Characteristics and Evolution of the Central Mobile Belt, Canadian ...

Characteristics and Evolution of the Central Mobile Belt, Canadian Appalachians Author(s): Ben A. van der Pluijm and Cees R. van Staal Reviewed work(s): Source: The Journal of Geology, Vol. 96, No. 5 (Sep., 1988), pp. 535-547 Published by: The University of Chicago Press Stable URL: http://www.jstor.org/stable/30053568 . Accessed: 28/02/2013 16:30 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp

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CHARACTERISTICS

BEN

AND EVOLUTION OF THE CENTRAL CANADIAN APPALACHIANS' A. VAN DER PLUIJM,

MOBILE BELT,

AND CEES R. VAN STAAL2

Department of Geological Sciences, Universityof Michigan 1006 C.C. Little Building,Ann Arbor,MI 48109, USA Lithosphere and Canadian Shield Division Geological Survey of Canada, Ottawa, Ontario, Canada KlA 0E8 ABSTRACT The Early to Middle Paleozoic stratigraphic,structural,and metamorphichistoriesof centralNewfoundland and New Brunswickshow remarkablesimilarities.These characteristicsshould be reflectedin zonal or terranesubdivisions of the Canadian Appalachians, and we thereforerecombine the Dunnage and Gander the CMB is a deformedOrdovizones/terranesinto the Central Mobile Belt (CMB). In our interpretation, cian ocean basin (Iapetus II), that was formed in between the Taconic magmatic arc and the Avalon continentduringMariana-typesubductionof the lapetus I ocean. The generalized stratigraphicsequence of the CMB records the formationof ophioliticcrust and continentalmarginsediments of lapetus II in Early Ordovician times, which was followed by a period of relativequiescence when spreading stopped (limestone and shale deposition). Late Ordovician to Silurian closure of lapetus II was accompanied by the deposition of a thick clastic sequence and locally volcanics. A complex history of thrusting,regional folding,and strike-slipfaultinghas been recognized. The major(Late Ordovician?-Early Silurian)thrusting episode records the formationof an accretionarycomplex that was subsequently deformedduringcontinuedregional shortening.Transcurrentfaultingmainlyoccurredin Silurian and Devonian times. Mineral assemblages in the area are representativeof two distinctmetamorphicevents. The firstevent produced mediumto highpressureassemblages and is associated withtheformationof the accretionarycomplex. The second, low pressure event is correlatedwiththe widespreadintrusionof graniticbodies. The widthof the Iapetus II ocean is not well established. Paleomagneticevidence indicates that eitherthisocean basin was 3500 km wide (orthogonal convergence) or that closure was oblique. In order to reduce the width of Iapetus II, sinistralstrike-slipmovementswould have to have accompanied finalclosure in Siluriantimes, but this is generallynot supported by fieldevidence whichindicatesdominantlydextralmovementsduring that time. INTRODUCTION

The Canadian Appalachians, comprising Newfoundland, Prince Edward Island, New Brunswick,Nova Scotia, and partof Quebec, formthe northernextension of the Appalachians of eastern North America. A variety of subdivisions have been proposed for this Paleozoic orogenic belt. In an early subdivision of Newfoundland by Williams (1964), three zones were distinguished: (1) Precambrian rocks and Paleozoic shelffacies in the northwest (later named the Western Platform;Kay and Colbert 1965), (2) the Central Paleozoic Mobile Belt in the central part of Newfoundland, and (3) Precambrian rocks and Paleozoic shelffacies (Avalon Platform; Kay and Colbert 1965) in the southeast. Sub1 Manuscriptreceived August 28, 1987; accepted April 6, 1988. 2 Geological Survey of Canada contributionno. 44487. [JOURNAL OFGEOLOGY,1988, vol. 96, p. 535-547] © 1988 by The University of Chicago. All rights

reserved.

0022-1376/88/9605-0010$1.00

sequently, subdivisions for Newfoundland and neighboringmainland Canada were proposed by Kay (1967), Poole (1967), Bird and Dewey (1970), Williams et al. (1972), Rast et al. (1976), Ruitenberg et al. (1977), Schenk (1978), Williams (1978, 1979), Dewey et al. (1983), and others. Williams (1979) recognized five major zones in the Canadian Appalachians; from northwestto southeast these are the Humber, Dunnage, Gander, Avalon, and Meguma zones. The Central Paleozoic Mobile Belt of Williams(1964) in this subdivision comprises the Dunnage and Gander zones. In more recent years tectonostratigraphiczones have been replaced by the Cordilleran concept of suspect terranes(Williams and Hatcher 1983; Keppie 1985). However, with only minor modification, the terrane boundaries and theirtectonic setting correspond closely to the earlier zonal subdivision (compare maps in Williams 1979 and Williams and Hatcher 1983). In figure 1 the terrane subdivision of Williamsand Hatcher (1983) is illustrated. Rast et al. (1976) correlated across the Cabot Strait between Newfoundland and

535


536

B. A. VAN DER PLUIJM AND C. R. VAN STAAL

FIG. 1.-Terranes in the northernAppalachians (fromWilliams and Hatcher 1983). PEI-Prince Island; DSRP-Deep Seismic ReflectionProfile.

New Brunswick and, in additionto the above fivezones, introduced a zone in between the Gander and the Avalon in New Brunswick, which they called the Frederictonzone. This zone was considered to be a fragmentof a Precambrian terrane that was continuous with the Gander zone to the northand the Avalon zone to the south. The Miramichi zone in north-centralNew Brunswick was thoughtby Rast et al. (1976) to representthe continuation of the Newfoundland Gander zone into New Brunswick, which is largely based on the presence of quartz-richturbidites that are similar to rocks in the NewfoundlandGander zone. Fyffe(1977), in a discussion of Rast et al. (1976), argued against this correlationon the basis of age, differencesin rock types, and deformationbetween these two areas (see also Rast 1983). Characteristic of the Miramichi zone is the presence of a mixed Late Arenigian to Middle Caradocian volcanicsedimentary rock assemblage and Caradocian (graptolitic) black shales (Fyffe 1977, 1982; van Staal 1987), which are generallynot found in the Newfoundland Gander zone. Similar rocks in Newfoundland are mainly present in the Dunnage zone (Kean et al. 1981; Dewey et al. 1983; Neuman 1984; Arnott et al. 1985; van der Pluijm et al. 1987), althoughWonderley and Neuman (1984) describe an isolated occurrence of Early Ordovician "Dunnage"-type volcanic and vol-

Edward

caniclastic rocks around Indian Bay Big Pond in the Newfoundland Gander zone, which they considered atypical of this zone. Ruitenberg et al. (1977) define five major zones in New Brunswick, which are locally subdivided into smaller zones. This interpretationwas largelyadopted by Williams (1978, 1979) on the tectonic-lithofaciesmap of the Appalachian Orogen. According to Williams (1979), the boundary between the Dunnage and Gander zones in New Brunswick is drawn at the Rocky Brook-MillstreamFault, and the boundary between Gander and Avalon zones is marked by the Fredericton (-Norumbega) Fault. Fyffeet al. (1983) disagreed with the latterboundary on the basis of similaritiesin lithology and paleontology across the FrederictonFault. They group the southwestern part of the Miramichi Highlands and the Cookson Inlier in southwestern New Brunswick withinthe same terrane. Recent studies in central Newfoundland (Nelson 1981; Karlstromet al. 1982; ColmanSadd and Swinden 1984; van der Pluijm 1986; Kirkham 1987) and northernNew Brunswick (van Staal and Williams 1984; van Staal 1987) have modified our view of the regional geological history. In van der Pluijm (1986, 1987) and van Staal (1987) the central part of the Canadian Appalachians is interpreted as a deformedEarly to Middle Paleozoic accretionary complex that was formed in a Mariana-type(Uyeda and Kanamori 1979) or


EVOLUTION NW

OF THE CENTRAL MOBILE BELT

537

ies over the past 50 yrs (e.g., Neuman 1984). Radiometricresults fromselected rock units AW confirmthese fossilages (e.g., Dungenerally EARLY et al. 1987). Following is a summaryof ning ORDOVICIAN our currentknowledge of the stratigraphyof the central parts of New Brunswick and CMB Newfoundland; in figure3 the Ordovician to AW Early Silurian stratigraphiesof these areas are shown. EARLY SILURIAN The centralportionsof New Brunswick are underlain by Cambro-Ordovician volcanosedimentary rocks known mainly as the Tetagouche Group (fig. 3, N. MIR), but also ContinentalCrust Gander and Lower includingcorrelativessuch as the Shin Brook Tetagouche Groups Oceanic Crust Formation (Neuman 1967) and the WinterAW AccretionaryWedge Upper Mantle ville Formation (Roy and Mencher 1976) in FIG.2.-The tectonicevolutionof thenorthern adjacent Maine. The base of the Tetagouche Appalachiansin Early to MiddlePaleozoictimes, Group is definedby a sequence of alternating withcrustand lithospheric uppermantleshown. quartzites and semi-pelitic phyllites, which CMB-Central Mobile Belt; see textforexplanaare generallyconsidered to be correlatives of tion. the Cambrian Grand Pitch Formation in Okinawa-type (Letouzey and Kimura 1985) Maine (Neuman 1967; Rast and Stringer convergentsetting,consistingof an eastward 1974), the lower part of the Cookson Formation in southern New Brunswick (Rast and dipping subduction zone below a magmatic arc withconcurrentback-arc spreadingto the Stringer1974; Fyffeet al. 1983) and the Gansoutheast (fig. 2). der Group in Newfoundland(Rast et al. 1976; In this paper we will compare the depoWilliams 1979). These dark gray to black, sitional, deformational, and metamorphic graphiticphyllitesand interbeddedquartzites events of New Brunswick and Newfoundland are overlain by coeval calcareous and tufand discuss some of the consequences forinfaceous siltstones in the northernpart of the terpretationsof the Appalachian orogenic Miramichizone (Fyffe 1976; Neuman 1984). history.We propose a revision of earlier reTremadocian and Arenigian graptoliteshave gional subdivisions and will furtherconstrain been found in these black phyllites in the our tectonicmodel (fig.2). We do notattempt Benton area of the southern part of the to present a review of the Canadian AppalaMiramichi zone (Fyffe et al. 1983) and in the Cookson Inlier in southern New Brunschians; for this the reader is referredto Williams (1979), McKerrow (1983), Dewey et al. wick (Cumming 1967). basalts (1983), and Rast (1983). Llandeilian-Early Caradocian (Nowlan 1983a) and associated red Fe/MnAND DEFORMATION, STRATIGRAPHY, rich shales follow, or are in part coeval with METAMORPHISM silicic volcanics and are spatiallyrelated with In this section, we will discuss and coma fragmentof oceanic crust preserved in the pare the stratigraphicsequences, and the deElmtree Inlier (fig. 3, ELMTREE; Rast and formationand metamorphic histories of the Stringer 1980). The composition of the central portion of the Canadian Appalabasalts ranges fromMORB-like tholeiites to alkaline within-plate basalts (van Staal chians, with the emphasis on results we obtainedfromthe eastern Notre Dame Bay area 1987). The Middle Ordovician volcanism was of north-central Newfoundland and the in part coeval with, and followed by, deponorthernpart of the Miramichi zone and sition of volcaniclastic graywackes, limeElmtree Inlier in northernNew Brunswick. stones, and gray to black shales duringLlandeilian to Caradocian times. Some of these Stratigraphyand Zonal Subdivision.-The Paleozoic rock units of the northernAppalalimestones (e.g., the Waterville Limestone) chians are relativelyrich in fossils, whichhas have an identical conodont fauna to the studallowed fordetailed chronostratigraphic Cobbs Arm Limestone on New World Island, SE

IAPETUS I

IAPETUS 11


538

B. A. VAN DER PLUIJM AND C. R. VAN STAAL NMB SIL 438

ELMTREE

N. MIR

NDB

LEGEND

LLANDOVERIAN Conglomerates

ASHGILLIAN

Greywackes

CARADOCIAN

Shales

LLANDEILIAN

Limestones

LLANVIRNIAN

Volcanics

ORDOVICIA ARENIGIAN

Sandstones Siltstones

TREMADOCIAN

SOphiolites

505

FIG. 3.-Generalized Ordovician and Early Silurianstratigraphicsequences of the CentralMobile Belt of New Brunswick,i.e., NorthernMatapedia Basin (NMB), the Elmtree Inlier(ELMTREE) and north-central the northernMiramichi(N.MIR), and the Notre Dame Bay area of northeasternNewfoundland(NDB). The volcanics are furthersubdivided into mafic(m) and felsic (f). Time scale is fromDNAG (Palmer 1983).

Newfoundland (Nowlan 1981). The black shales become a fairlycontinuous blanket in Caradocian times and are followed by turbiditic graywackes (Fyffe 1982) that may be proximal correlativesof the Grog Brook Formation turbidites(Rast and Stringer1980) of Late Caradocian to Ashgillianage to the west of the Miramichizone, which grade into calcareous turbidites of the Early Silurian Matapedia Group (fig. 3, NMB; St. Peter 1978; Nowlan 1983b). In northern New Brunswick the Grog Brook Formation appears to lie conformablyon Early Caradocian black shales and maficvolcanics (Potter 1964; Philpott 1988), which are lithologicalcorrelatives of the upper part of the Tetagouche Group. The overlying shallow water limestones, shales, sandstones, conglomerates and bimodal maficand silicic volcanics generally are of Late Silurian and Devonian age (e.g., Fyffe 1982). In the central part of Newfoundland a regional stratigraphywas proposed by Dean (1978), Kean et al. (1981), Dewey et al. (1983), and Arnott et al. (1985). In van der Pluijm et al. (1987) this generalized stratigraphy was discussed in view of structural complexities and is illustrated in figure 3 (NDB). Middle Ordovician and older rocks change laterally from ("Gander") sandstones, shales, and quartzites (Kennedy and McGonigal 1972; Blackwood 1982) to the southeast into ("Dunnage") maficand felsic volcanics, with limestone, sandstone, manganiferous chert, and shale interbeds (e.g., Exploits Group, Helwig 1969) to the northwest. Tremadocian graptoliteshave been re-

covered from dark shales in the Dunnage Melange (Hibbard and Williams 1979) similar to those in the black phyllites that overlie quartzites in the Miramichi zone of New Brunswick. The volcanic suite is conformably overlain by Llandeilian limestones(e.g., Cobbs Arm Limestone, Bergstrom et al. 1974), followed by Caradocian graptolitic black shales (e.g., Rodgers Cove Shale, Bergstromet al. 1974). Overlyingthis black shale unit is a generally coarsening upward graywacke and conglomerate sequence of Late Ordovician to Early Silurian age, known as the Sansom Graywacke and Goldson Conglomerate, respectively (cf. Milleners Arm Formation, Arnott 1983). This clastic sequence is in turn overlain by subaerial volcanics and sandstones of Early Silurian age (Botwood Group, Berry, and Boucot 1970). The generalized stratigraphiesof central New Brunswick and north-centralNewfoundlandare in close agreement(fig.3; see also Neuman 1984). In addition, similar stratigraphicsequences have been reported fromthe southern and central Appalachians (e.g., Shanmugam and Lash 1982; Hiscott et al. 1983) and the BritishIsles (e.g., Leggettet al. 1979), which suggests thatthe depositional settingwas similar over a large tract of the Appalachian/Caledonian chain. The rock types in the Newfoundland sequence described above are mainlyfound in the Dunnage zone, whereas those fromthe New Brunswick sequence are considered to be mainlypart of the Gander zone (Williams 1979; Williams and Hatcher 1983). Furthermore, the distinctionbetween Dunnage and


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539

FIG. 4.-Proposed revision of the zonal subdivision of the Canadian Appalachians. The Humber is the Early Paleozoic North American margin(miogeocline); the CentralMobile Belt comprises remnantsof the lapetus II ocean and an Ordovician magmaticarc; Avalon is the microcontinentat the eastern marginof lapetus II. Numbered boundaries and faultsare: 1-Dover-Hermitage Bay Fault System; 2-Baie VerteBromptonLine; 3-Canso Fault (McCutcheon and Robinson 1987); 4-Rocky Brook-MillstreamFault; 5Fredericton(-Norumbega)Fault; 6-Belleisle Fault System(includes Wheaton Brook Fault); 7-CobequidChedabucto Fault System. The locations of the New WorldIsland (NWI) and Bathurstareas (B) and Cape Breton Island (CBI) are indicated. "C" represents flat-lyingPaleozoic rocks outside the Appalachian orogenic belt, and "T" is for Triassic rocks.

Gander terranes is complicated by isolated occurrencesof Dunnage-typerocks thatstratigraphically overlie Gander-typerocks, such as at Indian Bay Big Pond in northeastern Newfoundland (Wonderley and Neuman 1984; Neuman pers. comm. 1987) and in the Hayesville area of New Brunswick (Fyffe 1982). These stratigraphicrelationshipswere subsequentlytectonized when Dunnage zone rocks were thrustover the Gander zone (see below). Thus Gander rocks representat least in part lateral equivalents of Dunnage rocks and were spatiallylinkedfromat least Middle Ordovician times onward. Therefore, the Dunnage and Gander zones do not represent two exotic terranes(cf. Hatcher and Williams 1983); rather,we interpretthemto be parts of a telescoped ocean basin which was locally flooredby oceanic crust, remnantsof which can be found in, for example, the Gander River Ultrabasic Belt (Blackwood 1982) and the Through Hill area (Colman-Sadd and Swinden 1984) in Newfoundland, and the Elmtree Inlier in New Brunswick (Pajari et al. 1977; Rast and Stringer1980). Remnants of the Taconian magmaticarc can be foundto the northwest (e.g., Twillingate Terrane, Hungry Mountain Complex in the Topsails Terrane), which combined withthe ocean ba-

sin to the southeast formthe Central Mobile Belt (CMB; fig. 4). On the basis of geochemistryand relative age of the volcanics, van Staal (1987) and Kirkham (1987) argue that this basin opened by back-arc spreading in continental crust, which seems supported by Whalen et al. (1987) who suggest that the Taconic magmatic arc was at least in part built on Precambrian continental crust. The northern boundary of the CMB is formed by the Baie Verte-Brompton line (Williams and St. Julien 1982). The southern boundaryis marked by the Dover-Hermitage Bay Fault in Newfoundland (Kennedy et al. 1982) and in New Brunswick this boundary is drawn at the Belleisle Fault (Rast 1983). McCutcheon (1981) has shown that in southwestern New Brunswick this contact is located 10-25 km to the northof the Belleisle Fault, at the Wheaton Brook Fault. However, Leger and Williams (1986) suggest that these faults are segments of a once continuous dextraltranscurrentfault,separated by a later northweststrikingtranscurrentfault. For the location of the CMB in Nova Scotia we tentatively follow Barr et al. (1987), who interpretthe presence of Grenville age rocks on northernCape Breton Is-


540


land (Blair River Complex) as the continuation of the Newfoundland Humber zone onto the mainland of Canada. Their interpretation necessitates significantthinningof the CMB on Cape Breton Island (fig.4), whereOrdovician volcanic or sedimentarysequences have not yet been identified(Barr pers. comm.). One interpretationfor this configurationis the irregularshape of the Grenvillianmargin of NorthAmerica (the St. Lawrence Promontory,Stockmal et al. 1987). Deformation Events and Tectonic History.-Detailed structural analysis of key areas in northcentral Newfoundland (e.g., Kennedy 1975; Nelson 1981; Karlstromet al. 1982; Elliott 1985; van der Pluijm 1986) and in northernNew Brunswick (e.g., Helmstaedt 1971; Irrinki 1979; van Staal and Williams 1984; van Staal 1987) show a remarkable similarityin deformationstyleand sequence. deformationgenIn this section, the different erationsand theircharacteristicsforthe New World Island area of Newfoundlandand the Bathurstarea of New Brunswickwill be discussed. We realize the problems and limitations associated withcorrelationof structures over any great distances in complexly deformedareas (Williams 1985) such as these. For example, differentfold generationsthat can be distinguished by overprintingrelationships do not necessarily imply discrete folding events, but may be due to progressive deformation.Furthermore,deformation across an orogen probably is diachronous. However, we feel thatthe similarityand consistency of especially the earlier deformation generations that we have recognized, stronglysupport our regional correlation. The earliest deformation(Dl) is markedly heterogeneous and characterized by recumbent folds and bedding-parallelfaultsthatare typical of thrusting.Major thrustshave been recognized in the CMB of Newfoundland,as well as New Brunswick (Pajari and Currie 1978; Nelson 1981; Thurlow 1981; Blackwood 1982; Fyffe1982; Karlstromet al. 1982; Nowlan and Thurlow 1984; Colman-Sadd and Swinden 1984; van Staal and Williams 1984; Elliott 1985; Kusky and Kidd 1985; van der Pluijm 1986; van Staal 1987; and others). The direction of earliest thrustingin northern New Brunswick was toward the southeast (van Staal 1987), and this directionhas been reported from several places in Newfound-

land (e.g., Colman-Sadd and Swinden 1984). Dl probably startedin Late Ordovician times and continuedinto the Silurian. Evidence for younger post-Early Silurian reverse faulting in a northerlydirection has been found in both New Brunswick (McCutcheon 1981; Fyffe 1982; Nowlan 1983b) and Newfoundland (Karlstrom et al. 1982). Dl has been related to underthrustingat the northwestern marginof lapetus II and the formationof an accretionarycomplex (fig. 2; van der Pluijm 1986; van Staal 1987). The Dl structures are refolded by large wavelength, tightto very tightF2 folds and related structures of probable Silurian to Early Devonian age. A regionalcleavage (S2) is generallyassociated with F2 folds, and locally we finda composite (S2 and rotated S1) foliation. F2 folds are overturned to the northwestand definealternatingbelts of flatlying recumbent folds and steeply inclined folds in the northernpart of the Miramichi zone of New Brunswick (van Staal and Williams 1984). It is possible thatthe above mentioned northwest-directedSilurian reverse faultingin Newfoundland and New Brunswick is associated with these early deformation events (van der Pluijm 1986), and may also be coeval with post-Early Silurian recumbentfolds and thrustsrecognized by Rast (1983). F3 folds are open to tight,generallyupright structures, that are associated with major ductile transcurrentfaulting in Newfoundland and northernNew Brunswick. Since F3 structuresaccommodate significantregional shorteningthey are related to a deformation regimeof transpression.The youngestdeformation(F4 and up) consists of chevron folds, box folds, and parallel and conjugate kinks with steeply to shallowly dipping axial surfaces. Chorlton and Dallmeyer (1986) describe a similardeformationsequence forrocks in the CMB of southern Newfoundland. Using radiometric techniques they determined that theiroldest deformationevent (thrustingand associated folding) was Late Ordovician to Early Silurian in age. Regional folding and foliation formation of latest Silurian-Early Devonian age was followed by strike-slip faulting (see also Wilton 1983). In New Brunswick, comparable deformation histories have been recognized in the Cookson


EVOLUTION


Inlier in southern New Brunswick (Stringer and Burke 1985), and in the southernpart of the Miramichizone (e.g., Fyffe 1982). Combined this suggests that, in addition to its depositional history, the Early to Middle Paleozoic deformationhistoryforthe CMB in the Canadian Appalachians was also remarkably similarover its entire length. Metamorphic History.-The metamorphic history of the CMB in New Brunswick is complex. A medium pressure, Barrovian type metamorphism (Ml), represented by porphyroblastsof albite, hornblende, clinozoisite, biotite, and grunerite is present in the stratigraphicallylower parts of the Tetagouche Group in northernNew Brunswick. Metamorphic peak conditions (400-450°C, 4±+1 kb; van Staal 1985) were achieved during or afterD1 deformation,but beforethe end of F2 (Helmstaedt 1971; van Staal 1985). Ml is thus bracketed by late Caradocian and late Siluriantimes. Ml was followed by retrogrademetamorphism that also took place before the end of F2 deformation.Late syn- or post-F2, pre-F3 porphyroblastsof andalusite, cordierite,and biotite occur in contact aureoles of some of the Silurian/Devonian granites (van Staal 1987b). The regional extent of this stage of low pressure, Buchan-type metamorphism (M2) is at present incompletely known and needs furtherstudy. A narrow belt of glaucophane, crossite, and epidote bearing schists that were formed during Ml, are locally found within metabasalts of structurallyhigher and generally also stratigraphicallyhigher parts of the Tetagouche Group (Skinner 1974, Trzcienski et al. 1984). The glaucophane schists are closely associated with andradite, aegirineaugite and pumpellyite bearing rocks (Whiteheadand Goodfellow 1978). Pumpellyite or pumpellyite-actinolitebearing assemblages are in fact relatively common in the metabasalts,whereas the absence of prehnite is conspicuous in Tetagouche Group basalts of northernNew Brunswick. These rocks probably experienced higherpressure conditions (see petrogenetic grid of Liou et al. 1985) and followed a P-T path thatwas different fromthe underlyingvolcanics and sediments of the Tetagouche Group (van Staal 1985). We interpret these higher pressure metabasalts and associated turbiditesas rel-

541

icts of an accretionary complex. In Maine, correlatives of these rocks (e.g., the WintervilleFormation) are also characterized by widespread pumpellyite, either in assemblages withactinoliteor prehnite(Richterand Roy 1974). This metamorphicevent probably occurred in latest Ordovician or earliest Silurian times, but there may have been more than one event (Roy 1980). Glaucophane and crossite-bearingschists are not known fromthe CMB of Newfoundland, but (sub-)greenschistfacies conditions defined by mineral assemblages containing combinations of pumpellyite,actinolite, andradite,epidote and prehniteare quite common in Ordovician volcanics and sediments fromnorth-centralNewfoundland(e.g., New Bay Formation, Franks 1974; Summerford Group, Reusch 1983; Moretons Harbour and Chanceport Groups, Armstrong written comm. 1986). As Zen (1974) indicated several years ago, it is attractiveto relate the widespread occurrence of pumpellyite-bearing assemblages in mafic volcanics and associated sediments of the northernAppalachians to the incorporation of these low temperature suites in a relatively high pressure orogenic wedge (see, for example, Hepworth et al. 1982). Following this suggestion, the Newfoundland rocks would represent the relatively higher structurallevels of the wedge compared to those in New Brunswick (blueschists). A yet unresolved problem is the age of metamorphismin this part of the CMB. Various timing schemes have been proposed (e.g., Dewey et al. 1983; Rast 1983), but few metamorphic events have been dated radiometrically or relative to deformation events. At the present, therefore,it is not possible to determine conclusively which metamorphicassemblages were formed during each metamorphic event. For example, Bostock 1988 describes amphibolite facies conditions superimposed on low-grade assemblages in the vicinityof plutons that intruded the Roberts Arm Group of northcentralNewfoundland. To the east, on New World Island, sub-greenschistto low greenschist assemblages (quartz, chlorite, muscovite, albite, clinozoisite) are unlikely to be related to burial or tectonic thickening,since these assemblages are uniformlydistributed across a nearly vertical, 8 km thick sequence


542


therefore disagree with the view that the Dunnage and Gander zones are exotic with respect to one another (cf. Williams and Hatcher 1983; Keppie 1985) and combine these two zones into the Central Mobile belt as was originally done in Williams (1964). Note that we stillrecognize the presence and consequences of strike-slip movements withinthis belt, which will be discussed below. In Karlstrom et al. (1982) and Karlstrom (1983) it was concluded that the Dunnage zone of Newfoundland is an allochthonous terranethat is largely underlainby continental crust (see also Colman-Sadd and Swinden 1984; Keen et al. 1986), rather than representing the rooted remnant of the incompletely closed lapetus Ocean (Williams 1979; Rast and Stringer1980; Williamsand Hatcher 1983). Furthermore,we interpretthe CMB to include an ocean basin that developed to the AND CONCLUSION DISCUSSION southeast of the pre-Middle Ordovician magOur comparison of the regional strati- matic arc. Earlier, a similartectonic setting for this area was inferredby, for example, graphicsequences, the timingand styleof deStevens et al. (1974). Following accretion of formationevents, and the metamorphichistories of central Newfoundland, northern the magmaticarc to the NorthAmerican continentin Middle Ordovician times("Taconian New Brunswick, and surrounding areas Orogeny"), this ocean basin was closed durreflectsthe remarkablesimilarityin the Early ing Late Ordovician to Siluriantimes, and an to Middle Paleozoic depositional, deformaaccretionary complex was formed. We protionaland metamorphichistoryof the Central Mobile Belt of the Canadian Appalachians. pose the name lapetus II to describe this baThis similarity,in part, has been recognized sin in contrast to lapetus I, which refersto the ocean basin that was closed during the by otherworkers (e.g., Rast et al. 1976; WilTaconic orogeny (fig. 2). The presence of liams 1979), but neverthelesshas resultedin a subdivisionthatlargelyignoresthe important early south(east) directed thrusts and, locally, high pressure metamorphic assempost-Middle Ordovician historyof the orogen blages such as glaucophane schists and and the close relationshipbetween the Dunpumpellyite-actinolitefacies rocks in the nage and Gander zones throughtime. We recaccretionary stack, suggest north(west)ognize thatLower Ordovician and older Gander zone rocks are lithologically different directedsubduction in lapetus II. This is supfromtime correlatives in the Dunnage zone; portedby the presence of Early Siluriancalcalkaline volcanics in eastern Quebec (David increasingly,however, evidence is presented and Gariepy 1986) and a coeval calc-alkaline thatthese two zones were closely associated suite in the Topsails Terrane of westernNewfrom at least Middle Ordovician times onward. For example, in Newfoundland it has foundland (Rainy Lake Complex, Whalen et al. 1987). Subsequent collision between the been suggested that equivalents of Dunnage zone rocks are locally deposited on, or are North American craton with the accreted lateral facies equivalents of Gander zone magmaticarc and the eastern continent(Avrocks (e.g., Kennedy and McGonigal 1972; alonia) resulted in what is generallyreferred to as the Acadian Orogeny, which we equate Pickerill et al. 1981; Blackwood 1982; Colwith our F2 deformationand possibly some man-Sadd 1980; Wonderley and Neuman 1984). Moreover, the structural-metamorphic of the younger deformationevents. A more recent plate tectonic analogue of histories of these terranes from Middle Orthe above tectonic scenario is the Japan redovician time onward are closely related. We of fault repeated Ordovician and Silurian graywackes and conglomerates. Instead, this metamorphism is attributed to Devonian igneous activity (van der Pluijm 1984; see also Nelson 1979; Dallmeyer et al. 1983). A Barrovian type of metamorphismis present in Gander zone rocks of Newfoundland (Kennedy 1975; Chorlton and Dallmeyer 1986) and is comparable to thatin the northern Miramichi zone of New Brunswick. Chorlton and Dallmeyer (1986) obtained Devonian hornblendecooling ages for this Barrovian metamorphism, whereas Kennedy (1975) concluded a pre-Caradocian age for this same event. The latter, however, is disputed by Pajari and Currie (1978), ColmanSadd (1980), and Hanmer (1981) who concluded that regional deformation and associated metamorphismmainlytook place duringSilurian and/orDevonian times.


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gion, where the initiationof eastward subduction of the Japan Sea underJapan is recorded by thrust-typefocal mechanism solutions (e.g., Kanamori and Astiz 1985). This eastward subduction of the Japan Sea is opposite to the westward subduction of the Pacific plate under Japan. In summary,we combine the Dunnage and Gander zones/terranesinto the CMB, which records the formation and subsequent destructionof the lapetus II ocean duringLate Ordovician to Early Devonian times. We view sedimentation and deformationin the CMB as roughly continuous, although diachronous processes, which are not representativeof discrete orogenic events (cf. Williams 1979; Williams and Hatcher 1983). Combining the Dunnage and Gander is supportedby a recentdeep seismic reflectionline along the northeasternshore of Newfoundland (Keen et al. 1986), which shows thatthe boundary between the Dunnage and Gander zones (the GRUB line) is not a deep crustal structurebut rathera relativelyshallow feature (thrust?) that superimposed Dunnage rocks onto Gander rocks. The Widthoflapetus H.-The widthof the Ordovician lapetus II ocean is not well known. However, we can obtain an estimate frompaleomagnetic data. Paleomagnetic determinationsof the Avalonian Dunn Point Group in Nova Scotia indicated that Avalon was at 42°S in Late Ordovician-EarlySilurian times, which is in general agreement with faunaldata (Cocks and Fortey 1982; Neuman 1984). At that time, the margin of North America was oriented approximatelynortheast-southwest, and the correspondingposition of Avalon on this marginwas located at approximately 150S (Van der Voo and Johnson 1985). However, this does not give the true width of lapetus II since the relative paleolongitudinal positions of Avalon and North America are not constrained by the paleomagnetic data. The convergence of Avalon and North America, and thus closure of lapetus II, started in Late Ordovician times and was completed by Middle Devonian times,which spans a timeintervalof approximately 50 m.y. If we assume orthogonal convergence between Avalon and North America the paleomagnetic data would suggest a width on the order of 3500 km, which requires a relativelyhighrate of convergence

543

of 7 cm/yrto close the basin. We can decrease the widthof the basin while maintainingpaleomagneticconstraintsby movingAvalon along its line of paleolatitude in the direction of the North American margin. Consequently,convergence would be oblique witha more north-southdirected movement vector,and finalclosure would have been accompanied by strike-slip movement. With this scenario, the sense of fault movement is constrainedby the relatively southern positionof Avalon withrespect to North America and would thereforehave to have been sinistral. Evidence of Silurian sinistral fault movements (e.g., Hanmer 1981) are rare in the northernAppalachians. Generally, dextral faultmovementshave been proposed (Webb 1969; Bradley 1982; Keppie 1982; Mawer and White 1986; Kusky et al. 1987), especially on faultsthat were active in Devonian and Carboniferous times. These data contrast with the sense of fault movements in the British part of the Appalachian/Caledonian chain, where large (>1500 km), Silurian to Early Devonian sinistral strike-slip motions are consideredto be associated withoblique convergence and closure of the lapetus ocean system(Dewey and Shackleton 1984; Soper and Hutton 1984; Murphy and Hutton 1986). It is clear that furtherdetailed work is needed to decipher the exceedingly complex history of the Central Mobile Belt. These studies should pay special attention to the timingand sense of displacement on reverse as well as on strike-slipfaults, the nature of the lapetus II ocean basin (trapped ocean vs. marginalsea) and the presence of tectonic elementssuch as ocean islands withinthisbasin. ACKNOWLEDGMENTS.-The data and views in this paper are based on our ongoing collaborative and independent research in the Canadian Appalachians. We acknowledge the many discussions with other workers in the area; their continued interest and criticism has helped to shape, clarifyand especiallymodifymanyof our views. In particular we thank Paul Williams for discussions and commentson numerous draftsof this paper. Versions of this paper were also read by Ken Currie, Colleen Elliott, Rex Johnson, Ron Pickerill,JohnRodgers, Peter Stringer,Reid


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