Appalachian Orogen in Canada

Appalachian Orogen in CanadaL HAROLD WILI,IAMS

Dt2prrrfnlt.~lt of Geology, Mc~noriul Unic;rr.\ify c?f ' lV~~~+:f i ,rrn~l/u1?~1, S t . Jolzn' 5 , 'l'fld., Crrnrrdtr, AIB 3x5

Received November 17, 1978

Accepted llecember 5, 1978

The Appalachian Orogen is divided into five broad zones based on stratigraphic and structura8 contl-asts between Cambriarm-Ordovician and older rocks. F~.orn west to east, these are the Hurnber, Dunnage, Gander, Avalon, and Megimnaa Zones.

The westerly three zones fit present models for the developnlent of the orogen thrc~ugh the generation and destruction of a late I'recambrian - Early Paleozoic Iapetims Ocean. Thus, the Hun~hcr Zone records the ~Pevelopment :and cfestruction on an Atlantic-type continental margin, i.ti., the ancient continelmtal margin of Eastern North America that lay to the west of Iapetus; the Dunnage Zone represents vestige:, of Iapetus with island are sequences and nailanges built upon oceanic crust; and the Gander Zone records the development and destruction s f n corntinental margin. at least in places of Andean type, that lay to the east of lapetus.

The Precambrian development of the Avalon Zone relates either to rifting and the initiation of Ikapetus or to subduction and a cycle that preceded the opening of Iapctus. During the Cambrian Pesiod, the Avalon Zone was a stable p1:atfornm or marine shelf.

C2amhriar~-(>rdovicinn socks of the Megu~na Zone represent either a remnant of the continental embankmeant of ancient Northwest Africa or the marine fill of a gsahen developed within the Avalon Zone.

Silurian and younger rocks of the Appalachian Orogen are mixed marine and terrestrial deposits that are unrelated to the earlier Paleozoic zontition of the system. Silurian s~md later development of the orogen is viewed as the history of deposition and deformation in successor basins that formed across the already destroyed margins and oceanic tract of Iapetus.

1,'orogkne des Appalaches se divise en cincg grandes zones baskes sul- les contrastes strati- gmphiques et structusaux entre les roches du Carnbro-(41-dovicien et eelles qili sont plus ancien- nes. ll'ouest en est, on distingue les 7.ones de Humber, dc Ilunnage, cfe Gander, d'Avalon et de Meguma.

Les trois zones occidentailes cor-respondermt aus snodelcs actuels pour le dkvcloppemcnt de I'orogene ii travers I;{ gknkration et la destruction de B'ockan Iapetus h la fin du Prkc:irnbrien et an dkbut du Palko7.o'ique. Ainsi, la zone de Hurnber enregistre le diveloppement et la destruction d'iane bol-ciure continentale c%c type Atlantiqire, c'est-i-dire I'ancienne bordure contirnentrale de I'est de 1'Anieriquedu Norci qui se trouve ri I'ouest de 1'Iapetus; la zone de Dunnage represente les vestige> de I'Iapetus avec ties sequences d'arcs insulaires ct de melanges constsuites sirr une croute oceaniqme: enfin, la zone dc Gander enregistre le dkveloppement et ia destruction d'rsne hordure continentale cfe type Andeen au lnoins par end)-oits et qili repose ri I'est de llIapelus.

Le developpernent du PI-dc:ambrien dans la zone d'.hvalon se rattache soit il I'effondrement et 5 I'initiation de I'Hapetus, soit 11 la subduction et 2 une cycle qui a precede I'ouvert~are de III:apetus. Au Gambrien, la zone d'AvaIon ktait un plateau stabae ou unc plate-forrne marine.

Ides roches du Cambro-(41-dovicien de Ba zone dc Meguma representent soit un vestige de 1 ~ 1

bordt~re continentale de I'anciennc Afrique du Nord-Ouest soit un remplissage marin ci'un grabern qui s'est developpi dans la zone dlAvalon.

Les roches siluriennes ou plus jeunes cie I'orogene des Appalaches sont un melange de d@pGts marins et terrestres etrangers ii la zonation du spstenle au debut du Paleozoi'que. On considPre le diveloppement de I'orogene au Silurien et subskquemment comme I'histoire cje depot et de deformation dans cies bassins successeurs qui se sont formes sur des bordures dejh detruites et une certain portion ockanique de I'lapetus.

Can. J. Earth Sci., 16,792-807 (1979) ['Traduit par le journal]

Preamble 'The initiation of the International Geodynarnics

Pro-ject coincided roughly with the wide acceptance of plate tectonic theory that has revolutionized the earth sciences and prcsvided a conceptual

"This paper fornis part of the final report of the Canadian Subcommittee for Geodynamnics on the result5 of the Canadian Program in the International Q;eodynrarnics Project.

framework for the siting and develop~nent of orogenic belts. During the lifespan of the Interna- tional Geod ynrimics Project (1970-78), there have been many major contributions to our understanding of the Canadian segment of the Appalachian Orogen. and its interpretation in terms of nloving plates. This account attempts to summarize our present knowledge of the Canadian Appalachians and points to a number- of now well-defined prob-

0008-4077/79/030792- 16$0 1 100/0 @ 1979 National Research Council of Canadald'onseil national de recherches ~ L I Canada

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WILLIAMS 793

lems, solutions to which are a necessary prerequi- site to further advances in our understanding of the development of the orcsgen.

Rocks of the Appalachian Orogen range from late Precambrian to Early h4esozoic. The older rocks. of Middle Ordovocian and earlier age, show sharp contrdsts in thickness, facies, and structural style in different parts of the orogen and this realization has led to several stratigraphic-tectwic zonations of the system. Most recent geologic syntheses are based upon such zonations.

The first zonal s~abdivision of the Canadian Ap- palachian$ was based upon the well-exposed northeast Newfoundland cross section (Williams 1964). The three hroad divisions recognized were named the Western Platform, the Central Volcanic Belt, and the Avalon Platfortn (Kay and Colbert 1965). This tripartite division of the orogen in the north was modified locally, e.g. Church 1969; Bil-cI and Dewey 1970; Church and Stevens 1971; but remained essentially intact until 1972 when the entire Canadian segment of the Appalachians was divided into nine zones (Williams et crl. 1972). Again based on the northern cross section, these were designated alphabetically A to I and assigned formzal geographic names in Newfoundlai~d (Wil- liams c-'t ril. 1974).

Although useful for descriptive purposes, this latest subdivision favoured not-thern latitudes and proved too specialized and restrictive for regional extrapolation of some zones along the interior part of the system. Consequently, there were modifica- tions and sharper definition of certain tectonic elements, especially in New Brunswick (Rast ct (11. 1976~1, 1976h; Fyffe 1977; Ruitenbel-g et ril. 1977) and Quebec (Beland 1974; St. Julien and Hubert 1975; St. Julien rt (11. 1976; Church 1977; Williaams and St. Julien 1978), but also locally in Newfound- land (Church 1972; Miller and Ileutsch 1976; Miller 1977; Dean and Strong 1977). In recent syntheses, the trend is toward fewer zones of broader definition (Williams 1976, 1978rr, 197%; Poole 1976; Schenk 1978).

Only five zones are used in the present discussion. 'I'hese are given local names in the north, i.e. from west to east: Humber, Dunnage, Gander, Av- alon, and Meguma, and are depicted in Fig. 1. Most of the lithic units that define these zones have been traced southwestward and delineated along the fill1 length of the Appalachian Orogen (Williams 1978a). As well, they can be extrapolated northeastward across the British Caledonides (Church 1969; Dewey 1969; Kennedy et (11. 1972; Williams 1978b). It must be emphasized that the present

5-fold zonation is based Lipon contrasts between Middle Ordovician and older rocks, and none of the subdivisions stresses the equally important Silurian-Devonian and Carboniferous development of the orogen.

Since the advent of plate tectonics, recent models for the development of the Canadian Appala- chians follow the suggestion of Wilson (1066) and in- volve the generation and destruction of a late Pre- cambrian - Early Paleozoic Iapetus Ocean (Dewey 1969: Bird and Dewey 1970; Stevens 1070; Williams et ( 8 1 . 1072; St. Julien and Hubert 1975, etc.). These models fit well with the present zonation of the Early Paleozoic rocks, especially those of the Humber, Dunnage, and Gander Zones. Thus, the Humber Zone records the develop~rient and destruction of an Atlantic-type continental margin, i.e. the ancient continental margin of Eastern North America, which lay to the west of the Iapetus Ocean (Rodgers 1968; Williams and Stevens 1974); the Dunnage Zone represents vestiges of Iapetus with island-arc sequences (Kean and Strong 1975) ancf melanges (Horne 1969; Kay 1976; Wibbard and Williams 1979) built upon oceanic crust (Up- adhyay et al. 1971 ; Smitheringale 1972; Kay 1975; Nornun and Strong 1975; Kidd 1977); and the Gan- der Zone records the development and destruction of a continental margin (Kennedy 1976), at least in places of Andean type (Pajari rt (11. 1977; Williams and Iloolan 1978), which lay to the east of Iapet us.

The Cambrian strata of the Avalon Zone indicate that it acted as a stable platform during Early Paleozoic time, when the Iapetus cycle was most in evidence to the west. More problematic is the late Precambrian history of the Avalon Zone and whether or not this earlier phase of its development related to rifting (Papezik 1970, 1972; Strong et (11. 1978) and the initiation of Iapetus, or else its PI-e- cambrian volcanic and sedimentary rocks reflect an ocean closing episode that preceded the Iapetus cycle (Hughes and Briickner 1971; Rast e~t (11. 1976~). The Meguma Zone also lay well east of Iapetus and east of the Avalon Zone, where its Early Paleozoic rocks may have formed a continental embankment to a craton now located in Africa (Schenk 1970, 1971, 1975; Keppie 1977a).

The 'Taconic Orogeny during Middle to Late Or- dovician time affected the Humber Zone, and movements of the same age affected the Dunnage and Gander Zones. This period of deformation is most reasonably interpreted to mark a closing of the Iapetus Ocean and the clestruction of its continental rnargin .

The history of Silurian-Devonian development

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794 CAN. J . EAR'I'H SCI. VOL.. 16. 1979

of the Appalachian Orogen is entirely different from that of Cambrian and Ordovician times. A sub-Silurian uncc~nfi9rrnity that is recognizecl across most of the system testifies to the earlier destruction of Hapetus and its margins. 'This and the lack of any stratigraphic evidence for the existence of an ocean and contemporaneous continental margins of S~~LII-ian-Devonian age within the orogen raises se r io~~s difficulties in attempts to relate the plutonic, rnetarnorphic, and deformative effects of the Devonian Acadian Orogeny to setbduction. Similarly, there is no evidence of oceanic rocks or continental margins in the on-land Carboniferous stratigraphic record to support the view that moving plates are the prime cause of Alleghanian- Hercynian deformation in the Appalachian 01.o- gen.

Irving (1979) and Kent and Opdyke (1978) inter- pret paleomagnetic data to indicate that at least part of the Northern Appalachians was situated 10 to 15 degrees south of its present position relative to North America in Devonian and Early Carbonifel-- 011s times. Major Carboniferous displacements are difficult to rationalize in view of the above state- ments. Movement of one part of the orogen relative to another is also difficult to reconcile with the continuity of Orciovician and earlier tectonic elements along its full length. If major Devonian-Car- lx-miferous displacements are real, significant zoiies of sinistral strike-slip motion must be envis- aged that parallel the stratigraphic-tectonic zonc boundaries proposed here.

?'his paper emphasizes the late Precambrian through Early Paleozoic history of the five rnajc~r divisions of the Canadian Appalachians. stressing present problems. This is followed ky an appraisal of the Silul-ian-Devonian and Carboniferous development of the orogen, and an analysis of the dit'ficulties in explaining its later development in terms of plate tectonics.

Late Precambrian and Early Paleozoic Development of the Appalachian Orogen

H~irnhor Zone: Assc-ie~rmt C7otl~ine7nt~il !Wut.gin of Etrstern Norfh Arracric-n

'I'he Hhamber Zone is underlain by a crystalline basement, which was formed during the Grenville 01-ogeny, overlain by mainly setlimentsa-y rocks. These show considerable fiacies variation and structural contrast across the width of the zone. The western margin of the Humber Zone is defined by the limit of Appalachian deformation and the eastern mas-gin is drawn at the Baie Verte-Brompton Line (St. %talien et 611. 1976: Williams and St. Julien 1978), r+ steep strtlctural zone characterized by

ophiolite occurrences. Toward the west, the Gren- villian basement is overlain unconformably by ;i

thin clastic unit with local lava flows and mafic dikes, ancl a prominent Canibria~i-Ordovician carbonate sequence. Eastward in Newfoundland. the Grenvillian basement is difficult to recognize where it has been deformed together with an overlying thick, polydeformed clastic sequence (DeWit 1 972).

Hn Newfoundlatid, the relatively undefor~~~ed carbonate sequence is overlain by easterly dee-ivcd clastic rocks, melange, and a variety of transported sequences, locally capped by an ophiolite nappe (Stevens 1970). Toward the east, the carbonate sequence and its alloch thonous cover are progres- sively deformed and structurally overlain by thrust slices of polydeformed schists and Grenvillian basement rocks, brought to the surface by coin- pressive structural telescoping that fc3llowed the soft emplacement of the earliest allochthons (Wil- liams 1977rr ; Williams and St. Julien 1978; Kennedy 1978).

'The evolution of the Humber Zone involved rifting of Grenvillian basement with conconiitarat mafic dike intrusion, matic volcanism, and the ac- cumulation of thick clastic sequences (Hubert vt a / . 1978; Strong and Williams 1972; Williams and Ste- vens 1974; Strong 1975). All of these events wcre initiated during Bate Precambrian time, as i~idicated by isotopic studies of the rift fiicies volcanics and dikes that yield ages from about 800 (Pringle ot ci/.

1971) to 600 hfa (Sttrkas and Reynolds 1974). A thinner sequence of mainly carbonate r-ocks,

locally supratidal, formed at the continental shelf from Early Cambrian to latest Early Ordovician time (Rodgers 2968, 1970). These sediments thicken eastward and record an upward ti-ansition from immature arkosic sandst "nes to m;ttua-c quartzites, and then limestones and dolomites.

Rocks deposited at the morphological maa-gin, between the carbonate platform sequence and thicker parts of the clastic prism, are now rep- resented in allochthonous sequences structurally above the carbonate platform. These consist of coarse limestone breccias of westerly provenance (Hubert ut a / . 1970; Hubert r t tr8. 19771, and turbidites and shales with clistinctive shelf-edge Faunas (Barnes and Frahraeus 1975; Shaw and Fortey 1977).

Uppermost slices of the allochthonous sequences consist of ophiolite suites up to I0 kni thick, which represent oceanic cl-~1st iilid mantle (Church and Stevens 1971; Dewey and Bird I971 : Williams 1971 ; hlalpas 1977; Salisbury and Chris- tensen 1978; Karson and Dewey l"P$), and provide

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evidence that the Appalachian Systein evolved during a cycle of oceanic growth and destruction, rather than one of ensialic rifting. Similar ophiolites along the Baie Verte-Brsmpton L,ine may be rem- nants of the same ophiolite sheet (Laurent 1975, 1977). Ophiolite transport and structural telescoping at an initiallj~ undeformed continental margin are suggested by the occurrence of ophiolitic mklange within the rise prism clastics that is affected by the full range of local deformations (Wil- liams 1977fl).

Destruction of the ancient stable margin began toward the end of the Early Ordovician Epoch. The first indication of instability is indicated by an ancient karst topography developed across an up- warped carbonate shelf (Collins and Smith 1972). Later subsidence is recorded first by the deposition of deeper-water carbonates across the disturbed bank, then by a flood of clastic rocks bearing ophicblite detritus from the east (Stevens et trl. 1969; Stevens 1970). These were structurally overriden, in turn, by a sequence of contrasting rock as- se~nblages in separate slices. The structurally low- est slices consist of sedimentary rocks from the nearby continental margin, and the highest ophiolitic slices represent farther travelled oceanic crust and n-mantle (St. Julien and Hubei-t 1975; Williams 1975). The geometry of the slices and fifties re- lationships indicate that the structural pile was as- sembled from the east: (Stevens 1970; Stevens and Williams 19731, possibly through peeling of succes- sively landward sections from the leading edge of the sinking continental plate. A contact between the stratigraphic base of the ophiolite sequence and metamorphic aureole of supracrustal rocks, now frozen into the folded ophiolite slice, represents the actual zone of obduction, along which sediments deposited at the continental margin became jux- taposed beneath the hot oceanic plate (Church and Stevens 1971 ; Williams and Smyth 1973; hlalpas et ( I / . 1973: Dallmeyer and Williams 1975; Archibald and Farrar 1976; Dewey 1976). The present con- tacts of the structural slices are marked by thin zones of shale melange with exotic blocks. These are the result of mass wastage aild tectonic mixing associated with latest gravity sliding emplacement (Briickner 1975).

In Newfoundland. Ordovician deformation and prograde metamorphism are recorded in eastern exposures of the carbonate shelf tel-rane (Lock 1972; Williams 1977b). Farther east, polyphase deformation and metamorphism in the continental rise prism probably accompanied ophiolite obduction as the leading edge of the North American continent moved under the oceanic crust along an

eastward-dipping subduction zone. Subsequent re- bound and later imbrication explains the present position of the metamorphosed and pol ydefc~rmed rise prism with respect to transported ophiolites to the west and less-deformed ophiolites at the Baie Vcrte-Brompton Line along the eastern boundary of the Humber Zone (Williams 197711; Fig. 2).

The sinuous course of the Humber Zone, ex- pressed in the Quebec Reentrant, St. Lawrence Promontory , and Newfoundland Reentrant (Fig. lB, is interpreted as the result of an initially orthogonal continental margin along which rectilinear rifts were linked orthogonally by transform faults (Burke and Dewey 1973; Thonlas 1977). Several important depositional and structural feature5 appear to be controlled by this orthogonal form. An- cient reentrants are the main sites of I-ift-related dikes and uolcanics rocks (Rankin 1976), they contain the t hickest and best preserved m-ift-related clastic sequences. they are the loc~ls of clastic wedges shed toward the craton during destruction of the margin (Thomas 1977), thcy harbour most sedimentary allochthons and accompanying highly allochthonous ophiolite suites (Williams and Doolan 1979), and they are marked by wide a;ones of thin-skinned deformation in cover rocks. In contrast. ancient promontories lack most of the above features and they are nlrtn-ked by narrow zones of thick-skinned dcfgbr~nation involving Grenvillian basement.

The mociern Atlantic margin parallels the ancient Paleozoic margin and the present promontory nlarked by the Grand Banks is a modern replica of the St. Lawrence Promontol-y. 'This coincidence of reentrants and promontories at both the Paleozoic and modern margins, suggests that modern continental breakup was contrc~lled by reactivation cbf ancient rifts and transforimas. Parallelism of the Humbet- Zone and the Grenvillian Structural Province further implies that the form of the Paleozoic margin may reflect even older structures.

The geology of the Humbei- Zone records the evolution and destruction of an Atlantic-type continental margin. There is no evidence for subduction beneath this margin and it remained as a stable platform up to the time of its destruction by col- lapse and t~-ansport of allochthonou5 sequences across it (Taconic Orogeny). Rocks and structures of the Humber Zone can be correlrited along the full length of the Appalachian Orogen (Williams 19780) and can be recognized in the British Caledonides on the opposite side of the Atlantic Ocean (Kennedy et ul. 1972; Williams 19786). Problems concerning the development of the Murnber Zone are therefore only of second-order importance, but some of them

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CAN. J. EARTH SCI. VOL. 16. 1979

LATE CAMBRIAN - EARLY ORDOVICIAN

?

Carbonate t e r r a n e R a f i c volcan ics

- - . Grenvi 11 i a n basement '-1'

- I l a T l C Q l K e S

-- " ' \ , l - / . ~ i ~ : - / - - , , -

A -=-. 1500 KM 'zr

EARLY ORDOVICIAN

Ophioli t i c m61ange Soda qran i t e and r e l a t e d rocks

EARLY TO RIDCLE GPDOVICIAN

PIDCLE ORDCVICIAN

Frc;. 2. Model for Taconic Bsogcny and the destruction of the ancient continental margin of eastern Noa-th Anmerica. western Newfoundland.

have serious implications to the development of are in the order of 800hIr1 (Pria~gle c7t ul. 1971). other parts of the system. However, the local rift-reiated sedimentary se-

A problem of nc~w long standing concerns the quences are thin and cc~nfc~rmable with overlying time of initiation of the ancient continental margin Cambrian strata. A morphological stable Atlantic of Eastern North America. The oldest isotopic ages type margin did not evolve until Early Cambrian of dikes and volcanic rocks that are related to rifting time, some 200 Ma after initial rifting. Younger

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WILLIAMS 797

isotopic ages of 600Ma (Stukas and Reynolds 19741, determined on the same Humbel- Zone igneous rocks in Newfoundland, appear more reason- aide in light of the stratigraphic record.

The choice of isotopic age for the initiation of the ancient continental margin of Eastern North America and Iapet~as Ocean bears heavily alpon models proposed for the development of the Av- alon Zone on the opposite side of the syste~n, e.g. do the Avalonian late Precambrian rocks relate to rifting and the initiation of Iapetus (Papezik 1970; 1972, Schenk 1971; Giles and Kuitenberg 1977; Strong et al. 1978), do they represent late Precam- brian convergence and arc volcanism at an early Andean Iapetus margin (Hatcher 1972; Hughes and Briickner 1971 ; Hughes 1972). or are they related at all to the lapetus cycle (Rast c2t 01. 1976(.)'? Obvi- ously, a 6OOMn age for the initiation of Iapetus negates the first two proposals and implies that the Precambrian history of the Avalon Zone related to an earlier separate cycle.

Another long-standing problem concerns the time of earliest deformation and metamorphism of the continental rise prism clastics with respect to the time of formation and obduction of nearby ophiolite suites, and the exact place of origin or root zone for ophiolite suites such as the Bay of Islands Complex. A plethora of models that bear upon the evolution of eastern parts of the Humber Zone, mainly in Newfoundland, suggest that each ophiolite occurrence originated in its own separcite ocean basin. Thus, the Ray of Islands Complex is stated to have originated in a small ocean basin within the Mutnber Zone at White Bay (Dewey and Bird 1971 ; Kidd 1977; Kidd ct crl. 1978), ophiolites along the Baie Verte Lineament are interpreted as the floor of a small ocean basin that opened within the undeformed (Kennedy 1975) or- the already deformed (Kidd 1977; Kidd et 01. 1978) continental rise PI-ism. and the more easterly Betts Cove ophiolite complex represents the floor of still another ocean basin (Kennedy 1975; Mattinson 1975, 1976). The op- posing view suggests that all of the Humber Zone ophiolites, together with occurrences along the Baie Verte-Brompton Line and eastward, originated in a single ocean (Church and Stevens 1971; Malpas and Strong 1975; Laurent 1977: Williams ct a%. 1977; Williams and Talkington 1977). Whether or not such an ocean was a marginal Japan Sea-type ocean or a major Atlantic-type ocean remains de- batable. Seismic velocity studies (Salisbury and Christensen 1978) and the recognition of a possible transform fault zone within the Ray of Islands ophiolites (Karson and Dewey 1978) suggest deri- vation from a major ocean. On the other hand, the associrition of many Appalachian csphiolites with

island arc volcanic sequences suggests formation in a marginal ocean (Upadhyay and Neale 1979). Re- gardless of the type of ocean, the recognition of ophiolitic melange within the polydeformed and n~etan~orphosed 'continental rise' clastics in~plies that ophiolite complexes, such as the Bay of Islands Complex, formed at the Baie Verte-BI-ompton Line or eastward.

Durzntlpre Zonc: Vestiges qfllrpctus Occcrn The Dunnage Zone represents vestiges of an

ocean domain, as its dominantly mafic volcanic rocks and associated marine sediments locally overlie the ophiolite suite of rock units. Volcanic rocks similar to those of the Dunnage Zone occur above sialic basement in thc Gander Zone toward the east, e.g. h4iramichi Anticlinon-ium of New Brunswick (Kast and Stringer, 1974; Rast ct nl., 1976h; Piijari et (ll., 1977; Ruitenbel-g ct crl., 1973), so that the true Dunnage-Gander distinction can only be made at the level of the under-lying basement.

Marine sequences underlain by ophiolitic rocks occupy a wide zone in northeast Newfoundland that becomes narrow toward the soaithwest and is entirely absent opposite the Cabot Pron~ontory at the so~athwest corner of the island (Brown 1973). Ophiolitic rocks of the Fournier Complex reappear to the south of the Raie Verte-Brompton Line in New Bn-unswick (Kast ut nl. 1976t-B; Pajari et srl. 1977; Rultenberg ut (11. 19771, and they arc extensive throughout the Easterr1 Townships of Quebec (Laurent 1975, 1977: Church 1977). Mafic-ultrci- mafic complexes c~f the Gander River Belt in Newfoundland may be allochthonc~us (Palari and Currie 1978), but the east limit of the Dunnage Zone is drawn east of these occurrences (Fig. I ) . The southern boundary of the zone is located at or near the Rocky Brook-Millstream Fault of New Bi~rnswick (Rast and Stringer 1974; Rast et (11. 1976t-B), and it is inferred to lie immediately south of the exposed ophiolite suites of Quebec (Williams 19780).

Rocks of the Dunnage Zone are much less deformed and metamorphosed than those of nearby parts of the Humber and Gander Zones. LJocal pres- ervation of the widest segment of the Dunnage Zone in Newfoundland appears to relate to the form of the margins of Iapetus. Thus, the Dunnage is widest and best preserved at the matching New- foundlrrind and Hermitage reentrants (Fig. l), whereas it is absent at the matching St. Lawrence and Cabot promontories.

Most ophiolite complexes of the Dunnage Zone are overlain by thick volcanic and vslcanicIastic sequences, which are interpreted as ancient

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798 CAN. J . bAKTH SC'I. VOL. 16, 1979

analogues of modern island arcs. In the Dunnage Zone of Newfoundland. the Betts Cove Complex (Upadhyay c't c l l . 1971) consists of a basal ultramafic member, transitionally overlain by a poorly developed gabbroic member, in turn overlain by a shected dike con~plex. 'The pillow-lava unit at the top of the ophiolite suite is ovei-lain by I,owel- Ordovician volcanic rocks, cherts, and ar- gillites lap to 4 km thick. 'Fhis transition from the ultramafic illember of the ophiolite complex to the top of the overlying volcanic s~lccession confirnls that m:irine volc~inic sequenceh in places directly overlie oceanic crust. Nearby volcailic sequences of Notre Dame Bay show lithic and chemical features of modern island arc sequences (Kean and Strcang 1975). Lowermost mafic dikes, gabbros, and pillow Iiivas pass upwards through marine cherts and turbidities illto pyroclastic rocks and volcaniclastic sedimentary rocks capped by limestone and subaerial tuffs. The overall deep- to shallow- water lit hic change is accompanied by geochemical changes in the volcanic stratigraphy from low- potassium tholeiites at the base to calc-alkaline low-silica andesites toward the top that show pro- gressive enrichment i n Al,O, and K,O and a de- crease in CaO and MgO (Kean and Str-ong 1975).

'I'he Dunnage Melange occupies a wide belt near the eastern margin of the Newfoundland Dunnage Zone. It consists csf a variety of volcanic and sedimentary blocks, some up to a kilometre in width, set in a black and green Lower (drdovician shale matrix (Horne 1969; Kay 1976; Williarl~s and Hibbard 1976; Hibbard clt ( 1 1 . 1977; Hibbard and Williams 1979). The chaotic tenane is the locus of a set of distinctive granitic porphyries and dioritic intrusions, some of which contain ult ramrific inclu- sions. A similar melange, possibly continuous with the Dunnage in subsurface, occurs 30km to the east. It contains ulti-amafic blocks, further implying an ophiol itic basenlent (Pa-jari and Currie 1978). The commonest interpretation of the Dunnage Melange and related rocks is that they mark the site of an oceanic trench and subduction zone (Dewey ant! Bird 1971; May 1976: Williams and Hibbard 1976; McKerrow and Cocks 1977, 1978). Recent detailed studies are less decisive (Hibbard and Wil- liams 1979), or else suggest deposition in a back arc basin (Pickerill et ( 7 1 . 1978; Pajari et (11 . 1979).

Ophiolite complexes along the Baie Verte- Brompton Line in Quebec and Newfoundland are overlain by shal y conglomerates and olistostromes with large ultramafic and gabbroic blocks (St. Ju- lien and Hubert 1975; Kidd 1977; Williams and St. Julien 1978). These are in turn overlain by greywackes and shale (Quebec) and mixed volcanic

and volcaniclastic rocks (Newfoundland). The chaotic shaly cover rocks are unique to ophiolite complexes along the Baie Verte-Brompton Line, and their time of formation and significance compared to nearby groups is an important local problem (Church 1977).

In New Brunswick, ophiolitic rocks of the Four- nier Complex exhibit complex Fabrics and the deformed igneous rocks are overlain by chaotic sedimentary and volcanic rocks. All of the 01-dvvi- cian structural features in thic area are equated with a subduction zone that dips southeastward beneath the Gander Zone (Kast et ~ 1 1 . 1976h; Pajari ct al. 1977; Ruitenbei-g 6.t ( 1 1 . 1977). Complex structiares and metamorphism in the Twillingate Granite and nearby rocks of Newfoundland may also I-elate to a subduction mechanism within the oceanic tract (Williams and Pciyne 1975). A similar style of deformation in transported plutonic rocks of western Newfoundlarld (Little Port Complex) has been interpreted recently to reflect transfoi-m faulting (Karson and Dewey 1978).

The age of volcanism and melange formation of the Dunnage Zone indicates that island arcs were growing east of the Humber Zone continental margin during emplacement of the Humber Zone al- lochtkons (Williams and Stevens 1974). A cover of Caradocian shale above the volcanic sequence and melanges indicates that the evolution of the volcanic islands ceased at the time of final emplacement of the allochthons. Possibly the Iapet~is oceanic tract was destroyed at this time.

Rocks of the Dunnage Zone, like those of the Hunlber Zone, fit well with the plate-tectonic model and only second-order problems remain. hc~bably the foremo\t question is whether or not the zone represents the vestiges of a ma~or ocean, a single marginal ocean basin, or many minor oceanic elements? This same questioil may be asked of its volcanic arc sequences and melanges, i.e. are they parts of a single complex (Strong and Riyne 1973; Strong 1977), or do they represent a composite ter-rane or orogenic collage of once widely separated elements (Williams and Payne 1975; Williams rt ul. 1976)'?

The location and polarity of subduction zones represents a further problem, and this has been the subject of some discussion (Strong et ul. 1974; Kay 1976; Haworth et d l . 1978). Certainly if the Dun- nage Melange is a subduction related phenomenon, then its forearc position relative to volcanic arc assemblages farther west implies westward subduction in northeast Newfoundland. On the othei- hand, the simplest model for ophiolite obduction across the Humber Zone is eastward subduction at

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the Baie Verte-Brompton Line (Church and Ste- vens 1971 ; Malpas and Strong 1975; Strong 1977). This would suggest that the Dunnage Melange lies in a backarc basin at the Gander Zone margin (Pr-bjari et ul. 1979). Likewise, volcanism upon a continental basement to the Gander Zone of New Brunswick demands southeastward subduction beneath this area (Pa-iari rt ctl. 1977; Ruitenberg et ctl. 1977). Possibly more than one subduction zone was active during Early Ordovician time; if so, most activity seerns to have ceased by Caradocian time. 1,ocally in southeast New Brunswick, Muitenberg et ctl. (1977) suggest there is evidence of marine Silurian island-arc volcanism associated with northwesterly subduction.

Gcnncier- Zone: Ectsterrz rWcrs-gin of lcrpetrls O(.ecan The Gander Zone consists mainly of pre-Middle

Ordovician arenaceous rocks (Gander Group of Newfoundland; Lower Tetagouche Group of New Brunswick), which are in most places polp- deformed and metamorphosed, thus resembling clastic rc~cks at the east margin of the Mumber Zone on the opposite side of Iapetus. Migmatites, granitic gneisses, and some foliated rnegaci-ystic gr-anites along the eastern margin of the zone in Newfoundland are interpreted either as basement to the clastic cover rocks (Kennedy and McGonigal 1972; Kennedy 1976). or else deeper migmatitic levels of the cover sequence (Fyffe 1977; Black- blood 1978; Pajari and Gurrie 1978). Paleozoic rnegacrystic biotite granites and garnetiferous muscovite leucogranites and a5sociated pegrnatites are everywhere common throughout this zone (Bell and Blenkinsop 1975; Bell et al. 1977). The eastern boundary of the Gander Zone is drawn at the Dover Fault (Rlackwood and Kennedy 1975; Blackwood and 0' Driscoll 1976) in northeast Newfoundland. This fault separates metamorphic rocks to the west from slightly deformed and essentially un- metamorphosed late Precambrian sedimentary and volcanic rocks of the Avalon Zone to the east. In New Brunswick, the Gander-Avalon Zone boundary is covered by younger Silurian rocks of the Fredericton 'Frough (McKerrow and Ziegler 1971; Rast et al . 1976~ ; Ruitenberg and Ludman 1978), and Carboniferous strata.

A thick polydeformed arenaceous sequence of the Gander Zone is interpreted as a prism of sediment built up parallel to an existing shoreline (bTil- liams 1964), the eastern margin of Iapetus (Ken- nedy 1976). Deformation of the cover sequence into southeast-facing recumbent structures in New- foundland may relate to ophiolite obduction along the Dunnage-Gander Zone boundary (Pajari and

Curl-ie 1978). The time of earliest deformation along this margin of Iapetus is interpreted as Early Ordovician in Newfoundland on the basis of Caradocian conglomerates with deformed clasts which overlie ultr-amafic rocks with local unconforinity (Kennedy 1976). An unconformitp of similar age is inferrcd within the 'I'etagouche succession of New Rrunswick between the lower clastic unit and the upper volcanic unit (Rljari et al. 1977).

In New Brunswick and southward for 300 km along the U.S.A. part of the Appalachians. volcanic rocks similar to those of the uppel- 'Tetagouche Group either overlie the clastic sequence or lie upon an older sialic metamorphic basement (M1illiams 1978ca). 'i'hese relr-btionships suggest that volcanism was initiated directly upon the eastern margin of lapetus so that it evolved locally as an Andean margin (Williams and Doolan 1978). Increasing K,O content in granitic rocks from west to east across thc Newfoundland Gander Zone also supports eastward subduction beneath this margin (Strong et crl. 1974).

'I'he foremost problem of the Gander Zone is the nature and age of its basement rocks. Almost all of the deformed and foliated rocks of the Newfound- I2111d Gander Zone yield Paleozcsic isotopic ages (Bell ~t crl. 1977) rather than Precambrian ages. Even more confusing, some of the most careful Mb/Sr work indicates Val-bonifesous ages for foliated megacrystic biotite granites, yet the nearest Devoniran ancl Chi-boniferous strata in Newfoundland are nearly hor-izontal molasse deposits .

Time of deformation of the Gander Group clastic sequence with respect to the time of formation of nearby melange of the Dunnage Zone, and the ~alti- mate cause of deformatic~~ within the Gander Zone are also pn-oblems of'lociil concern.

A Z I U ~ O ~ I Zone. 'I'he Avalon Zone consists mainly of late Pre-

cambrian volcanic and sedimentary rocks that are relatively ~~nmetamc~i-phosed and undeforn-med coinpared to rocks of nearby parts of the Gander Zone. Minor unconforn-mities O C C L I ~ in the late Pre- cambrian successions and locally the rocks are cut by Precambrian granite. In other places, the late Precambrian rocks pass upwards conformably into Cambrian and Lower Ordovician shales and sandstones. Late Precambrian rocks such as those of [he Avalon Peninsula in Newfc~undland occur in the southeastern part of Gape Breton Island and in eastern New Brunswick. Further correlatives may be present in the Antigonish Highlalids and Cc~bequid Mornntains of Nova Scotia (Keppie 1978;

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800 CAN. 3 . EARTH SCI. V01>. 16. 1979

Williams 197th). The oldest Prec:tmbrian rocks of the Avalon Zone are marbles and quartzites, urhich are interpreted to underlie the late Precambrian seciimentary and volcat-tic rocks in Cape Breton Island (George River Group) and in eastern New Brunswick (Green Head Group). A still older basement granite has been discovered recently beneath marbles identified as Green Head equivalents (W. L. Dickson and N. Kitst. personal communica- t ion, 1978).

Underwater sampling of the Virgin Mocks and Eastern Shoals in the eastern part of the Grand Banks (1,illy 1965) and farther east at Flemish Cap (Pelletier 1971) has revealed late Precambrian rocks, which resemble those of the Avalon Zone. If these Precambrian rocks are continuous in snb- sui-fiace beneath the Grand Ranks. then the Avaion Zone is far wider than the combined width of other zones in the Appalachian Orogen. Avalon cort-ela- tives occur throughout the full length of the Appa- lachian Orogen in the solitheastern United States, and they continue through southern parts of the British Isles, Brittany, Northwest Africa, and Europe on the eastern side of the Atlantic Ocean (Kast ct (11. 1976~; Willianls 19786).

The late Precambrian rocks of the Avalon Zone in Newfoundland can be broadly subdivided into three groups: (a) a basal assemblage of predomi- n~tntly subaerial volcanic rocks, but including some sedimentary units (Harbour hlaitl and Love Cove Groups); (b) an intermediate assemblage of marine siliceous slates and greywacke? (Conception Group), with local volcanic interlayers and including a tillite unit near its base (Gaskiers Formation), and beds containing late Precambrian fossils at its top (Mistaken Point Formation); and (c) an upper assemblage of mainly \edimentary rocks, which in places includes a thick volcanic unit near its base and everywhere includes large amounts of arkose and red sandstone and conglomerate of shallow- water continental facies. The upper Precambrian beds are overlain by white quartzite (Kandom For- mation), in turn followed by Lower Cambrian fos- siliferous shales with Atlantic trilobite faunas. The Cambrian shales are remarkably uniform throughout the zone and represent a fairly complete history of deposition. Locally, these are overlain by Or- dovician and younger rocks.

Late Precambrian Avalonian rocks of Cape Breton Island consist mainly of volcanic rocks (Fourchu Group) overlain by red beds (Morrison River Formation) and then Cambrian shales. A similar succession occurs in southeast New Wrunswick where volcanic rocks of the Coldbrook Group are succeeded by red beds of the Ratcliff Brook Formation anci then by Cambrian shales.

The geology of the Avalon Zone is well known locally. but the controlling features of its late Pre- cambrian evolution are still doubtful. 'Fhe Avalon Zone is separated froin the Dunnage Zone by the Gander Zone. indicating that the Avalon Zone did not directly face or abut the Iapetus Ocean. Fur- thermore, the Avalon Zone was a stable platform during the Cambrian Period and locally during the Ordovician Period when thc generation and destruction of Iapetus was most active.

The Avalon Zone in Newfoundland was linked to the Gander Zone during the Silurian and Devonian Periods, and the two may have evolved side by side upon a common basenrent since late Precambrian time (Blackwood and Kennedy 1975).

The controlling feature of the Precambrian development of the Avalon Zone was either an as- chipelago of volcanic islands related to subduction and a cycle that pre-dated the opening of Iapetus (Wast c2t ~ l l . 1976c), or else its late Precambrian volcanic rocks relate to rifting and the initiation of Iapetus (Papezik 1972; Strong ct ul. 1978; Giles and Kuitenberg 1977).

The common occurrence of terrestrial redbeds and associated subaerial volcanic rocks in late Pre- cambrian sequences of the Avalon Zone and ra pre- ponderance of similar sedimentary-volcanic assemblages in Silurian an(% later Paleozoic sequences almost everywhere within the Appala- chian Orogen, implies similar prevailing conditions during late Precambrian and mid to late Paleozoic -

times. 'The Paleozoic deposits are localized mainly in successor basins developed upon the Taconic and Acadian deformed zones, where they represent the final stages of the Appalachian orogenic cycle. Possibly the Precambrian rocks of the Avalon Zone have the same meaning and represent the final stages of a Precambrian orogenic cycle, i.e. an Av- alonian cycle, related only spatially to Paleozoic rocks of the Appalachian Orogen.

The time and significanceof late Precambrian development of the Avalon Zone are first-orcler

in understanding the development of the Appalachian Orogen. 'The scale of this problem has emerged only recently. Avalonian rocks of the Canadian Appalachians extend southward to eastern Massachusetts and they comprise the extensive Carolina Slate Belt of the Southern Appalachians (Williams 1978a). Correlatives in Wales, on the eastern side of the present Atlantic, appear to continue into Brittany (Brioverian) and beyond to the Iberian Peninsula, Northwest Africa, Czecho- slovakia, and even Turkey. The extent and position of Avalonian rocks on the eastern side of the Atlan- tic presents further problems, for although the Av- alon Zone parallels the Humber Zone throughout

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WILLIAMS 80 1

the entire length of the Appalachians and British Caledonides (Williams 19786), the Avalon Zone clearly follows a separate course beyond the British Isles and passes to the southcast, rather than northwest, of the Baltic Shield. A clear understanding of the Avalon Zone in Canada and its proper synthesis can only be attempted when the extent of Avalonian rocks is known and the scope of the problem fully realized. 'Ioward this end. international cooperation such as that provided by the International Geodynamics Project and the In- ternational Geological Correlation Program is essential.

Meglrnlri Zone: Continc~ntal Elrlbat7/ina~t7t of AH- caie n t A.fiic*ci K

The Meguma Zone is restricted to mainland Nova Scotia, where it is separated from the Avalon Zone by a major transcurrent fault between Minas Basin and Chedabucto Bay (Fig. 1). 'The zone is made up mostly of a confot-mable succession of Cambrian-Ordovician strata up to 13 kin thick (Meguma Group). 'The succession consists of a lower greywacke unit overlain by a shaly unit that contains Lower Ordovician graptolites in its upper part (Schenk 1970, 1976). The Meguma Group is in turn overlain conformably by undated mixed sedimentary and volcanic rocks that include a possible Ordovician tillite unit at its base (Schenk 1972; Lane 1976). 'This sequence is in turn overlain by Devonian sediments.

Studies of the Meguma Group indicate that its source area was a low-lying met~imorphic terrane to the southeast, and the volume of sediments suggests a provenance of continental dimensions (Schenk 1970, 1971, 1976). Accordingly, the Me- guma Group is interpreted as a remnant of the continental embankment of ancient Northwest Africa that remained against North America after Mesozoic opening of the present Atlantic Ocean (Schenk 1971; Keppie 197761). The possible Or- dovician tillite above the Meguma is interpreted further to reflect Saharan glaciation (Schenk 1972; Lane 1976).

If the Megunla Zone represents another ancient continental margin, now contained within the Ap- palachian Orogen, t hen t his implies a Paleozoic ocean that once existed between the Avalon Zone and Meguma Zone (Keppie 1977n. 1977b3. No vestiges of such an oceanic tract remain, and evidence of its western margin, i.e. along the east side of the Avalon Zone, is entirely lacking.

Cambrian greywackes of the Welch Basin on the southern side of the British Caledonides have also been suggested as Meguma equivalents (Rast et ul. 1976~). These are interpreted as graben de-

posits and deep-water correlatives of nearby Cambrian shales and limestones. By analogy with Wales, the Meguma Group of Nova Scotia may have filled a faulted trough that developed within the Avalon Zone.

Paleom~ignetic studies of Meguma Zone rocks are inconclusive, and some results suggest bizarre arrangements of continental blocks at the eastern seaboard of North America in Early Paleozoic time (Morris 1976; Keppie 1977b; Ziegler et (11. 1977; Kent and Opdyke 1978). Conceivably, the rocks of the Megunla Zone have been displaced significantly by major transcurrent fi1~11ts.

Continental reconstructions and the identifica- tion of natural continuations of the Meguma Group are necessary prerequisites to the solution of these problems. Meguma equivalents may be rep- resented in the Eastern Slate Belt of the Southern Appalachians in NOI-t h Carolina (Williams 1978~).

Silurian and Later Development of the Appalachian Orogen

Silurian and youngem- rocks of the Appalachian Orogen are mixed marine and terrestrial deposits. with local facies distribution that is entirely unrelated to the earlier Rileozoic zonation of the system. Therefore, this later period of development of the orogen is entirely different from its Cam- brian-Ordoviciiin development. A sub-Silurian or sub-Middle Ordovician (Caradocian) unconformity across most of the system indicates almost complete destruction of earlier continental inargins and the oceanic tract related to the Iapetus cycle. 'The Silurian-Devonian and Carboniferous development of the system is viewed, therefore, as the history of deposition and deformation of successor basins, which formed across the already destroyed margins and oceanic tract of Iapetus.

The common view that a wide Silurian or Devon- ian Iapetus closed eventually in late Silurian or Devonian times to produce Acadian deforn~ation, magmatism, and metamorphism (Dewey 1969; McKerrow and Ziegler 1971, 1972; Schenk 1971; Dewey and Kidd 1974; McKerrow and Cocks 1977, 1978; Keppie 197761, 1977b) is based more on the ruling theory that orogeny is the I-esult of moving plates and continental collision, rather than on the facts of the stratigraphic record. The Cam- brian-Ordovician evidence for development of the continental margins and crust of Iapetus is virtually unassailable. However, no comparable evidence exists for the role of an Iapetus Ocean in the Silurian-Ilevonian development of the Appala- chians, and most tectonic elements essential to such a continuing role, e.g. ophiolite suites, melanges, continuous continental margins, vol-

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802 CAN. J . EAK'I'H SCI. VOL. 86, I979

canic arc sequences, deep marine secliments, or volcanic sequences built upon oceanic crust, are unknown in the Silurian-Devonian record of the orogen .

SillsrPcrn-Dcuot~iun Silurian rocks of southeast Quebec in the Gaspe

Sy nclinorium unconformabl y overlie the 'Faconic deformed zone of the Quebec Appalachians, and the axis of the synclinorium cuts acutely across the Baie Verte-Brompton Line (Beland 1974; St. Ju- lien and H~lbert 1975; Williams and St. Julien 1978). Similarly in northern Newfoundland, shallow marine to terrestrial Silul-ian volcanic and sedimentary rocks unconti)rn~ably overlie transported rocks of the Humber Zone in western White Bay (Williams 1977b3, and undated correlatives are unconfo1-mable upon ophiolltic rocks and Lower Ordovician island-arc volcanics of the nearby Dun- nage Zone (Schroeter 1973; Neale et ul. 1975; Kidd 1977). In local areas of the Newfoundland Dunnage Zone where the Ordovician-Silurian record is mn- interrupted, marine Ordovician rocks pass upwards into Silurian conglomerates and coralline shales capped by continental red beds. Similarly in the far removed Meguma Zone of Nova Scotia, the Meguma Croup is overlain by undated sedimentary and volcanic rocks overlain in turn by mainly terrestrial Devonian rocks (Schenk 1975; Jensen 1976).

The idea of a Siluririn-Devonian ocean with marine Silurian rocks deposited upon an undis- turbed ophiolite suite is permissive only in a few plrxes in the Appalachian Orogen where the base of the Silurian is unexposed. Thus, the marine Silur- ian turbidites of the Fredericton Trough of New Brunswick are interpreted as the fill of a Silurian ocean (McKerrow and Ziegler 1971). The possibil- ity of an ophiolite basement of Silurian or earliei- age beneath the axis of the Fredericton Trough seems remote, but if an ocean ever existed along this axis, it developed entirely within the Can- der-Avalon Zones and was removed from the real axis of Iapetus (Dunnage Zone), which lay far to the north.

A Silurian suture representing a former ocean has also been proposed along the Reach Fault of the northeast Newfoundland Dunnage Zone (McKer- row and Cocks 1977, 1978). This proposal is among the more attractive, since the local Silurian section there contains olistostromes that are sited directly above the Lower Ordovician Dunnage Melange. However, rather than a suture. this area of the Newfoundland Dunnage Zone at Notre Dame Bay represents the widest and best preserved vestige of Iapetus within the Appalachian Orogen. The Silur-

ian history of the region is mainly one of terrestrial volcanism and red bed deposition. A local marine olistostrome more likely marks the axis of a trough inherited from the Ordovician development of this area.

C~1~botlfe1'01~s ut1~I TYI'NSS~C" Carboniferous rocks of the Appalachian Orogen

are mainly terrestrial sandstones and conglomer- d e se- ates with local thick marine sandstone-sh, I

quences (Howie and Barss 1974, 1975). The Car- boniferous rocks are essentially undeformed, ex- cept in southeast New Brunswick, where a narrow zone of rocks displaying intense Hercynian deformation is parallel to the present coastline (Kuiten- berg 46 crl. 1973; Kast and Grant 1973; Rast et (il. 1976b), and in Nova Scotia and Newfoundland, where a narrow zone of deformation extends from Minas Basin to Chedabucto Bay, and from Cape Ray to White Bay, respectively.

Penetrative Carboniferous deformation in New Brunswick and amphibolite-facies metamorphism and plutonism in Gal-boniferous rocks along strike in eastern Massachusetts appear to be confined to areas underlain by Avalonian basement. The same is true for certain deformed zones in the Carolina Slate Belt of the United States Appalachians and Hercynian deformed zones of Europe. 'Ihis cir-- cumstance may reflect the deep structure and crustal nature of the Avalon Zone.

Triassic rocks of the exposed Canadian Appala- chians are restricted to a fault bounded half-graben in the vicinity of Minas Basin (Fig. 1). These are red beds and basalts that relate to rifting and onset of the modern episode of Atlantic spreading (Ballard and Uchupi 1975). In Nova Scotia, the Triassic graben coincides with a faulted zone of deformed Carboniferous rocks and the still earlier Ava- Ion-h4eguma Zone boundary, which may mark the site of a Paleozoic ocean.

Structure, Metamorphism and Plutonism Structural style, plutonism, and metamorphism

in the Canadian Appalachians are in some respects peculiar to certain tectono-stratigraphic zones. The effects of late Precambrian (Avalonian) orogeny are confined to the Avalon Zone and appear to have been localized in some parts of it, whereas uninter- rupted PI-ecambrian sections occur in nearby parts. Ordovician (Taconic) orogeny is most evident along the eastern margin of the Hunlber Zone and nearby parts of the Dunnage Zone, where it is man- ifested by early west-facing recumbent structures, and by regional metamorphism that may be related to ophiolite abduction and the destruction of the ancient continental margin of Eastern North

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WILLIAMS 803

America (Fig. 2). A similar structural and Wright. persoilal communication 1978). All of these metamorphic'style in parts of the Gander Zone geophy\ical parameters are in general agreement might reflect synchroneity of deformation on oppo- with the present zonation and models for the Early site sides of the ancient Tapetus Ocean, but this Paleozoic development of a Iapetus cycle. remains to be established. Intervening rocks of the Dunnage Zone vary from little deformed and Concluding Remarks - metamorphosed, where the zone is widest (match- since the suggestion of Wilson (1966) that the ing reentrants), to intensely deformed and meta- Appalachian Olugell was the result of generation, morphosed at constrictions (matching promon- then clestruction. of :in old Ocean. a numbel- of tories).

Devonian (Acadian) 01-ogeny affected the entire orogen, and it seems to have keen most intense in its interior parts. It is characterized by upright folds. which reflect a shortening across the system. rather than recumbent structures so typical of hori- zontal transport during the 'Faconic Orogeny. Un- like the effects of the Taconic Orogeny, the pat- terns of the Acadian Orogeny provide little evidence for the location of Silurian-Devonian continental margins or suture zones of Devonian age.

Granitic intrusions, mainly related to Acadian orogenesis, are abundant in the Canadian Appala- chians; they are commonest in the Dunnage and more easterly zones. Those intruding the mafic volcanic and sedimentary rocks of the Dunnage Zone are composite batholiths dominated by calc- alkaline hornblende-biotite granodiorite, quartz diorite, and granite. Those intruding meta- sedimentary rocks and gneissic terranes of the Gander Zone are mainly two-mica gal-netiferous leucogranites and megacrystic biotite granites, respectively. Megacrystic biotite granitesare common also in parts of the Avalon and Meguma Zones. The coi-respondence between synchronous intrusions and crustal rocks of different zones is interpreted as evidence for the formationofthe intrusions by partial melting of crustal rocks (Strong and Dickson 1978; Osberg 1978).

Geophysics Geophysical measurements across the Appala-

chian Orogen closely reflect bedrock geology and the present zonal subdivision. Thus, (1) the Bouguer anomaly field over the oceanic rocks of the Dunnage Zone has a general level tens of milli- gals higher than that of the nearby Humber and Gander Zones (Weaver 1967; Haworth 1974, 1975; Haworth and MacIntyre 1976); (2) seismic refrac- tion studies indicate a thick dense crust beneath the Dunnage Zone, which contrasts with a thinner lighter crust beneath bounding zones (Dainty et (11. 1966; Sheridan and Drake 1968); (3) the Baie Verte-Brompton Line is characterized by a sharp gravity gradient and magnetic signature (Haworth 1974; Miller and Beutch 1976), and (4) heat flow in the Dunnage Zone is in places extremely low (9.

workers have attempted to explain various facets of Appalachian evolution in terms of lithosphere plate tectonics. Major differences ainong some models arise mainly because of the emphasis placed upon local details and disregard for broad regional generalities. The plate-tectonic pendulum may have already swung too far, and although plate tectonics fits well with the Caii~brian-Ordovician development of the Appalachian Orogen, its appli- cation during later development seems less satis- factory. Acadian and Hercynian deformation, metamorphism, and inagmatisnl that affect the Ap- palachian Orogen may indicate, more than any- thing else, that these processes persist long after the destruction of continental margins and the oceanic tract.

Probably the surest way towards a better understanding of the Appalachian Orogen is through further international cooperation and, more specifically. by the preparation of a regional map portfolio covering the entire orogen and allowing quick comparison between such parameters as age. lit hof'acies, time of deformation, tectonic elements, style of deformation, basement re1 ationship, met:illogeny, plutonism, metamorphism, isotopic data, gravity, magnetics, seismics, heat flow, etc. Compilations of this type would do much to im- prove understanding of the existing data base and, hopefully, would provide the impetus for further coopel-ation, discussion, and reflection.

Acknowledgments I wish to thank P. St. Julien, B. L. Doolan, C.

Hubert, A. Ruitenberg, N, Rast, P. Schenk, J. D. Keppie, and all of the contributors to the Tectonic Lithofacies Map of the Appalachian Orogen (Wil- liams 1978~1) for discussion and help in extrapolat- ing zonal subdivisions throughout the Appalachian Orogen. R. A. Price and E. R. W. Neale kindly reviewed the manuscript and suggested many im- prove~nents. I wish also to acknowledge financial support of my own field projects, map compilations, and travel through grants by the National Research Council of Canada, the Killm Program of the Canada Council, and the Department of Energy, Mines and Resources.

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Appalachian Orogen in Canada