt, (7) Benner Hill belt, (8) St. Croix-Ellsworth belt, (9) Mascarene belt, and (10) Avalon belt. The decision as to whether each of these belts represents a separate terrane is at present reserved. In the coastal Maine zone the situation is particularly complex, and belts 6 through 10 can be recognized there. In Massachusetts, we interpret the Merrimack Trough belt as in fault contact with both the Kearsarge-Central Maine and Bronson Hill belts to the northwest, and in Connecticut, with the Bronson Hill belt alone. Additionally, the Merrimack Trough belt is in fault contact with the Putnam-Nashoba belt to the southeast. The latter shows mainly a Taconian metamorphism and extensive intrusion of granites; clear evidence for Acadian orogenic effects in the Putnam-Nashoba belt is lacking. In Newfoundland the main orogeny appears to be Silurian in age, and the same is true of New Brunswick, whereas in the Meguma of Nova Scotia the Devonian deformation and intrusive activity continue from the Devonian to the Carboniferous. Correlations with the south-central Appalachians indicate a possibility of significant Acadian transpressional effects. The most recent evidence of a new microfossil find, however, implies that considerable Acadian deformation occurred in the Southern Appalachians, although it may have been directly continuous with earlier Taconian events. The Acadian metamorphism in the Northern Appalachians is associated with numerous granites, in general ranging in age from the Silurian to the Carboniferous. The earlier Silurian granites may have originated along the Iapetus suture or may be associated with transcurrent faults. The plate tectonic interpretation of the orogenic system is based on a model of successive blocks (terranes) approaching and colliding with North America and squeezing intervening sediments and volcanics. This took place over a fairly prolonged period of time.

The Penobscot orogeny in the Appalachians, the Grampian orogeny in the British Isles, and the Finnmarkian orogeny in the Scandinavian Caledonides are Late Cambrian–Early Ordovician events that characterize terranes having similar pre- and post-deformation features. Together they record a major orogen-wide closing of the Cambrian Iapetus Ocean basin. The Penobscottian is the principal pre-Acadian deformation event in the Gander and related terranes of the Appalachians. The upper Proterozoic–Cambrian rocks deformed by it include subduction-related mélanges. In the northern Appalachians its timing is best constrained in northern Penobscot County, Maine, where polydeformed slate and sandstone (Grand Pitch Formation), dated Cambrian(?) by Oldhamia, are overlain by the singly deformed Arenig-age Shin Brook Formation. Basal conglomerate of the Shin Brook contains Grand Pitch clasts, and higher tuff contains Early Ordovician (late Arenig) Celtic province shelly fossils that indicate deposition around a volcanic island in cool waters of moderate to high latitude, remotely distant from contemporaneous warm equatorial waters of the North American (Laurentian) continental margin. The Grampian, in the Scottish–Irish orthotectonic Caledonides, deformed and metamorphosed miogeoclinal, upper Precambrian to lower Middle Cambrian Dalradian rocks that were largely derived from the Laurentian craton. An early tectonothermal phase, probably associated with subduction that produced blueschists, was followed by the main Barrovian metamorphism (510 to 480 Ma). Fossils in the Arenigian post-tectonic rocks of the South Mayo trough have strong North American affinities. Dalradian equivalents in the paratectonic Caledonides (Howth Peninsula, Ireland; Anglesey, Wales) suggest rifting of the Cambrian miogeocline, confirmed by the cool-water Celtic province Arenig shelly fauna of Anglesey. The Finnmarkian event deformed and metamorphosed rocks of the Late Proterozoic–Cambrian(?) Baltic continental-rise prism of Baltic provenance and obducted slabs of ophiolite. The minimum of its isotopic age range, 540 to 480 Ma, is confirmed by the Llanvirnian Otta serpentinite conglomerate that unconformably overlies an ophiolite remnant. Fossils from the conglomerate suggest cool-water Baltic and ocean-island affinities. The sequences deformed by these penecontemporaneous events record a complex history of the Late Proterozoic–Cambrian Iapetus Ocean, including miogeoclinal sedimentation on the margins of Laurentian, Baltic, and Armorican cratons, and the rifting and closing of intervening oceanic tracts. At the time of their deformation, these sedimentary sequences occupied the margins of a very large Early Ordovician Iapetus Ocean.

Abstract Recent plays like the Middle Devonian Marcellus Shale and possible prospects like the Upper Ordovician Utica Shale point out the significance of dark-shale source rocks in the Appalachian Basin. Mapping the distribution of such shales in space and time throughout the basin shows that periods of dark-shale deposition coincided with orogenies and the related formation of foreland basins. The fact that foreland basins form and become repositories for organic-rich dark-shale source rocks is mostly the result of deformational loading in the adjacent orogen. Tectonism mostly exerts its control through the flexural effects of deformational loading and subsequent relaxation in the orogen. These flexural processes generate sedimentary responses in the foreland basin that are reflected in a seven-part unconformity-bound cycle, of which dark shales are a major component. Because orogenies comprise a series of smaller deformational events, or tectophases, and each tectophase generates a similar cycle, many foreland basins typically exhibit a cyclic array of dark-shale and intervening clastic units, called tectophase cycles. Thirteen such third-order tectophase cycles, formed during four orogenies, are present in the Appalachian Basin. Using examples of foreland-basin dark-shale units formed during the Ordovician-Silurian Taconian and Devonian-Mississippian Acadian/Neoacadian orogenies, the timing of cycles and migration of successive dark-shale units within them relative to the progress of orogeny are presented as evidence of causal relationships between tectonism and dark-shale sedimentation. However, tectonic influence may extend well beyond the confines of the foreland basin in the form of far-field tensional and compressional forces. This may impel the yoking of foreland and intracratonic basins as well as the reactivation of foreland basement structures—the former allowing dark-shale depositional conditions to move from one basin to the other, and the latter, inaugurating new basins for dark-shale accumulation.

Recent discoveries in the North American Cordillera of composite exotic terranes that had become accreted to the Cordillera during its evolution require reexamination of the older Appalachian mountain systems for evidence of possibly similar history. In the New England segment of the Appalachian orogen, the three Paleozoic orogenies (Taconian, Acadian, Alleghanian) must be separately examined. Evidence for Taconian orogeny supplies the best support for subduction processes at the margin of a continent-ocean plate junction. Definition of ancestral North America prior to the completion of that subduction process is the starting point for a search of Taconian exotic terranes. On the basis of such criteria as age of basement, occurrence of in-place ophiolite, melange, blueschist, continental-margin facies, and island-arc rocks, this margin is proposed to be best preserved in northern Maine, where it runs from the Jim Pond-Boil Mountain ophiolite south of the Chain Lakes massif northeast to the Elmtree ophiolite in New Brunswick. Rocks of the Weeksboro-Lunksoos Lake and Miramichi anticlinoria are southeast of this boundary. In Maine, this boundary, which was the trace of a subduction zone, was marked by a residual marine basin in Late Ordovician and Early Silurian time. No Taconian accreted terrane has been detected on the North American craton side except for the Chain Lakes massif, which is suggested to be an obducted allochthon derived from the opposite side of Iapetus Ocean; this opposite side is labeled “Craton X” and is otherwise largely unknown. The Merrimack synclinorium is interpreted to have formed on Craton X. Acadian orogeny probably resulted from a continent-continent collision. The nature and extent of the Silurian and Devonian flysh sequences demand basins of deposition much larger than present geologic relations allow; these sequences may or may not be in mutual sedimentary contact, and may not have been even before their deformation and metamorphism. This fact and the anomalous paleomagnetic pole position for the Merrimack synclinorium suggest possible large-scale tectonic transport during the Acadian orogeny. In that sense, the terrane now occupied by the synclinorium may be exotic, both because its basement was originally Craton X and because the Taconian suture may have been disrupted by younger longitudinal transport of unknown extent. The coastal belt of Rhode Island, Massachusetts, and Maine contains rocks in distinct lithotectonic blocks. These blocks are best defined in northeast Massachusetts and around Penobscot Bay in Maine, where they are mutually separated and also separated from the Acadian version of North America by large faults. These blocks appear to be exotic; they may have arrived at their present locations since the peak of the Acadian orogeny and thus have been largely unaffected by it. This coastal belt includes the Avalonian terrane; it may have been emplaced during latest Acadian to early Alleghanian deformations. If the Avalonian terrane did arrive late, then it could not have constituted Craton X during the Taconian event. The three Paleozoic orogenies led to three types of accreted terranes: (1) Taconian, thrust allochthons directly attributable to subduction-induced collision during the closing of Iapetus Ocean; (2) Acadian, continent-continent collision and possible large concomitant transcurrent displacement; (3) Alleghanian, oblique-slip high-angle faulting, the concomitant formation of a sedimentary basin having no immediately identifiable sediment source, and the formation of a microplate collage. For ancient mountain belts, the detection of microplate accretion is at best difficult. The use of a combination of geological, geochemical, and geophysical methods is necessary. Sedimentologic analysis may furnish the best clue to the arrival of new terranes; criteria to detect root zones of transcurrent faults are needed. Geochemical study may lead to definition of discrete blocks and the nature of sutures between them. Geophysical data are generally corroborative rather than definitive; even paleomagnetic data need geologic confirmation and are best used to sniff out suspect land and eventually to define the extent of motion. The hard middle part of establishing an exotic terrane must remain a geologic task.