The Central Maine Terrane (CMT) includes the rocks that extend northeasterly from Connecticut to Maine and from the Monroe Fault on the west to the Campbell Hill-Nonesuch River Fault Zone on the east. A four-phase sequence of Acadian regional deformation is recognized for the CMT cover sequence. D 1 , the earliest phase, is characterized by F 1 nappes that have east or west vergence; the sense of vergence switches at the Central New Hampshire anticlinorium (CNHA). D 1 is also characterized by early, rarely observed, low-angle and “blind” T 1 thrust faults. The CNHA (or “dorsal zone”) is analogous to a “pop up” structure and is the likely root zone for both east- and west-verging Acadian D 1 thrust-nappes. D 2 is characterized by abundant F 2 tight to isoclinal, inclined to recumbent folds with northeast-trending axes and east-southeast vergence. Most of these folds face downward, a reflection of D 2 refolding the inverted limbs of D 1 structures, and these structures are identifiable chiefly in eastern New Hampshire. F 2 folds define a regional map-scale fold, the Lebanon antiformal syncline. During D 3 broad, open, upright to inclined F 3 folds with west- or northwest-trending axes were developed across the entire belt. F 3 map-scale syntaxial folds are well defined by the outcrop pattern of the metasedimentary rocks. D 4 , the last phase of deformation, is characterized by F 4 , tight to isoclinal, inclined folds with north-northeast-trending axes and east vergence and is restricted to the western part of the CMT. F 4 folds refolded the earlier structures and significantly modify the map pattern, tightening some of the earlier major structures in the CMT, for example the Kearsarge-Central Maine synclinorium. D 2 and D 4 are similarly oriented but spatially and temporally distinct. Deformation phases D 1 through D 4 are geographically restricted. This uneven distribution of structures is critical to correlations of deformation sequences across the orogen. Any local sequence of deformation in the CMT of central New Hampshire will commonly have only three of the four regional phases preserved in outcrop.

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 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 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.

The Boundary Mountains terrane is defined primarily by a sialic basement consisting of a distinctive suite of diamictites, which were metamorphosed in late Precambrian time to granofels, gneiss, and schist. These rocks make up the Chain Lakes massif, exposed in the Boundary Mountains along the southwestern part of the Maine–Québec border, and large blocks of similar lithology exposed in mélange of the St. Daniel Formation, Eastern Townships of Québec. Rocks of similar lithology and age stand out as megaclasts in ophiolitic mélange near the northwest margin of the Macquereau dome, southeastern Gaspé Peninsula. The cratonal basement of the Boundary Mountains terrane may extend from central or northern New Hampshire and northeastern Vermont roughly 1,000 km to the western part of the Gulf of St. Lawrence, southeast of Gaspé. Collectively, these basement rocks are unlike those composing the Grenville tectonic province of the Laurentian Shield, and unlike high-grade gneisses exposed in the Miramichi Highlands of New Brunswick and in lithotectonic assemblages of Avalonian aspect bordering the Gulf of Maine and the Bay of Fundy. The accretionary history of the Boundary Mountains terrane is believed to have begun in Middle to Late Cambrian time. It therefore may represent one of the earliest of accretionary events in the prolonged orogenic history of the northern Appalachians. Two parallel mélange belts, the Hurricane Mountain and St. Daniel, of the Maine and Québec portions, respectively, of the northern Appalachians, are interpreted as suture zones that define the southeast and northwest margins of the Boundary Mountains Terrane. They are named for the predominant lithotectonic units in each belt—the Hurricane Mountain Formation, in the Lobster Mountain anticlinorium of Maine, and the St. Daniel Formation, which crops out along the southeast margin of the Baie Verte–Brompton line in Québec. The tectonic history of the Hurricane Mountain mélange belt is interpreted as expressing the amalgamation, during Late Cambrian to Early Ordovician time, of the Boundary Mountains terrane to a second terrane on its southeastern margin, probably the Gander. Sparse paleontologic and isotopic ages along the Hurricane Mountain belt indicate that suturing progressed from present-day southwest to northeast, along an ensimatic convergent plate boundary. Volcanogenic flysch deposits of the Dead River Formation, overlying the Hurricane Mountain Formation to the southeast, are believed to have formed in a forearc-basin environment. Polarity of subduction is inferred to have been toward present-day southeast. This diachronous event provides a tectonic driving mechanism, in time and space, for the Penobscottian orogeny. The Penobscottian event preceded the Taconian collision of the composite Boundary Mountains–Gander terrane to the Laurentian (North American) margin. Amalgamation of individual terranes, therefore, in this part of the northern Appalachians, did not proceed in a regular, craton-outward succession.