characteristics and evolution of the central mobile - global change

Characteristics and Evolution of the Central Mobile Belt, Canadian AppalachiansAuthor(s): Ben A. van der Pluijm and Cees R. van StaalReviewed work(s):Source: The Journal of Geology, Vol. 96, No. 5 (Sep., 1988), pp. 535-547Published by: The University of Chicago PressStable URL: http://www.jstor.org/stable/30053568 .

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CHARACTERISTICS AND EVOLUTION OF THE CENTRAL MOBILE BELT, CANADIAN APPALACHIANS'

BEN A. VAN DER PLUIJM, AND CEES R. VAN STAAL2

Department of Geological Sciences, University of 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 metamorphic histories of central Newfound- land and New Brunswick show remarkable similarities. These characteristics should be reflected in zonal or terrane subdivisions of the Canadian Appalachians, and we therefore recombine the Dunnage and Gander zones/terranes into the Central Mobile Belt (CMB). In our interpretation, the CMB is a deformed Ordovi- cian ocean basin (Iapetus II), that was formed in between the Taconic magmatic arc and the Avalon continent during Mariana-type subduction of the lapetus I ocean. The generalized stratigraphic sequence of the CMB records the formation of ophiolitic crust and continental margin sediments of lapetus II in Early Ordovician times, which was followed by a period of relative quiescence 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-slip faulting has been recognized. The major (Late Ordovician?-Early Silurian) thrusting episode records the formation of an accretionary complex that was subsequently deformed during continued regional shortening. Transcurrent faulting mainly occurred in Silurian and Devonian times. Mineral assemblages in the area are representative of two distinct metamorphic events. The first event produced medium to high pressure assemblages and is associated with the formation of the accretionary complex. The second, low pressure event is correlated with the widespread intrusion of granitic bodies. The width of the Iapetus II ocean is not well established. Paleomagnetic evidence indicates that either this ocean basin was 3500 km wide (orthogonal convergence) or that closure was oblique. In order to reduce the width of Iapetus II, sinistral strike-slip movements would have to have accompanied final closure in Silurian times, but this is generally not supported by field evidence which indicates dominantly dextral movements during that time.

INTRODUCTION

The Canadian Appalachians, comprising Newfoundland, Prince Edward Island, New Brunswick, Nova Scotia, and part of Quebec, form the northern extension of the Appala- chians 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) Precam- brian rocks and Paleozoic shelf facies in the northwest (later named the Western Plat- form; Kay and Colbert 1965), (2) the Central Paleozoic Mobile Belt in the central part of Newfoundland, and (3) Precambrian rocks and Paleozoic shelf facies (Avalon Platform; Kay and Colbert 1965) in the southeast. Sub-

1 Manuscript received August 28, 1987; accepted April 6, 1988.

2 Geological Survey of Canada contribution no. 44487.

[JOURNAL OF GEOLOGY, 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 neighboring mainland 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 northwest to 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 tectonostratigraphic zones 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 their tectonic 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 Williams and Hatcher (1983) is illustrated.

Rast et al. (1976) correlated across the Cabot Strait between Newfoundland and

535



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

FIG. 1.-Terranes in the northern Appalachians (from Williams and Hatcher 1983). PEI-Prince Edward Island; DSRP-Deep Seismic Reflection Profile.

New Brunswick and, in addition to the above five zones, introduced a zone in between the Gander and the Avalon in New Brunswick, which they called the Fredericton zone. This zone was considered to be a fragment of a Precambrian terrane that was continuous with the Gander zone to the north and the Avalon zone to the south. The Miramichi zone in north-central New Brunswick was thought by Rast et al. (1976) to represent the continuation of the Newfoundland Gander zone into New Brunswick, which is largely based on the presence of quartz-rich turbidites that are similar to rocks in the New- foundland Gander zone.

Fyffe (1977), in a discussion of Rast et al. (1976), argued against this correlation on the basis of age, differences in rock types, and deformation between these two areas (see also Rast 1983). Characteristic of the Mira- michi zone is the presence of a mixed Late Arenigian to Middle Caradocian volcanic- sedimentary rock assemblage and Carado- cian (graptolitic) black shales (Fyffe 1977, 1982; van Staal 1987), which are generally not 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; Ar- nott et al. 1985; van der Pluijm et al. 1987), although Wonderley and Neuman (1984) describe an isolated occurrence of Early Or- dovician "Dunnage"-type volcanic and vol-

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 interpretation was largely adopted by Williams (1978, 1979) on the tectonic-lithofacies map 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-Millstream Fault, and the boundary between Gander and Av- alon zones is marked by the Fredericton (-Norumbega) Fault. Fyffe et al. (1983) dis- agreed with the latter boundary on the basis of similarities in lithology and paleontology across the Fredericton Fault. They group the southwestern part of the Miramichi High- lands and the Cookson Inlier in southwestern New Brunswick within the same terrane.

Recent studies in central Newfoundland (Nelson 1981; Karlstrom et al. 1982; Colman- Sadd and Swinden 1984; van der Pluijm 1986; Kirkham 1987) and northern New 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 deformed Early to Middle Paleozoic accretionary complex that was formed in a Mariana-type (Uyeda and Kanamori 1979) or

536



EVOLUTION OF THE CENTRAL MOBILE BELT

NW SE

IAPETUS I IAPETUS 11 AW

EARLY

ORDOVICIAN

CMB

AW

EARLY

SILURIAN

Continental Crust Gander and Lower

Oceanic Crust Tetagouche Groups

Upper Mantle AW Accretionary Wedge

FIG. 2.-The tectonic evolution of the northern Appalachians in Early to Middle Paleozoic times, with crust and lithospheric upper mantle shown. CMB-Central Mobile Belt; see text for explana- tion.

Okinawa-type (Letouzey and Kimura 1985) convergent setting, consisting of an eastward dipping subduction zone below a magmatic arc with concurrent back-arc spreading to the southeast (fig. 2).

In this paper we will compare the depositional, deformational, and metamorphic events of New Brunswick and Newfoundland and discuss some of the consequences for in- terpretations of the Appalachian orogenic history. We propose a revision of earlier regional subdivisions and will further constrain our tectonic model (fig. 2). We do not attempt to present a review of the Canadian Appala- chians; for this the reader is referred to Wil- liams (1979), McKerrow (1983), Dewey et al. (1983), and Rast (1983).

STRATIGRAPHY, DEFORMATION, AND

METAMORPHISM

In this section, we will discuss and compare the stratigraphic sequences, and the deformation and metamorphic histories of the central portion of the Canadian Appala- chians, with the emphasis on results we obtained from the eastern Notre Dame Bay area of north-central Newfoundland and the northern part of the Miramichi zone and Elmtree Inlier in northern New Brunswick.

Stratigraphy and Zonal Subdivision.-The Paleozoic rock units of the northern Appala- chians are relatively rich in fossils, which has allowed for detailed chronostratigraphic stud-

537

ies over the past 50 yrs (e.g., Neuman 1984). Radiometric results from selected rock units generally confirm these fossil ages (e.g., Dun- ning et al. 1987). Following is a summary of our current knowledge of the stratigraphy of the central parts of New Brunswick and Newfoundland; in figure 3 the Ordovician to Early Silurian stratigraphies of these areas are shown.

The central portions of New Brunswick are underlain by Cambro-Ordovician volcano- sedimentary rocks known mainly as the Tetagouche Group (fig. 3, N. MIR), but also including correlatives such as the Shin Brook Formation (Neuman 1967) and the Winter- ville Formation (Roy and Mencher 1976) in adjacent Maine. The base of the Tetagouche Group is defined by a sequence of alternating quartzites and semi-pelitic phyllites, which are generally considered to be correlatives of the Cambrian Grand Pitch Formation in Maine (Neuman 1967; Rast and Stringer 1974), the lower part of the Cookson Forma- tion in southern New Brunswick (Rast and Stringer 1974; Fyffe et al. 1983) and the Gan- der Group in Newfoundland (Rast et al. 1976; Williams 1979). These dark gray to black, graphitic phyllites and interbedded quartzites are overlain by coeval calcareous and tuf- faceous siltstones in the northern part of the Miramichi zone (Fyffe 1976; Neuman 1984). Tremadocian and Arenigian graptolites have been found in these black phyllites in the Benton area of the southern part of the Miramichi zone (Fyffe et al. 1983) and in the Cookson Inlier in southern New Bruns- wick (Cumming 1967).

Llandeilian-Early Caradocian basalts (Nowlan 1983a) and associated red Fe/Mn- rich shales follow, or are in part coeval with silicic volcanics and are spatially related with a fragment of oceanic crust preserved in the Elmtree Inlier (fig. 3, ELMTREE; Rast and Stringer 1980). The composition of the basalts ranges from MORB-like tholeiites to alkaline within-plate basalts (van Staal 1987). The Middle Ordovician volcanism was in part coeval with, and followed by, deposition of volcaniclastic graywackes, limestones, and gray to black shales during Llan- deilian to Caradocian times. Some of these limestones (e.g., the Waterville Limestone) have an identical conodont fauna to the Cobbs Arm Limestone on New World Island,



SIL

ORDOVICIA

NMB ELMTREE

438

505

LLANDOVERIAN

ASHGILLIAN

CARADOCIAN

LLANDEILIAN

LLANVIRNIAN

ARENIGIAN

TREMADOCIAN

N. MIR NDB LEGEND

Conglomerates

Greywackes

Shales

Limestones

Volcanics

Sandstones Siltstones

SOphiolites

FIG. 3.-Generalized Ordovician and Early Silurian stratigraphic sequences of the Central Mobile Belt of north-central New Brunswick, i.e., Northern Matapedia Basin (NMB), the Elmtree Inlier (ELMTREE) and the northern Miramichi (N.MIR), and the Notre Dame Bay area of northeastern Newfoundland (NDB). The volcanics are further subdivided into mafic (m) and felsic (f). Time scale is from DNAG (Palmer 1983).

Newfoundland (Nowlan 1981). The black shales become a fairly continuous blanket in Caradocian times and are followed by tur- biditic graywackes (Fyffe 1982) that may be proximal correlatives of the Grog Brook For- mation turbidites (Rast and Stringer 1980) of Late Caradocian to Ashgillian age to the west of the Miramichi zone, 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 ap- pears to lie conformably on Early Caradocian black shales and mafic volcanics (Potter 1964; Philpott 1988), which are lithological correlatives of the upper part of the Tetagouche Group. The overlying shallow water limestones, shales, sandstones, conglomerates and bimodal mafic and silicic volcanics generally are of Late Silurian and Devonian age (e.g., Fyffe 1982).

In the central part of Newfoundland a regional stratigraphy was 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") mafic and felsic volcanics, with limestone, sandstone, man- ganiferous chert, and shale interbeds (e.g., Exploits Group, Helwig 1969) to the northwest. Tremadocian graptolites have 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, Berg- strom et al. 1974). Overlying this 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 Con- glomerate, 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 stratigraphies of central New Brunswick and north-central New- foundland are in close agreement (fig. 3; see also Neuman 1984). In addition, similar stratigraphic sequences have been reported from the southern and central Appalachians (e.g., Shanmugam and Lash 1982; Hiscott et al. 1983) and the British Isles (e.g., Leggett et al. 1979), which suggests that the depositional setting was similar over a large tract of the Appalachian/Caledonian chain.

The rock types in the Newfoundland sequence described above are mainly found in the Dunnage zone, whereas those from the New Brunswick sequence are considered to be mainly part of the Gander zone (Williams 1979; Williams and Hatcher 1983). Further- more, the distinction between Dunnage and

B. A. VAN DER PLUIJM AND C. R. VAN STAAL 538




FIG. 4.-Proposed revision of the zonal subdivision of the Canadian Appalachians. The Humber is the Early Paleozoic North American margin (miogeocline); the Central Mobile Belt comprises remnants of the lapetus II ocean and an Ordovician magmatic arc; Avalon is the microcontinent at the eastern margin of lapetus II. Numbered boundaries and faults are: 1-Dover-Hermitage Bay Fault System; 2-Baie Verte- Brompton Line; 3-Canso Fault (McCutcheon and Robinson 1987); 4-Rocky Brook-Millstream Fault; 5- Fredericton(-Norumbega) Fault; 6-Belleisle Fault System (includes Wheaton Brook Fault); 7-Cobequid- Chedabucto Fault System. The locations of the New World Island (NWI) and Bathurst areas (B) and Cape Breton Island (CBI) are indicated. "C" represents flat-lying Paleozoic rocks outside the Appalachian orogenic belt, and "T" is for Triassic rocks.

Gander terranes is complicated by isolated occurrences of Dunnage-type rocks that stratigraphically overlie Gander-type rocks, 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 stratigraphic relationships were subsequently tectonized when Dunnage zone rocks were thrust over the Gander zone (see below). Thus Gander rocks represent at least in part lateral equivalents of Dunnage rocks and were spatially linked from at least Middle Ordovician times onward. Therefore, the Dunnage and Gander zones do not represent two exotic terranes (cf. Hatcher and Williams 1983); rather, we interpret them to be parts of a telescoped ocean basin which was locally floored by oceanic crust, remnants of 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 Stringer 1980). Remnants of the Taconian magmatic arc can be found to the northwest (e.g., Twillingate Terrane, Hungry Mountain Complex in the Topsails Terrane), which combined with the ocean ba-

sin to the southeast form the Central Mobile Belt (CMB; fig. 4). On the basis of geochemistry and 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 boundary is 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 north of the Belleisle Fault, at the Wheaton Brook Fault. How- ever, Leger and Williams (1986) suggest that these faults are segments of a once continuous dextral transcurrent fault, separated by a later northwest striking transcurrent fault.

For the location of the CMB in Nova Scotia we tentatively follow Barr et al. (1987), who interpret the presence of Gren- ville age rocks on northern Cape Breton Is-

539




land (Blair River Complex) as the continuation of the Newfoundland Humber zone onto the mainland of Canada. Their interpretation necessitates significant thinning of the CMB on Cape Breton Island (fig. 4), where Ordovi- cian volcanic or sedimentary sequences have not yet been identified (Barr pers. comm.). One interpretation for this configuration is the irregular shape of the Grenvillian margin of North America (the St. Lawrence Promon- tory, Stockmal et al. 1987).

Deformation Events and Tectonic His- tory.-Detailed structural analysis of key areas in northcentral Newfoundland (e.g., Kennedy 1975; Nelson 1981; Karlstrom et al. 1982; Elliott 1985; van der Pluijm 1986) and in northern New Brunswick (e.g., Helmstaedt 1971; Irrinki 1979; van Staal and Williams 1984; van Staal 1987) show a remarkable similarity in deformation style and sequence. In this section, the different deformation generations and their characteristics for the New World Island area of Newfoundland and the Bathurst area of New Brunswick will be discussed. We realize the problems and limita- tions associated with correlation of structures over any great distances in complexly deformed areas (Williams 1985) such as these. For example, different fold generations that can be distinguished by overprinting relationships do not necessarily imply discrete folding events, but may be due to progres- sive deformation. Furthermore, deformation across an orogen probably is diachronous. However, we feel that the similarity and con- sistency of especially the earlier deformation generations that we have recognized, strongly support our regional correlation.

The earliest deformation (Dl) is markedly heterogeneous and characterized by recumbent folds and bedding-parallel faults that are typical of thrusting. Major thrusts have been recognized in the CMB of Newfoundland, as well as New Brunswick (Pajari and Currie 1978; Nelson 1981; Thurlow 1981; Black- wood 1982; Fyffe 1982; Karlstrom et 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 thrusting in northern New Brunswick was toward the southeast (van Staal 1987), and this direction has been reported from several places in Newfound-

land (e.g., Colman-Sadd and Swinden 1984). Dl probably started in Late Ordovician times and continued into the Silurian. Evidence for younger post-Early Silurian reverse faulting in a northerly direction has been found in both New Brunswick (McCutcheon 1981; Fyffe 1982; Nowlan 1983b) and Newfound- land (Karlstrom et al. 1982). Dl has been related to underthrusting at the northwestern margin of lapetus II and the formation of an accretionary complex (fig. 2; van der Pluijm 1986; van Staal 1987).

The Dl structures are refolded by large wavelength, tight to very tight F2 folds and related structures of probable Silurian to Early Devonian age. A regional cleavage (S2) is generally associated with F2 folds, and locally we find a composite (S2 and rotated S1) foliation. F2 folds are overturned to the northwest and define alternating belts of flat- lying recumbent folds and steeply inclined folds in the northern part of the Miramichi zone of New Brunswick (van Staal and Wil- liams 1984). It is possible that the above men- tioned northwest-directed Silurian reverse faulting in Newfoundland and New Bruns- wick is associated with these early deformation events (van der Pluijm 1986), and may also be coeval with post-Early Silurian recumbent folds and thrusts recognized by Rast (1983).

F3 folds are open to tight, generally upright structures, that are associated with major ductile transcurrent faulting in Newfound- land and northern New Brunswick. Since F3 structures accommodate significant regional shortening they are related to a deformation regime of transpression. The youngest deformation (F4 and up) consists of chevron folds, box folds, and parallel and conjugate kinks with steeply to shallowly dipping axial sur- faces.

Chorlton and Dallmeyer (1986) describe a similar deformation sequence for rocks in the CMB of southern Newfoundland. Using radiometric techniques they determined that their oldest deformation event (thrusting and 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

540




Inlier in southern New Brunswick (Stringer and Burke 1985), and in the southern part of the Miramichi zone (e.g., Fyffe 1982). Com- bined this suggests that, in addition to its depositional history, the Early to Middle Paleozoic deformation history for the CMB in the Canadian Appalachians was also remark- ably similar over 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 porphyroblasts of albite, hornblende, clinozoisite, biotite, and grunerite is present in the stratigraphically lower parts of the Teta- gouche Group in northern New Brunswick. Metamorphic peak conditions (400-450°C, 4±+ 1 kb; van Staal 1985) were achieved during or after D1 deformation, but before the end of F2 (Helmstaedt 1971; van Staal 1985). Ml is thus bracketed by late Caradocian and late Silurian times.

Ml was followed by retrograde metamorphism that also took place before the end of F2 deformation. Late syn- or post-F2, pre-F3 porphyroblasts of 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 further study.

A narrow belt of glaucophane, crossite, and epidote bearing schists that were formed during Ml, are locally found within metabasalts of structurally higher and generally also stratigraphically higher parts of the Tetagouche Group (Skinner 1974, Trzcienski et al. 1984). The glaucophane schists are closely associated with andradite, aegirine- augite and pumpellyite bearing rocks (Whitehead and Goodfellow 1978). Pumpelly- ite or pumpellyite-actinolite bearing assemblages are in fact relatively common in the metabasalts, whereas the absence of prehnite is conspicuous in Tetagouche Group basalts of northern New Brunswick. These rocks probably experienced higher pressure conditions (see petrogenetic grid of Liou et al. 1985) and followed a P-T path that was different from the underlying volcanics and sediments of the Tetagouche Group (van Staal 1985). We interpret these higher pressure metabasalts and associated turbidites as rel-

icts of an accretionary complex. In Maine, correlatives of these rocks (e.g., the Win- terville Formation) are also characterized by widespread pumpellyite, either in assemblages with actinolite or prehnite (Richter and Roy 1974). This metamorphic event probably occurred in latest Ordovician or earliest Silu- rian times, but there may have been more than one event (Roy 1980).

Glaucophane and crossite-bearing schists are not known from the CMB of Newfound- land, but (sub-)greenschist facies conditions defined by mineral assemblages containing combinations of pumpellyite, actinolite, andradite, epidote and prehnite are quite common in Ordovician volcanics and sediments from north-central Newfoundland (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 attractive to relate the widespread occurrence of pumpellyite-bearing assemblages in mafic volcanics and associated sediments of the northern Appalachians 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 New- foundland rocks would represent the relatively higher structural levels of the wedge compared to those in New Brunswick (blue- schists).

A yet unresolved problem is the age of metamorphism in this part of the CMB. Vari- ous 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 metamorphic assemblages were formed during each metamorphic event. For example, Bostock 1988 describes amphibolite facies conditions superimposed on low-grade assemblages in the vicinity of plutons that in- truded the Roberts Arm Group of north- central Newfoundland. To the east, on New World Island, sub-greenschist to low greenschist assemblages (quartz, chlorite, musco- vite, albite, clinozoisite) are unlikely to be related to burial or tectonic thickening, since these assemblages are uniformly distributed across a nearly vertical, 8 km thick sequence

541




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 metamorphism is present in Gander zone rocks of Newfoundland (Kennedy 1975; Chorlton and Dallmeyer 1986) and is comparable to that in the northern Miramichi zone of New Brunswick. Chorlton and Dallmeyer (1986) obtained De- vonian hornblende cooling ages for this Bar- rovian metamorphism, whereas Kennedy (1975) concluded a pre-Caradocian age for this same event. The latter, however, is dis- puted by Pajari and Currie (1978), Colman- Sadd (1980), and Hanmer (1981) who concluded that regional deformation and associated metamorphism mainly took place during Silurian and/or Devonian times.

DISCUSSION AND CONCLUSION

Our comparison of the regional stratigraphic sequences, the timing and style of deformation events, and the metamorphic histories of central Newfoundland, northern New Brunswick, and surrounding areas reflects the remarkable similarity in the Early to Middle Paleozoic depositional, deformational and metamorphic history of the Central Mobile Belt of the Canadian Appalachians. This similarity, in part, has been recognized by other workers (e.g., Rast et al. 1976; Wil- liams 1979), but nevertheless has resulted in a subdivision that largely ignores the important post-Middle Ordovician history of the orogen and the close relationship between the Dun- nage and Gander zones through time. We recognize that Lower Ordovician and older Gan- der zone rocks are lithologically different from time correlatives in the Dunnage zone; increasingly, however, evidence is presented that these two zones were closely associated from at least Middle Ordovician times onward. For example, in Newfoundland it has been suggested that equivalents of Dunnage zone rocks are locally deposited on, or are lateral facies equivalents of Gander zone rocks (e.g., Kennedy and McGonigal 1972; Pickerill et al. 1981; Blackwood 1982; Col- man-Sadd 1980; Wonderley and Neuman 1984). Moreover, the structural-metamorphic histories of these terranes from Middle Or- dovician time onward are closely related. We

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 still recognize the presence and consequences of strike-slip movements within this 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 terrane that is largely underlain by continental crust (see also Colman-Sadd and Swinden 1984; Keen et al. 1986), rather than repre- senting the rooted remnant of the incompletely closed lapetus Ocean (Williams 1979; Rast and Stringer 1980; Williams and Hatcher 1983). Furthermore, we interpret the CMB to include an ocean basin that developed to the southeast of the pre-Middle Ordovician magmatic arc. Earlier, a similar tectonic setting for this area was inferred by, for example, Stevens et al. (1974). Following accretion of the magmatic arc to the North American continent in Middle Ordovician times ("Taconian Orogeny"), this ocean basin was closed during Late Ordovician to Silurian times, and an accretionary complex was formed. We propose the name lapetus II to describe this basin in contrast to lapetus I, which refers to the ocean basin that was closed during the Taconic orogeny (fig. 2). The presence of early south(east) directed thrusts and, locally, high pressure metamorphic assemblages such as glaucophane schists and pumpellyite-actinolite facies rocks in the accretionary stack, suggest north(west)- directed subduction in lapetus II. This is supported by the presence of Early Silurian calc- alkaline volcanics in eastern Quebec (David and Gariepy 1986) and a coeval calc-alkaline suite in the Topsails Terrane of western New- foundland (Rainy Lake Complex, Whalen et al. 1987). Subsequent collision between the North American craton with the accreted magmatic arc and the eastern continent (Av- alonia) resulted in what is generally referred to as the Acadian Orogeny, which we equate with our F2 deformation and possibly some of the younger deformation events.

A more recent plate tectonic analogue of the above tectonic scenario is the Japan re-

542




gion, where the initiation of eastward subduction of the Japan Sea under Japan is recorded by thrust-type focal 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/terranes into the CMB, which records the formation and subsequent de- struction of the lapetus II ocean during Late Ordovician to Early Devonian times. We view sedimentation and deformation in the CMB as roughly continuous, although diachronous processes, which are not representative of discrete orogenic events (cf. Wil- liams 1979; Williams and Hatcher 1983). Combining the Dunnage and Gander is supported by a recent deep seismic reflection line along the northeastern shore of Newfound- land (Keen et al. 1986), which shows that the boundary between the Dunnage and Gander zones (the GRUB line) is not a deep crustal structure but rather a relatively shallow fea- ture (thrust?) that superimposed Dunnage rocks onto Gander rocks.

The Width oflapetus H.-The width of the Ordovician lapetus II ocean is not well known. However, we can obtain an estimate from paleomagnetic data. Paleomagnetic de- terminations of the Avalonian Dunn Point Group in Nova Scotia indicated that Avalon was at 42°S in Late Ordovician-Early Silurian times, which is in general agreement with faunal data (Cocks and Fortey 1982; Neuman 1984). At that time, the margin of North America was oriented approximately northeast-southwest, and the corresponding posi- tion of Avalon on this margin was located at approximately 150S (Van der Voo and John- son 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 Av- alon and North America, and thus closure of lapetus II, started in Late Ordovician times and was completed by Middle Devonian times, which spans a time interval of 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 relatively high rate of convergence

of 7 cm/yr to close the basin. We can de- crease the width of the basin while maintain- ing paleomagnetic constraints by moving Av- alon along its line of paleolatitude in the direction of the North American margin. Consequently, convergence would be oblique with a more north-south directed movement vector, and final closure would have been accompanied by strike-slip movement. With this scenario, the sense of fault movement is constrained by the relatively southern posi- tion of Avalon with respect to North America and would therefore have to have been sinistral.

Evidence of Silurian sinistral fault movements (e.g., Hanmer 1981) are rare in the northern Appalachians. Generally, dextral fault movements have been proposed (Webb 1969; Bradley 1982; Keppie 1982; Mawer and White 1986; Kusky et al. 1987), especially on faults that were active in Devonian and Car- boniferous 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 considered to be associated with oblique convergence and closure of the lapetus ocean system (Dewey and Shackleton 1984; Soper and Hutton 1984; Murphy and Hutton 1986).

It is clear that further detailed work is needed to decipher the exceedingly complex history of the Central Mobile Belt. These studies should pay special attention to the timing and sense of displacement on reverse as well as on strike-slip faults, the nature of the lapetus II ocean basin (trapped ocean vs. marginal sea) and the presence of tectonic elements such as ocean islands within this basin.

ACKNOWLEDGMENTS.-The data and views in this paper are based on our ongoing col- laborative and independent research in the Canadian Appalachians. We acknowledge the many discussions with other workers in the area; their continued interest and criti- cism has helped to shape, clarify and especially modify many of our views. In particular we thank Paul Williams for discussions and comments on numerous drafts of this paper. Versions of this paper were also read by Ken Currie, Colleen Elliott, Rex Johnson, Ron Pickerill, John Rodgers, Peter Stringer, Reid

543




Wellensiek and Hank Williams and the 'final' version benefited from comments made by three anonymous reviewers. Research was supported by the Division of Earth Sciences, the National Science Foundation Grant EAR 86-12469 (BvdP), the Geological Survey of

Canada, the Canada-New Brunswick Mineral Development Agreement 1984-1989 and a Natural Sciences and Engineering Research Council of Canada post-graduate scholarship (CvS), and A7419 (P. F. Williams).

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characteristics and evolution of the central mobile - global change

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