Full article available in PDF version.

Full article available in PDF version.

Full article available in PDF version.

Full article available in PDF version.

Full article available in PDF version.

Full article available in PDF version.

Full article available in PDF version.

Full article available in PDF version.

Full article available in PDF version.

Full article available in PDF version.

Full article available in PDF version.

Full article available in PDF version.

Full article available in PDF version.

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.

With the recognition of the Hope Valley shear zone (HVSZ) as a terrane boundary, the Esmond-Dedham terrane (EDT) was subdivided, and the western division was named the Hope Valley terrane (HVT). The oldest rocks of the HVT consist of schist, gneiss and quartzite (Plainfield Formation), and metavolcanic and metaplutonic gneisses and amphibolites (Waterford Group), some of the latter yielding a radiometric age of 620 Ma. Members of the Sterling Plutonic Suite, consisting of granite gneiss and alaskite gneiss, intrude these older units. An exact radiometric age could not be determined for the alkaline pluton, Joshua Rock Granite Gneiss, but is assigned to the broad age range from c. 380 to 280 Ma. The Narragansett Plutonic Suite yields a radiometric age of c. 273 Ma, is a terrane-linking plutonic sequence cutting through the HVSZ, and links the HVT to the EDT. The EDT has a stratigraphic sequence that in many respects is similar to that of HVT, but has pronounced differences that mainly consist of a wider range of rock units and ages represented. Additionally, the rocks of HVT, and especially those near coastal Connecticut, have been elevated more generally to higher metamorphic grades than the EDT. The Harmony Complex and the Blackstone Group predominantly consist of plutonic and volcanic rocks, and schist, quartzite, and basaltic volcanics, respectively, into which have been intruded members of the Esmond Plutonic Suite or rocks correlated with them. The Price Neck Formation, of the Newport Basin, contrasts notably with the Harmony and Blackstone, but is intruded by the Cliff Walk Granite, similar in age and composition to the Esmond, and consists predominantly of fine-grained graded sedimentary rocks with volcanogenic beds. Fossiliferous limestone, phyllite, and siltstone make up Lower and Middle Cambrian rocks of the Pirate Cave Formation and the Conanicut Group of the Newport Basin, rocks unknown in HVT. A large part of the EDT is underlain by alkaline plutonic and volcanic rocks of the Scituate Plutonic Supersuite, whose radiometrically determined age is c. 373 Ma. Fluvial coal-bearing sedimentary rocks (Rhode Island Group) of the Narragansett and related basins contain a rich floral assemblage, which permits accurate dating to Westphalian and Stephanian stages of the Carboniferous. These rocks are unrepresented in the HVT. On the basis of structural and metamorphic data for the above stratigraphic units, a pre-Mesozoic evolutionary history has been outlined from late Proterozoic through Permian events. Compressional tectonic events within the Avalon superterrane and the composite Avalon terrane include the late Proterozoic Avalonian orogeny and the Alleghanian orogeny; mid-Paleozoic rifting events are interpreted for the alkaline plutonic rocks. Collisions involving the Avalon composite terrane with terranes farther to the west were responsible for Acadian and possibly late-stage Taconian orogenic events elsewhere in southern New England.

Boston Basin is an outcrop of metasedimentary and metavolcanic strata bordered by faults mainly against late Precambrian and Paleozoic granitoids. No unit-by-unit stratigraphic correlations can be made between this basin and others of the same general age in the circum-Atlantic region. Its sedimentary, mainly clastic rocks, named the Boston Bay Group, contain a diamictite frequently referred to as the Squantum tillite. The Boston Bay Group is underlain and partly interbedded with mainly felsic meta-volcanics. In the Boston Basin the sediments are of Vendian age determined by microfossils and isotopic dating. The surrounding calc-alkaline granitoids are closely associated with gabbro-diorites that often show close (“acid-basic”) relations between products of coexisting mafic and felsic magmas. These lithological associations are typical of late Proterozoic Avalonian terranes in the North Atlantic. H. Williams’ original concept of the Avalon domain in Newfoundland was that of a platform at the edge of a continent. Thereafter, the platform was successively referred to as a prong, a microcontinent, a plate, and then a terrane. Keppie (1985) and ourselves advocated that the terrane, in turn, consists of a collage of linked (accreted) blocks. Yet the broad lithological similarity of rock associations among such blocks in Precambrian and possibly earliest Cambrian time indicates that they were parts of an originally distinct major lithotectonic unit such as an island arc. The collage of the closely related blocks is referred to as the Avalon superterrane. It is conjectured that the superterrane broke up in late Proterozoic and possibly Early Cambrian time into several blocks, which drifted apart during the opening of the Iapetus Ocean. From mid-Paleozoic time onward the ocean started closing. The blocks were assembled and accreted to the Laurentian continent as a new collage, referred to as the Avalon composite terrane. It is proposed that the thick terrigenous-volcanic Boston Bay Group accumulated in a graben-like structure within the late Proterozoic superterrane, although now it is a fragmented part of the Boston block that is a constituent of the composite terrane.

There is no completely accepted definition of a mélange, and the papers in this volume reflect this fact. In our regional assessment, the term mélange is principally used for a technically fragmented and mixed body of rock. A different term, olistostrome, is used for a chaotic and mixed rock body that formed by sedimentary processes such as slumping or gravity sliding. The term olistostromal mélange is used here if sedimentary processes and tectonic deformation were both involved in the fragmentation and mixing. In some cases there is evidence that these were effectively concurrent. Four main belts of Paleozoic mélanges and olistostromes have been recorded in the Appalachians of the northeastern United States and mainland Canada. These include: (1) olistostromes and olistostromal mélanges along Logan’s line and the Taconic allochthons, which are related to thrusting during the Taconian orogeny; (2) mélanges associated with ophiolite fragments along the Baie Verte–Brompton line, which are thought to represent a Taconian suture; (3) mélanges containing ophiolite fragments along the Hurricane Mountain mélange belt (Boone and Boudette, this volume), which are thought to represent a Penobscottian terrane boundary; and (4) Acadian mélanges and olistostromes such as the Silurian Deadman Harbour mélange, an olistostromal mélange that probably formed at the front of an Acadian overthrust. In addition, Precambrian olistostromes have been recognized in southeastern New England and in the Green Head Formation of New Brunswick. These may have originated along normal faults on the rifted continental shelf of Gondwanaland, or they may relate to an earlier Pan-African cycle. These Precambrian olistostromes, therefore, accompany the extensional development of the Iapetus Ocean, whereas the Paleozoic olistostromes and mélanges mark its progressive closure. Five main sets of Paleozoic mélanges and olistostromes have been recorded in the central and southern Appalachians. These are: (1) a composite mélange-olistostrome belt in the Piedmont of Maryland and Virginia that includes olistostromes such as the Sykesville Formation (of unknown age) and mélanges such as those of the Morgan Run Formation (of debated age) and Mine Run Complex, some of which contain possible ophiolite fragments; (2) mélanges in the Blue Ridge Province from Virginia to Alabama, which contain possible ophiolite fragments; (3) Silurian or Early Devonian olistostromes of the Lay Dam Formation in the Talladega slate belt of Alabama; (4) mélanges, including the Falls Lake and Juliette mélanges in the Carolinas and Georgia, that border the Carolina terrane as defined by Secor and others (1983); and (5) broken formations and mélanges along major faults such as the Pulaski and Brevard, which are mostly related to Alleghanian thrusting. In addition, there is good evidence in the Ocoee Supergroup of the Blue Ridge Province for Late Proterozoic olistostromes related to the initiation of grabens prior to the opening of the Iapetus Ocean.

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.