Nick Rast and the recognition of the Avalonian Arc

Journal of Geodynamics 37 (2004) 437–455 www.elsevier.com/locate/jog

Nick Rast and the recognition of the Avalonian Arc R. Damian Nancea,*, J. Brendan Murphyb, J. Duncan Keppiec a Department of Geological Sciences, Ohio University, 316 Clippinger Laboratories, Athens, OH 45701, USA Department of Earth Sciences, St. Francis Xavier University, PO Box 5000, Antigonish, Nova Scotia, Canada B2G 2W5 c Instituto de Geologı´a, Universidad Nacional Auto´noma de Me´xico, 04510 Me´xico D.F., Mexico

b

Abstract Recognition of the eastern (Avalonian) margin of the northern Appalachian orogen as a Late Precambrian microcontinental arc terrane, rather than the opposing passive margin of the Proto-Atlantic (Iapetus) Ocean to that of eastern Laurentia, constituted a fundamental advance in Appalachian geology that profoundly influenced subsequent models for the orogen’s plate tectonic evolution. This advance was first clearly articulated by Nick Rast and his students in 1976, who, by correlating rocks of the Avalon Platform with those of the British Midlands, established the Avalonian volcanic belt as a Japan-like microcontinent. Contrary to contemporary views of the Avalon Platform, which favored an extensional, Basin and Range-like setting for its volcanism, Rast argued on the basis of this correlation that the association of Avalonian volcanism with compressional orogeny, widespread calc-alkaline plutonism and, in Angelsey, with blueschists and ophiolitic rocks, indicated a convergent plate margin setting. Rast further proposed that the Avalonian volcanic belt was ensialic, and was bordered to the northwest and southeast by Precambrian oceanic domains. Contemporary reconstructions of the Avalonian and Cadomian belts as fragments of a Cordilleran-like accretionary orogen that developed along an active margin of Neoproterozoic Gondwana owe their origin to these early ideas and, while far removed from the tectonic model that Rast envisaged, are a direct heritage of his recognition of the Avalonian volcanic belt as a microcontinental arc terrane. # 2004 Elsevier Ltd. All rights reserved.

1. Introduction With the passing of Nick Rast on August 28, 2001, the earth science community lost one of its most erudite and colorful geologists. A devoted mentor, a loyal friend, and a formidable adversary, Rast began his career in geology in 1956 with a doctoral degree from the University of Glasgow. Over the next 15 years, Rast would have a profound impact on British geology, initially as a

* Corresponding author. Tel.: +1-740-593-1107; fax: +1-740-593-0486. E-mail address: [email protected] (R.D. Nance). 0264-3707/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jog.2004.02.015

438

R.D. Nance et al. / Journal of Geodynamics 37 (2004) 437–455

lecturer at the University of Aberystwyth in Wales, and later at the University of Liverpool, where he was rapidly promoted to Reader. Rast’s contributions to British geology covered a wide range of topics, from field studies in Scotland, Ireland and Snowdonia describing structural (e.g., King and Rast, 1955, 1959; Rast, 1958a, 1961a, 1963), volcanic (e.g., Rast et al., 1958; Rast, 1961b, 1969), stratigraphic (e.g., Smith and Rast, 1958; Harper and Rast, 1964; Rast and Litherman, 1970) and metamorphic (e.g., Rast, 1958b; Harris and Rast, 1960a) relationships, to process-oriented investigations of small-scale folding (e.g., King and Rast, 1956; Rast and Platt, 1957; Rast, 1964), boudinage (Rast, 1956), the growth of metamorphic minerals (e.g., Rast and Sturt, 1957; Harris and Rast, 1960b; Rast et al., 1962; Rast, 1965, 1966), and the ascent and emplacement of magmas (Rast, 1970). To each of these topics he brought an insight gained from both a keen observational eye and a perceptive ability to recognize the large-scale implications of his observations. But in 1971, Rast left England to become Chair of Geology at the University of New Brunswick in Canada, a position he would hold until 1979. In doing so, Rast brought his expertise, his notable linguistic abilitites, and his wide knowledge of Caledonian geology, to the geology of the northern Appalachians. The impact was profound and immediate, and would continue into the central and southern Appalachians following his prestigious appointment as Hudnall Chair of Structural Geology and Tectonics at the University of Kentucky, a position he held until his retirement in 2001. Rast stormed into northern Appalachian geology by recognizing the existence, in southern New Brunswick, of a Late Paleozoic fold-thrust belt that could be correlated with the Variscan front of southern Britain and Europe (e.g., Rast and Grant, 1973; Rast and Currie, 1976; Mosher and Rast, 1984; Rast, 1983, 1984). As a result, the term ‘‘Maritime Disturbance,’’ previously used to describe the deformation by Poole (1967), was sometimes applied to Rast himself! Rast similarly plunged into central and southern Appalachian geology with a pivotal paper on the origin of the Ocoee Supergroup (Rast and Kohles, 1986). However, his most lasting contribution to our understanding of the Appalachian orogen was his recognition of the nature of its eastern (Avalonian) margin and the significance of this belt to plate tectonic models for Appalachian evolution. In the following retrospective essay we re-examine this contribution and the impact it has had on subsequent models for the development of the Appalachian orogen.

2. Role of Avalon Platform in early models of Appalachian evolution At the time that Rast joined the University of New Brunswick in 1971, plate tectonic principles, then recently established on the basis of present-day geodynamics and Cenozoic geology, were being applied for the first time to older geologic provinces. The Appalachian orogen was an obvious candidate for such test cases and plate tectonic models for its evolution had already been proposed, most notably in the landmark papers of Wilson (1966), Dewey (1969), and Bird and Dewey (1970). While representing an enormous step forward from earlier geosynclinal models for Appalachian evolution (e.g., Kay, 1947, 1951; Dietz, 1963; Dietz and Holden, 1967; Poole, 1967), all such reconstructions were initially based on the belief that the southeastern flank of the orogen, defined in Newfoundland as the Avalon Platform (Williams, 1964), represented its stable southeastern margin.

R.D. Nance et al. / Journal of Geodynamics 37 (2004) 437–455

439

Plate tectonic models consequently viewed the orogen as a symmetrical system in which the Avalon Platform (and its presumed correlatives in New England, Maritime Canada, the British Midlands and northwest France; Zone C of Dewey, 1969, and Bird and Dewey, 1970) first rifted away from Laurentia as Iapetus (then the Proto-Atlantic Ocean) opened in the Late Precambrian, and was later re-accreted to Laurentia as the Iapetus Ocean closed, a collision attributed at that time to the Acadian orogeny and so assigned to the Devonian (Fig. 1). In this way, the plate tectonic evolution of the Appalachian orogen was the prototype Wilson cycle in which the

Fig. 1. Plate tectonic evolution of the Caledonide orogen in Britain proposed by John Dewey in 1969. (a) Spreadingexpanding phase, (b) sequential onset of subduction beneath southeast and (c) northwest margins, (d) development of flysch wedges that (e) swamp oceanic realm as ocean contacts, and (f) final stages of ocean closure. T=Torridonian, L=Longmyndian, M=Moinian, D=Dalradian, DL=Durness Limestone, GV=Gwna Volcanics, MC=Mona Complex, BZ 1-3=Benioff zones in order of development, MNT=Moine Thrust, BV=Ballantrae Volcanics, ISH= Irish Sea Horst, PHF=Pontesford Hill Fault, CSF=Church Stretton Fault, SMT=South Mayo Trough, HBS=Highland Boundary Serpentinite, BBWV=Borrodale, Balbriggan and Waterford Volcanics, DF=Dinorwic Fault, SV=Snowdon Volcanics. Zones A, B and C (Fig. 1f) represent, respectively, the northwestern continental shelf and rise, oceanic domain, and stable southeastern margin of the Proto-Atlantic Ocean. As such, they correspond closely to the Humber Zone, Central Mobile Belt (Dunnage and Gander Zones), and Avalon Platform of Newfoundland and the northern Appalachians. Figure modified from Dewey (1969).

440

R.D. Nance et al. / Journal of Geodynamics 37 (2004) 437–455

Laurentian Platform on the orogen’s northwestern flank and the Avalon Platform to the southeast, represented conjugate passive margins of the Proto-Atlantic (Iapetus) Ocean. Since reconstructions of Pangea (e.g., Bullard et al., 1965) had clearly placed Africa against eastern North America, the Avalonian margin of the Appalachian orogen was logically considered part of West Africa, a view strengthened by Schenk’s (1971) correlation of the outboard Meguma Zone in southern Nova Scotia with similar rocks in Morocco. Consistent with its interpretation as the stable southeastern margin of the Appalachian orogen, accounts of the Lower Paleozoic geology of the Avalon Platform by Williams (1964), McCartney (1967), Greene and Williams (1974) and others, described a Cambrian-Ordovician platformal succession dominated by shallow-marine siliciclastics and minor limestones. The Avalon Platform and its correlatives were also known to possess a distinctive ‘‘Atlantic’’ trilobite fauna that contrasted with the contemporary ‘‘Pacific’’ fauna of the Laurentian Platform (e.g., Cowie, 1974), as would be expected of the opposing margins of a major ocean. However, unlike the Laurentian Platform, the Lower Paleozoic passive margin succession of which is floored by North American continental basement of Grenville (ca. 1 Ga) age, the much thinner Lower Paleozoic succession on the Avalon Platform passes transitionally downward into volcanic rocks and molasse-like and flyschoid sedimentary sequences of Late Precambrian age. Beneath these sequences, no unequivocal basement exposures were known. Instead, granitoid emplacement and the presence of unconformities within the Precambrian sequences provided evidence of a Late Precambrian Avalonian orogeny (e.g., Lilly, 1966; Poole, 1967; Hughes, 1970). Despite their Precambrian age and apparent record of an orogenic event, these volcanic-sedimentary sequences were considered to represent volcanic islands within a pre-existing ProtoAtlantic Ocean (e.g., Poole, 1967; Hughes and Bru¨ckner, 1971), a permissible interpretation given the available Rb/Sr age data (e.g., Fairbairn et al., 1966), much of which was anomalously young, and the uncertain Late Precambrian timing for initial rifting and ocean opening. Indeed, Dewey (1969) attributed correlative volcanic-sedimentary sequences in Britain to the onset of Late Precambrian subduction within an already wide Proto-Atlantic Ocean (Fig. 1). However, based on comparisons between the Avalonian geology of southern New Brunswick and Cape Breton Island, and that of the Midland and Breton platforms in southern Britain and northwestern France (Rast and Crimes, 1969), Rast et al. (1976) proposed that the Avalon Platform and its correlatives represented a Japan-like microcontinental volcanic arc, the evolution of which preceded the Appalachian orogenic cycle. Hence, rather than representing the opposing Proto-Atlantic margin to that of eastern Laurentia, the Avalon Platform was, for the first time, recognized as a ‘‘suspect terrane’’ within the Appalachian orogen.

3. Recognition of the Avalonian Arc Although subsequent mapping and the application of terrane analysis (e.g., Dallmeyer and Nance, 1992; Barr and White, 1996; White and Barr, 1996; Currie, 1997) has since introduced far greater complexity, Rast et al. (1976) provided the first description of a simple four-tier stratigraphy in rocks considered equivalent to those of the Avalon Platform in southern New Brunswick. The succession involved (1) metacarbonates and gneisses (Greenhead Group), structurally overlain by (2) a succession of acid and intermediate volcanic rocks (Coldbrook Group) that were

R.D. Nance et al. / Journal of Geodynamics 37 (2004) 437–455

441

unconformably overlain by (3) molasse-type redbeds (Radcliffe Brook Formation), which passed conformably up into (4) fossiliferous Cambrian-Ordovician platformal sandstones and shales (Saint John Group). No radiometric ages existed for the rocks beneath the unconformity. However, a Precambrian age was considered probable based on the tentative assignment of stromatolites in the metacarbonates to the Middle Riphean (Hofmann, 1974) and the available K/Ar radiometric ages for granitoid bodies that intrude these rocks, the oldest of which was Cambrian (e.g., Leech et al., 1963). Rast et al. (1976) further suggested that each of these four assemblages could be correlated with comparable units in Cape Breton Island (Fig. 2). The correlative units respectively include the George River Group, the Fourchu Group, the Morrison River Formation, and a CambrianOrdovician succession, the fauna of which were known to be similar, not only to that of southern New Brunswick, but also to those of Massachusetts, Newfoundland, and the British Isles (Hutchinson, 1952). Newfoundland was considered to lack equivalents of the gneiss complexplatformal metasedimentary rock association, but had comparable molasse-type and CambrianOrdovician successions, and numerous volcanic and flyschoid sedimentary assemblages (e.g., Love Cove, Harbour Main, Connecting Point, and Conception groups) that Rast et al. (1976)

Fig. 2. Table showing correlations proposed by Nick Rast and his co-workers in 1976 between composite stratigraphic sections along the length of the Avalonian margin. Although subsequent radiometric dating has modified the timescale (ages in parentheses are from Okulitch, 1999) and telescoped most Precambrian units into the interval ca. 685–550 Ma (see Nance et al., 2002 for comparison), the broad correlations proposed in this table have largely withstood the test of time. Figure modified from Rast et al. (1976).

442

R.D. Nance et al. / Journal of Geodynamics 37 (2004) 437–455

considered correlative with the Coldbrook Group. But in Newfoundland, the volcanic assemblages were, in addition, locally intruded by the Holyrood granite of known Precambrian age (57411 Ma, Rb/Sr, McCartney et al., 1966; 60911 Ma, Rb/Sr, Frith and Poole, 1972), and the molasse-type facies were known to be associated with younger, more alkaline volcanic rocks (Bull Arm Formation; e.g., Hughes and Bru¨ckner, 1971). In the British Isles, Rast et al. (1976) identified successions analogous to those of the Avalon Platform of Newfoundland in South Wales, the Welsh Borderlands, and the English Midlands (Fig. 2). Each of these regions exposed Late Precambrian volcanic-sedimentary successions (Benton and Pebidian groups, Uriconian and Longmyndian, and Charnian, respectively) interpreted to be underlain by higher-grade plutonic-metamorphic assemblages (Malverns and Johnston complexes) and generally overlain, locally by way of intervening molasse-type redbeds (e.g., Brand Series), by unconformable Lower Paleozoic platformal strata (e.g., Rast and Crimes, 1969; Baker, 1971). As in Newfoundland, two periods of volcanic activity had been recognized, an older calc-alkaline suite (e.g., Charnian) associated with greywackes, and a younger, more alkaline suite (e.g., Warren House Group) that accompanied the redbeds (Thorpe, 1972). In addition, available isotopic data (e.g., Lambert and Rex, 1966) gave ca. 600 Ma ages similar to those of the Avalon Platform, whereas the geochemical data suggested that the underlying crust was continental. Rast et al. (1976) further argued (1) that the Late Precambrian successions of Anglesey (Mona Complex) and southern Ireland (Cullenstown and Bray groups) were correlative with the Gander Group of Newfoundland and the Grand Pitch Formation of Maine, and (2) that they represented a shelf apron that bordered the Avalonian volcanic belt to the northwest, a view supported by the basinal rather than platformal facies of the overlying Cambrian strata. The occurrence of blueschists and ophiolitic rocks in the Anglesey succession further suggested that the ensialic volcanic belt was bordered to the northwest by an oceanic domain. In addition, a Late Precambrian episode of polyphase deformation and metamorphism (Monian orogeny), broadly coeval with the milder Avalonian orogeny, had been established in Anglesey (Shackleton, 1969), the limited extent of which was taken by Rast et al. (1976) as indicative of arc-continent rather than continent-continent collision. The southeastern margin of the Avalonian volcanic belt is obscured in the Appalachians by the Atlantic Ocean and its coastal plain. Southeast of the British Avalonian belt, however, a thick succession of Late Precambrian volcanic rocks and turbidites known as the Brioverian is exposed in northwest France, in which a major Late Precambrian orogenic event (Cadomian orogeny) had been described by Cogne´ (1972). A major east-west fault zone that is associated with blueschists and eclogites, and forms the southern boundary of the Brioverian, had been described as the junction between Precambrian Europe and Africa by Shackleton (1974), and was considered by Rast et al. (1976) to represent the suture line of the Cadomian orogeny. In this way, Rast et al. (1976) defined an Avalonian volcanic belt extending from the English Midlands to Massachusetts. They further proposed that the volcanic belt was floored by continental basement and bordered to the northwest and southeast by Precambrian oceanic domains, a relationship that would later be used (Rast and Skehan, 1983) to define an Avalonian plate. In addition, Rast et al. (1976) correctly recognized that the Avalonian volcanic belt, so defined, represented a Late Precambrian volcanic arc. Although Dewey’s (1969) plate reconstruction of the Caledonian orogen had clearly attributed the Avalonian rocks of Britain to Late Precambrian

R.D. Nance et al. / Journal of Geodynamics 37 (2004) 437–455

443

subduction (Fig. 1), a view later supported by geochemical data (Thorpe, 1974), interpretations of the Avalonian volcanic belt in Atlantic Canada at this time (e.g., Schenk, 1971; Papezik, 1972) favored an extensional setting like that of the Basin and Range based on available geochemical data, and attributed Avalonian volcanism to prolonged rifting of an Avalonian microcontinent from eastern Laurentia. While acknowledging that the younger period of volcanism (e.g., Bull Arm Formation, Warren House Group) was essentially alkaline and bimodal, Rast et al. (1976) argued that the association of older volcanism with compressional orogeny, the common and widespread occurrence of calc-alkaline granitoids, and the existence of blueschists and ophiolitic rocks in Anglesey, were all indicative of a compressional plate margin. Rast et al. (1976) therefore proposed a volcanic arc model for Avalonian volcanicity (Fig. 3). They viewed the Cadomian orogen as the major site of Late Precambrian orogenic activity and so placed the major ocean to the southeast. The Avalonian volcanic belt was consequently seen as the magmatic arc that developed above the northwestward dipping subduction zone of this closing Cadomian Ocean. The Monian orogeny, on the other hand, was attributed to tectonism at the edge of a back-arc basin that opened between the ensialic Avalonian arc and cratonic North America in a similar fashion to the present-day Japan Sea. Uplift and erosion of the arc in the wake of orogeny led to molasse-type sedimentation and, following erosion to base level, to the development of a stable platform across which the Cambrian sea transgressed. By this time, the former Avalonian arc was bordered to the northwest by the Proto-Atlantic (Iapteus) Ocean, which had developed approximately at the site of the former back-arc basin, and so became a component of the Appalachian orogenic cycle.

4. Definition of the Avalonian Plate With his longtime colleague, Father Jim Skehan of Boston College, Rast was later able to extend the Avalonian volcanic belt southward into the Carolinas and Florida, and eastward into Morocco and, possibly, the Mauritanides and Rokelides of West Africa (Rast and Skehan, 1983), as new research on these areas became available (Fig. 4). In the southern Appalachians, Avalonian-like volcanic-sedimentary rocks intruded by Late Precambrian granitoids had been described in the Carolina Slate Belt (e.g., Costello et al., 1981), where they record a Late Precambrian deformation (Glover and Sinha, 1973) and pass upward into Cambrian rocks containing trilobites akin to the ‘‘Atlantic’’ fauna of the Avalon Platform (Samson et al., 1982). Precambrian volcanic rocks intruded by granitoids had also been described from the Florida basement (e.g., Bass, 1969; Smith, 1982). In the Anti-Atlas of Morocco, a three-stage Precambrian stratigraphic succession closely resembling that of the Avalonian volcanic belt in Atlantic Canada had been described by Leblanc and Lancelot (1980) and Pique´ (1981). Unconformably overlying basement gneisses of the West African craton (Precambrian I), the succession comprises (1) quartzites and carbonates (Precambrian II) like those of the Green Head and George River groups, (2) volcanic and volcaniclastic rocks (Precambrian II-III) similar to those of the Coldbrook, Fourchu, Love Cove, Harbour Main, Connecting Point and Conception groups, and (3) unconformably overlying molasse-like redbeds (Precambrian III) resembling those of the Radcliffe Brook, Morrison River and Bull Arm formations. As in Atlantic Canada, these redbeds pass gradationally up into

444

R.D. Nance et al. / Journal of Geodynamics 37 (2004) 437–455

Cambrian strata. A similar succession had also been described in the Mauritanides of West Africa (e.g., Lecorche´ and Sougy, 1978), which continue southward into the Rokelides (e.g., Culver and Williams, 1979). Rast and Skehan (1983) also expanded the Cadomian orogen eastward into central Europe to incorporate the Late Precambrian Moldanubian basement of the Variscan belt (e.g., Khain,

Fig. 3. Late Precambrian-Early Paleozoic evolution of the Avalonian volcanic belt proposed by Rast and coworkers in 1976. (a) Subduction of main (Cadomian) ocean produces Avalonian volcanic arc and opens back-arc basin that separates arc from North American craton. (b) Avalonian arc collides with proto-Africa to produce the Cadomian and Avalonian orogenies while back-arc basin opens into small ocean (Iapetus). (c) Uplift and erosion of arc are associated with molasse-type sedimentation, emplacement of post-tectonic granitoids, and block faulting. (d) Arc and sutured Cadomian orogen are reduced to base level and cratonized to form platformal margin of newly developing Iapetus (Proto-Atlantic) Ocean to northwest. Figure modified from Rast et al. (1976).

R.D. Nance et al. / Journal of Geodynamics 37 (2004) 437–455

445

1977), and southward into the Central Iberian Zone of Spain (Fig. 4), the Late Precambrian stratigraphy of which closely resembles that of the Cadomian belt in northwestern France and likewise shows evidence of a Late Precambrian orogenic event (e.g., Julivert et al., 1974). They further argued that the Cadomian belt, so defined, was a probable continuation of the Late Precambrian Pan-African orogenic system and so was likely to be equivalent to the ophiolite-bearing Pharusian-Dahomeyan belt on the eastern margin of the West African craton (e.g., Bertrand and Caby, 1978). In making these broad correlations, Rast and Skehan (1983) refined the evolution of the Avalonian volcanic belt and introduced the concept of an Avalonian microplate (Fig. 4). Characterized by Late Precambrian to Cambrian arc volcanic, granitoid, and marine to terrestrial volcaniclastic rocks, and bordered by orogenic belts (Cadomian and Avalonian-Monian) that marked the sites of ocean and back-arc basin closure, the Avalonian plate was considered to have developed between the North American and West African cratons as a carapace over older continental fragments (see Fig. 3). In this way, the carapace was initially built upon both ensialic and ensimatic basement, the continental blocks having formed by the earlier fragmentation of Africa (c.f., Burke and Dewey, 1971). Prior to the Avalonian-Monian and Cadomian orogenies, however, it had become unified into a microcontinental arc. Following post-orogenic volcanism and molasse deposition that lasted into the Cambrian, renewed rifting near the sites of former ocean closure produced the Iapetus and Theic (Rheic) oceans, respectively, and incorporated the former Avalonian plate into the Armorican microcontinent (Fig. 5). Following Van der Voo (1982), Rast and Skehan (1983) envisaged this microcontinent to have collided with Laurentia during the Taconic orogeny, after which, in the Devonian or Early Carboniferous, it was displaced leftlaterally to become the rampart that would halt Variscan movements in the northern Appalachians

Fig. 4. The Avalonian microplate and Cadomian-Pharusian orogen of Rast and Skehan (1983) drawn on a Permian reconstruction. AND=Andalusia, AR=Ardennes, BF=Black Forest, Boh=Bohemia, BM=British Midlands, CAR=Carolinas, Cau=Caucasus, GAL=Galicia, Mas=Massachusetts, NA=Noires-Auvergne region, NB=New Brunswick, NF=Newfoundland, NSc=Nova Scotia, PB=Penobscot Bay, R=Romania, V=Vosges.

446

R.D. Nance et al. / Journal of Geodynamics 37 (2004) 437–455

and Europe during the Late Paleozoic assembly of Pangea. By contrast, in the southern Appalachians, remnants of the former Avalonian plate were engulfed by Variscan deformation (c.f., Dewey and Kidd, 1974) and fragmented by Variscan thrust slices.

5. Impact on subsequent research Although contemporary models for the development of the Avalonian volcanic belt (since variously renamed the Avalon Zone, the Avalon terrane or, simply, Avalonia) differ from those of Rast et al. (1976) and Rast and Skehan (1983) in many important aspects, recognition of the volcanic belt as an arc terrane proved to be the foundation upon which all subsequent tectonic models were built (e.g., Murphy and Nance, 1989; Keppie and Dostal, 1991; Nance et al., 1991; Murphy et al., 1999; Nance et al., 2002). Recognition of the Avalon Platform as a Late Precambrian microcontinental arc raised two fundamental questions regarding the evolution of the Appalachian orogen and challenged the

Fig. 5. Paleozoic development of Armorican microcontinent following Cadomian-Avalonian collision in the Late Precambrian, as reconstructed by Rast and Skehan (1983). See text for details. A=Armorica, B=Baltica, G=Gondwana, L=Laurentia.

R.D. Nance et al. / Journal of Geodynamics 37 (2004) 437–455

447

notion that it developed as a result of a full Wilson cycle of orthogonal ocean opening and closure. The first concerned the nature of the Proto-Atlantic (Iapetus) Ocean. If Iapetus was the ocean produced by the rifting of eastern Laurentia, then what was the ocean whose subduction had previously produced the Avalonian arc? The second concerned the nature of the opposing margin of Iapetus to that of eastern Laurentia. If the Avalonian plate was a Late Precambrian microcontinental arc, was it not, by definition, a suspect terrane within the Appalachian orogen? If so, it need not have formed the rifted eastern margin of Iapetus and, indeed, need not have had a provenance in West Africa. In contrast to Dewey’s (1969) model (Fig. 1), and later works that continued to attribute the Avalonian volcanic belt to a subducting Iapetus Ocean (e.g., Thorpe et al., 1984), Rast et al. (1976) and Rast and Skehan (1983) assigned subduction to an older ‘‘Cadomian’’ Ocean, a view that is widely held today, although not in the sense they envisaged. Subsequent studies of the Cadomian orogen (e.g., D’Lemos et al., 1990, Strachan et al., 1996; Egal et al., 1996; Balle`vre et al., 2001; Chantraine et al., 2001) have shown that it, too, is a magmatic arc complex coeval with, but distinct from, that of the Avalonian plate. Hence, subduction of the Cadomian Ocean was needed to produce the Cadomian arc, and so could not have formed the Avalonian volcanic belt in the manner proposed by Rast et al. (1976) and Rast and Skehan (1983). The solution to this problem lay in the Late Precambrian continental reconstruction of Bond et al. (1984), which positioned eastern Laurentia against western South America, an assembly later developed by Hoffman (1991) and Dalziel (1992, 1997). A West African provenance for the Avalonian volcanic belt placed it along the northern periphery of Gondwana (Fig. 6), and so

Fig. 6. Cordilleran model for the evolution of Avalonia proposed by Nance et al. (2002) illustrating the influence of the recognition of the Avalonian volcanic belt (Avalonia) as an ensialic arc on contemporary reconstructions of its tectonic evolution. (a) Late Neoproterozoic (ca. 635–590 Ma) subduction along the periphery of Gondwana (Ch=Chortis Block, Ox=Oaxaquia, Y=Yucatan Block, F=Florida). (b) Latest Neoproterozoic (ca. 590–540 Ma) arc-trench collision with diachronous termination of subduction and generation of continental transform fault. (c) Early Paleozoic (ca. 510–480 Ma) continental rifting and separation of Avalonia and Carolina from Gondwana following shift of transform inboard and transfer of terranes to the oceanic plate.

448

R.D. Nance et al. / Journal of Geodynamics 37 (2004) 437–455

divorced it from Iapetus, which was interpreted to have formed between the North American and Amazonian cratons. In such a configuration, the Avalonian and Cadomian belts could be viewed as accretionary orogens that formed portions of the same magmatic arc and faced the same Panthalassa-like ocean surrounding the Gondwanan supercontinent (Murphy and Nance, 1989, 1991; Nance et al. 1991). Seen as just two examples of numerous peri-Gondwanan terranes (e.g., Keppie, 1985, 1989; Nance and Thompson, 1996), the subsequent juxtaposition of the Avalonian and Cadomian belts could be attributed to orogen-parallel transform motion (e.g., Ferna´ndezSua´rez et al., 2002a, b). In this way, the ocean whose subduction produced the Cadomian orogen, that is, the Cadomian Ocean of Rast et al. (1976) and Rast and Skehan (1983), could indeed be viewed as the ocean responsible for the development of the Avalonian volcanic belt. Although its recognition as a suspect terrane no longer required this to be true, the assumption of a West Africa provenance for the Avalonian volcanic belt proved to be more resilient. For example, while recognizing the Avalonian plate as a terrane, a concept previously introduced by Keppie (1982) and Williams and Hatcher (1982), Rast and Skehan (1983) continued to regard its provenance as West African, a view strengthened by the similarity of Avalonian geology to that of Pan-African orogenic belts (e.g., O’Brien et al., 1983), and the occurrence within the Cadomian orogen of ca. 2.0 Ga (Icartian) basement (Auvray et al., 1980) that closely resembled the Eburnian basement of the West African craton. Indeed, a West African provenance for the volcanic belt continued to be assumed in tectonic models for the Avalon terrane (e.g., Nance, 1987) and the Appalachian orogen (e.g., Hatcher, 1987, 1989) for more than a decade after its recognition as a microcontinental arc. However, basement like that of the Icartian could not be found in the Avalonian volcanic belt, geochronology having repeatedly shown all potential candidates to be early components of the Avalonian arc (e.g., Bevier et al., 1990; Tucker and Pharoah, 1991; Doig et al., 1993; Keppie et al., 1998). Indeed, as inherited and detrital zircon data accumulated for rocks of the Avalonian arc, their U-Pb age populations proved inconsistent with the age provinces of the West African craton, suggesting instead a provenance like that of Amazonia (Keppie and Krogh, 1990, Keppie, 1992). Based on these age data and the neodymium isotopic compositions of felsic igneous rocks that are the product of basement melting, Nance and Murphy (1994, 1996) further developed a peri-Amazonian location for the Avalonian volcanic belt, and proposed a similar provenance for the peri-Gondwanan rocks of Carolina and the Florida basement. They additionally proposed, based on neodymium data, that the Avalonian arc had developed upon juvenile basement of ca. 1 Ga age. By contrast, the available neodymium data supported a West African-like basement and, hence, a peri-West African position for the Cadomian belt, a location later substantiated by detrital zircon data (Samson et al., 1999; Ferna´ndez-Sua´rez, et al., 2002a). These contrasting basement isotopic signatures enabled Nance and Murphy (1994, 1996) to position the peri-Gondwana terranes along the northern margin (present coordinates) of Gondwana (Fig. 6), and have since been used both to distinguish rocks of Avalonian and Cadomian affinity and to further refine the paleogeography of the peri-Gondwanan arc (e.g., Friedl et al., 2000; Kro¨ner et al., 2001; Zeh et al., 2001; Linnemann and Romer, 2002). In this way, the Avalonian and Cadomian belts have come to be regarded as fragments of a Cordilleran-like accretionary orogen that developed along the active northern margin of Neoproterozoic Gondwana (e.g., Keppie et al., 1996; Keppie and Ramos, 1999; Murphy et al., 1999; Unrug et al., 1999; Nance et al., 2002; Keppie et al., 2003). Many other fragments of this active margin have also been recognized, as a result of which

R.D. Nance et al. / Journal of Geodynamics 37 (2004) 437–455

449

so-called peri-Gondwanan terranes are now known to be distributed throughout the younger Appalachian-Caledonian, Variscan, and Alpine orogens (e.g., Nance and Thompson, 1996; Hibbard et al., 2002; Murphy et al., 2002; Neubauer, 2002; von Raumer et al., 2002). Although far removed from the model proposed by Rast et al. (1976), these contemporary reconstructions are directly descended from their recognition of the Avalonian volcanic belt as a microcontinental arc. However, in contrast to the tentative reconstructions of Rast and Skehan (1983), the Avalonian and Cadomian belts, while sharing similar tectonic settings in the Late Precambrian, are now thought to have experienced quite different Paleozoic histories. Whereas the Cadomian belt (as part of Armorica) eventually collided with Laurussia during the Devonian closure of the Rheic Ocean (e.g., Torsvik et al., 1996; Tait et al., 1997; von Raumer et al., 2002), the Avalonian volcanic belt is thought to followed a similar path to that proposed by Rast and Skehan (1983) (Fig. 5), first separating from Gondwana with the Late Cambrian-Early Ordovician opening of the Rheic Ocean, and then colliding with eastern Laurentia and southern Baltica during the Late Ordovician-Early Silurian closure of the Iapetus Ocean and Tornquist Sea (e.g., Trench and Torsvik, 1992; Cocks, 2000; Torsvik and Rehnstro¨m, 2003).

6. Conclusions By correlating the Late Precambrian volcanic rocks of the Avalon Platform in eastern Canada with those of the Midland Platform in southern Britain (where they are bordered to the northwest and southeast by the broadly coeval Monian and Cadomian orogenic belts), Rast et al. (1976) were the first to establish the Avalonian volcanic belt as an ensialic volcanic arc. Although contemporary interpretations of the Avalon Platform favored an extensional environment for its volcanic rocks, Rast et al. (1976) argued for an active continental margin setting based on their association with calc-alkaline granitoid intrusions and compressional orogenic belts that locally contain blueschists and ophiolitic rocks. Rast further proposed that the Avalonian volcanic belt was ensialic and bordered to the northwest and southeast by oceanic domains, factors that later led Rast and Skehan (1983) to propose an Avalonian microcontinental plate. This recognition of the eastern (Avalonian) margin of the Appalachian orogen as an ensialic arc rather than a passive Proto-Atlantic (Iapetan) margin conjugate to that of eastern Laurentia, as had previously been envisaged, represented a milestone in the study of the Appalachians, the impact of which is evident in all subsequent tectonic models for the orogen’s evolution. Although current interpretations of Avalonian and Cadomian terranes as fragments of an active continental margin of Neoproterozoic Gondwana differ significantly from those of Rast et al. (1976) and Rast and Skehan (1983), they ultimately stem for their initial recognition of the Avalonian volcanic belt as a Japan-like microcontinental arc.

Acknowledgements The paper is a contribution to IGCP Projects 453: Ancient and Modern Orogens and 497: The Rheic Ocean, and has greatly benefited from the constructive comments of Rachel Burks, Meg Thompson, and Alec Gates.

450

R.D. Nance et al. / Journal of Geodynamics 37 (2004) 437–455

References Auvray, B., Charlot, R., Vidal, P., 1980. Donne´es nouvelles sur le prote´rozoı¨que infe´rieur du domaine nordarmoricaine (France): age et signification. Canadian Journal of Earth Sciences 17, 532–538. Baker, J.W., 1971. The Proterozoic history of southern Britain. Proceedings of the Geologists’ Association 82, 249– 266. Balle`vre, M., Le Goff, E., He´bert, R., 2001. The tectonothermal evolution of the Cadomian belt in northern Brittany, France: a Neoproterozoic volcanic arc. Tectonophysics 331, 19–43. Barr, S.M., White, C.E., 1996. Contrasts in late Precambrian-early Paleozoic tectonothermal history between Avalon composite terrane sensu stricto and other possible peri-Gondwanan terranes in southern New Brunswick. In: Nance, R.D., Thompson, M.D. (Eds.), Avalonian and Related Peri-Gondwanan Terranes of the Circum-North Atlantic. Geological Society of America Special Paper 304, 95–108. Bass, M.N., 1969. Petrography and ages of crystalline basement rocks of Florida—some extrapolations. American Association of Petroleum Geologists Memoir 11, 283–310. Bertrand, J.M.L., Caby, R., 1978. Geodynamic evolution of the central Sahara Pan-African mobile belt: a new interpretation of the Hoggar Shield (Algerian Sahara). Geologische Rundschau 67, 357–388. Bevier, M.L., Barr, S.M., White, C.E., 1990. Late Precambrian U-Pb ages for the Brookville Gneiss, southern New Brunswick. Journal of Geology 98, 955–965. Bird, J.M., Dewey, J.F., 1970. Lithosphere plate-continental margin tectonics and the evolution of the Appalachian orogen. Geological Society of America Bulletin 81, 1031–1060. Bond, G.C., Nickeson, P.A., Kominz, M.A., 1984. Breakup of a supercontinent between 625 Ma and 555 Ma: new evidence and implications for continental histories. Earth and Planetary Science Letters 70, 325–345. Bullard, E., Everett, J.E., Smith, A.G., 1965. The fit of the continents around the Atlantic region. Philosophical Transactions of the Royal Society of London, Series A 258, 41–51. Burke, K.C., Dewey, J.F., 1971. Orogens in Africa. In: Dessauvagie, T.F.J., Whiteman, A.J. (Eds.), Implications of Continental Drift to the Earth Sciences, 2. Academic Press, London, pp. 1035–1045. Chantraine, J., Egal, E., Thie´blemont, D., Le Goff, E., Guerrot, C., Balle`vre, M., Guennoc, P., 2001. The Cadomian active margin (North Armorican Massif, France): a segment of the North Atlantic Panafrican belt. Tectonophysics 331, 1–18. Cocks, L.R.M., 2000. The Early Palaeozoic geography of Europe. Journal of the Geological Society (London) 157, 1–10. Cogne´, J., 1972. Le Briove´rien et le cycle oroge´nique cadomien dans le cadre des oroge`nes fini-pre´cambriens. In: Colloque International sur les Corre´lations du Pre´cambrien. Notes et Me´moires Service Ge´ologique de Maroc 236, 193–218. Costello, O.P., Secor Jr., D.T., Snoke, A.W., 1981. Structural relationships as a key to stratigraphic sequence in the Carolina Slate Belt, Lake Murray, South Carolina. Southeastern Geology 22, 139–147. Cowie, J.W., 1974. The Cambrian of Spitzbergen and Scotland. In: Holland, C.H. (Ed.), Cambrian of the British Isles, Norden and Spitzbergen. Lower Paleozoic Rocks of the World, vol. 2. Wiley, London, pp. 123–156. Culver, S.J., Williams, H.R., 1979. The Late Precambrian and Phanerozoic geology of Sierra Leone. Journal of the Geological Society (London) 136, 605–618. Currie, K.L., 1997. Geology of the Saint John-Saint George area, New Brunswick (revised). Geological Survey of Canada, Open File 3418. Dallmeyer, R.D., Nance, R.D., 1992. Tectonic implications of 40Ar/39Ar mineral ages from late PrecambrianCambrian plutons, Avalon composite terrane, southern New Brunswick. Canadian Journal of Earth Sciences 29, 2445–2462. Dalziel, I., 1992. On the organization of American plates in the Neoproterozoic and the breakout of Laurentia. GSA Today 2 (11), 238–241. Dalziel, I., 1997. Overview: Neoproterozoic-Paleozoic geography and tectonics: review, hypotheses and environmental speculations. Geological Society of America Bulletin 109, 16–42. Dewey, J.F., 1969. Evolution of the Appalachian/Caledonian orogen. Nature 22, 124–129.

R.D. Nance et al. / Journal of Geodynamics 37 (2004) 437–455

451

Dewey, J.F., Kidd, W.S.F., 1974. Continental collisions in the Appalachian-Caledonian orogenic belt: variations related to complete and incomplete suturing. Geology 2, 543–546. Dietz, R.S., 1963. Collapsing continental rises: an actualistic concept of geosynclines and mountain building. Journal of Geology 71, 314–333. Dietz, R.S., Holden, J.C., 1967. Miogeoclines in space and time. Journal of Geology 65, 566–583. D’Lemos, R.S., Strachan, R.A., Topley, C.G., 1990. The Cadomian Orogeny. Geological Society of London Special Publication 51, 423 p. Doig, R., Murphy, J.B., Nance, R.D., 1993. Tectonic significance of the Late Proterozoic Economy River Gneiss, Cobequid Highlands, Avalon composite terrane, Nova Scotia. Canadian Journal of Earth Sciences 30, 474–479. Egal, E., Guerrot, C., Le Goff, D., Thie´blemont, D., Chantraine, J., 1996. The Cadomian orogeny revisited in northern France. In: Nance, R.D., Thompson, M.D. (Eds.), Avalonian and Related Peri-Gondwanan Terranes of the CircumNorth Atlantic. Geological Society of America Special Paper 304, 281–318. Fairbairn, H.W., Bottino, M.L., Pinson Jr., W.H., Hurley, P.M., 1966. Whole rock age and initial 87Sr/86Sr of volcanics underlying fossiliferous Lower Cambrian in the Atlantic Provinces of Canada. Canadian Journal of Earth Sciences 3, 509–521. Ferna´ndez-Sua´rez, J, Gutie´rrez Alonso, G., Jeffries, T.E., 2002. The importance of along-margin terrane transport in northern Gondwana: insights from detrital zircon parentage in Neoproterozoic rocks from Iberia and Brittany. Earth and Planetary Science Letters 204, 75–88. Ferna´ndez-Sua´rez, J., Gutie´rrez Alonso, G., Cox, R., Jenner, G.A., 2002. Assembly of the Armorican microplate: a strike-slip terrane delivery? Evidence from U-Pb ages of detrital zircons. Journal of Geology 110, 619–626. Friedl, G., Finger, F., McNaughton, N., Fletcher, I.R., 2000. Deducing the ancestry of terranes: SHRIMP evidence for South America-derived Gondwanan fragments in central Europe. Geology 28, 1035–1038. Frith, R.A., Poole, W.H., 1972. Late Precambrian rocks of eastern Avalon Peninsula, Newfoundland—a volcanic island complex. Discussion. Canadian Journal of Earth Sciences 9, 1058. Glover III, L., Sinha, A.K., 1973. The Virgilina deformation: a Late Precambrian to early Cambrian (?) orogenic event in the central Piedmont of Virginia and North Carolina. American Journal of Science 273, 234–251. Greene, B., Williams, H., 1974. New fossil localities at the base of the Cambrian in southeastern Newfoundland. Canadian Journal of Earth Sciences 11, 319–323. Harper, J.C., Rast, N., 1964. The faunal succession and volcanic rocks of the Ordovician near Bellewstown, County Meath. Proceedings of the Royal Irish Academy 64, 1–23. Harris, A.L., Rast, N., 1960a. The evolution of quartz fabrics in the metamorphic rocks of Central Perthshire. Transactions of the Royal Society of Edinburgh 18, 51–78. Harris, A.L., Rast, N., 1960b. Oriented quartz inclusions in garnets. Nature 185, 448–449. Hatcher Jr., R.D., 1987. Tectonics of the southern and central Appalachian internides. Annual Review of Earth and Planetary Sciences 15, 337–362. Hatcher, R.D., Jr., 1989. Tectonic synthesis of the U.S. Appalachians. In: Hatcher, R.D., Thomas, W.A., Viele, G.W., (Eds.), The Appalachian-Ouachita Orogen in the United States. Boulder, Colorado, Geological Society of America, The Geology of North America F-2, 511–535. Hibbard, J.P., Stoddard, E.F., Secor, D.T., Dennis, A.J., 2002. The Carolina Zone: overview of Neoproterozoic to Early Paleozoic peri-Gondwanan terranes along the eastern flank of the southern Appalachians. Earth-Science Reviews 57, 299–339. Hoffman, P.F., 1991. Did the breakout of Laurentia turn Gondwanaland inside out? Science 252, 1409–1412. Hofmann, H.J., 1974. The Stromatolite Archaezoon acadiense from the Proterozoic Green Head Group of Saint John, New Brunswick. Canadian Journal of Earth Sciences 11, 1098–1115. Hughes, C.J., 1970. The late Precambrian Avalonian Orogeny in Avalon, southeastern Newfoundland. American Journal of Science 269, 183–190. Hughes, C.J., Bru¨ckner, W.D., 1971. Late Precambrian rocks of eastern Avalon Peninsula, Newfoundland—a volcanic island complex. Canadian Journal of Earth Sciences 8, 899–915. Hutchinson, R.D., 1952. The stratigraphy and trilobite faunas of the Cambrian sedimentary rocks of Cape Breton Island, Nova Scotia. Geological Survey of Canada Memoir 263, 124.

452

R.D. Nance et al. / Journal of Geodynamics 37 (2004) 437–455

Julivert, M., Fontbote, J.M., Ribeiro, A., Conde, L., 1974. Mapa Te´cto´nico de la Penı´nsula Ibe´rica y Balearas. Scale: 1:1,000,000. Instituto Tecnologico GeoMinero de Espan˜a. Kay, M., 1947. Geosynclinal nomenclature and the craton. American Association of Petroleum Geologists Bulletin 31, 1289–1291. Kay, M., 1951. North American geosynclines. Geological Society of America Memoir 48, 143. Keppie, J.D., 1982. Tectonic map of Nova Scotia, Scale 1:500,000. Nova Scotia Department of Mines and Energy, Halifax, Nova Scotia. Keppie, J.D., 1985. The Appalachian Collage. In: Gee, D.G., Sturt, B.A. (Eds.), The Caledonide Orogen: Scandinavia and Related Areas. Wyllie and Sons, New York, pp. 1217–1226. Keppie, J.D., 1989. Northern Appalachian terranes and their accretionary history. In: Dallmeyer, R.D. (Ed.), Terranes in the Circum-Atlantic Paleozoic Orogens. Geological Society of America Special Paper 230, 159–192. Keppie, J.D., 1992. North-South American trade in the Silurian. Geological Association of Canada, Abstracts Volume 17, A56. Keppie, J.D., Dostal, J., 1991. Late Proterozoic model for the Avalon terrane in Maritime Canada. Tectonics 10, 842–850. Keppie, J.D., Krogh, T.E., 1990. Detrital zircon ages from Late Precambrian conglomerate, Avalon Composite Terrane, Antigonish Highlands, Nova Scotia. Geological Society of America Abstracts with Programs 22, 27–28. Keppie, J.D., Ramos, V.S., 1999. Odyssey of terranes in the Iapetus and Rheic oceans during the Paleozoic. In: Ramos, V.S., Keppie, J.D. (Eds.), Laurentia-Gondwana Connections before Pangea. Geological Society of America Special Paper 336, 267–276. Keppie, J.D., Dostal, J., Murphy, J.B., Nance, R.D., 1996. Terrane transfer between eastern Laurentia and western Gondwana in the Early Paleozoic: constraints on global reconstructions. In: Nance, R.D., Thompson, M.D. (Eds.), Avalonian and Related Peri-Gondwanan Terranes of the Circum-North Atlantic. Geological Society of America Special Paper 304, 369–380. Keppie, J.D., Davis, D.W., Krogh, T.E., 1998. U-Pb geochronological constraints on Precambrian stratified units in the Avalon Composite Terrane of Nova Scotia: tectonic implications. Canadian Journal of Earth Sciences 35, 222– 236. Keppie, J.D., Nance, R.D., Murphy, J.B., Dostal, J. 2003. Tethyan, Mediterranean, and Pacific analogues for the Neoproterozoic-Paleozoic birth and development of peri-Gondwanan terranes and their transfer to Laurentia and Laurussia. Tectonophysics, 365, 195–219. Khain, V.E., 1977. The new international tectonic map of Europe. In: Ager, D.V., Brooks, M. (Eds.), Europe from Crust to Core. Wiley, London, pp. 19–40. King, B.C., Rast, N., 1955. Tectonic styles in the Dalradian and Moines of parts of the Central Highlands of Scotland. Proceedings of the Geologists’ Association 66, 243–269. King, B.C., Rast, N., 1956. The small-scale structures of southeastern Cowal, Argyllshire. Geological Magazine 93, 185–195. King, B.C., Rast, N., 1959. Structural geometry of Dalradian rocks at Loch Leven, Scottish Highlands: a discussion. Journal of Geology 67, 244–246. Kro¨ner, A., Jaeckel, P., Hegner, E., Opletal, M., 2001. Single crystal ages and whole-rock Nd isotopic systematics of early Paleozoic granitoid gneisses from the Czech and Polish Sudetes (Jizerske´ hory, Krkonosˇ e Mountains and Orlice-Snı`zˇnı´k Complex). International Journal of Earth Sciences (Geologische Rundschau) 90, 304–324. Lambert, R.St.J., Rex, D.C., 1966. Isotopic ages of minerals from the Precambrian complex of the Malverns. Nature 209, 605. Leblanc, M., Lancelot, J.R., 1980. Interpre´tation ge´odynamique du domaine pan-africain (pre´cambrien terminale) de l’Anti-Atlas (Maroc) a` partir de donne´es ge´ologiques et ge´ochronologiques. Canadian Journal of Earth Sciences 17, 142–155. Lecorche´, J.P., Sougy, J., 1978. Les Mauritanides, Afrique, occidentale. Essai de synthe`se. In: Tozer, E.T., Schenk, P.E. (Eds.), Caledonian-Appalachian Orogen of the North Atlantic Region. Geological Survey of Canada Paper 78–13, 231–239. Leech, G.B., Lowdon, J.A., Stockwell, C.H., Wanless, R.K., 1963. Age determinations and geologic studies. Geological Survey of Canada Paper 63-17, 103–104.

R.D. Nance et al. / Journal of Geodynamics 37 (2004) 437–455

453

Lilly, H.D., 1966. Late Precambrian and Appalachian tectonics in light of submarine exploration on the Great Bank of Newfoundland and in the Gulf of St. Lawrence. Preliminary views. American Journal of Science 264, 569–574. Linnemann, U., Romer, R.L., 2002. The Cadomian Orogeny in Saxo-Thuringia, Germany: geochemical and Nd-Sr-Pb isotopic characterization of marginal basins with constraints to geotectonic setting and provenance. Tectonophysics 352, 33–64. McCartney, W.D., 1967. Whitbourne map-area, Newfoundland. Geological Survey of Canada Memoir 341, 135 p. McCartney, W.D., Poole, W.H., Wanless, R.K., Williams, H., Loweridge, W.D., 1966. Rb/Sr age and geologic setting of the Holyrood granite, southeast Newfoundland. Canadian Journal of Earth Sciences 3, 947–958. Mosher, S., Rast, N., 1984. The deformation and metamorphism of Carboniferous rocks in Maritime Canada and New England. In: Hutton D.H.W., Sanderson, D.J., (Eds.), Variscan Tectonics of the North Atlantic Region. Geological Society (London) Special Publication 14, 233–243. Murphy, J.B., Nance, R.D., 1989. Model for the evolution of the Avalonian-Cadomian orogenic belt. Geology 17, 735–738. Murphy, J.B., Nance, R.D., 1991. Supercontinent model for the contrasting character of Late Proterozoic orogenic belts. Geology 19, 469–472. Murphy, J.B., Keppie, J.D., Dostal, J., Nance, R.D., 1999. Neoproterozoic-early Paleozoic evolution of Avalonia. In: Ramos, V.A., Keppie, J.D. (Eds.). Laurentia-Gondwana Connections Before Pangea. Geological Society of America Special Paper 336, 253–266. Murphy, J.B., Eguiluz, L., Zulauf, G., (Eds.), 2002. Cadomian Orogens, peri-Gondwanan correlatives and LaurentiaBaltica connections. Tectonophysics 352, 259 p. Nance, R.D., 1987. Model for the Precambrian evolution of the Avalon terrane in southern New Brunswick. Geology 15, 753–756. Nance, R.D., Murphy, J.B., 1994. Contrasting basement isotopic signatures and the palinspastic restoration of peripheral orogens: example from the Neoproterozoic Avalonian-Cadomian belt. Geology 22, 617–620. Nance, R.D., Murphy, J.B., 1996. Basement isotopic signatures and Neoproterozoic paleogeography of AvalonianCadomian and related terranes in the circum-North Atlantic. In: Nance, R.D., Thompson, M.D. (Eds.), Avalonian and Related Peri-Gondwanan Terranes of the Circum-North Atlantic. Geological Society of America Special Paper 304, 333–346. Nance, R.D. Thompson, M.D. (Eds.), 1996. Avalonian and Related Peri-Gondwanan Terranes of the Circum-North Atlantic. Geological Society of America Special Paper 304, 390 p. Nance, R.D., Murphy, J.B., Strachan, R.A., D’Lemos, R.S., Taylor, G.K., 1991. Late Proterozoic tectonostratigraphic evolution of the Avalonian and Cadomian terranes. Precambrian Research 53, 41–78. Nance, R.D., Murphy, J.B., Keppie, J.D., 2002. A Cordilleran model for the evolution of Avalonia. Tectonophysics 352, 11–31. Neubauer, F., 2002. Evolution of late Neoproterozoic to early Paleozoic tectonic elements in Central and Southeast European Alpine mountain belts: review and synthesis. Tectonophysics 352, 87–103. O’Brien, S.J., Wardle, R.J., King, A.F., 1983. The Avalon zone: a Pan-African terrane in the Canadian Appalachians. Geological Journal 18, 195–222. Okulitch, A.V., 1999. Geological time scale, 1999. Geological Survey of Canada, Open File 3040 (National Earth Science Series, Geological Atlas)-REVISION. Papezik, V.S., 1972. Late Precambrian ignimbrites in eastern Newfoundland and their tectonic significance. 24th International Geological Congress, Section 1, 147–152. Pique´, A., 1981. Northwestern Africa and the Avalon plate: relations during the late Precambrian and late Paleozoic time. Geology 9, 319–322. Poole, W.H., 1967. Tectonic evolution of the Appalachian region of Canada. Geological Association of Canada Special Paper 4, 9–51. Rast, N., 1956. The origin and significance of boudinage. Geological Magazine 93, 401–408. Rast, N., 1958a. The tectonics of the Schichallion Complex. Quarterly Journal of the Geological Society (London) 114, 25–46. Rast, N., 1958b. The metamorphic history of the Schichallion Complex (Perthshire). Transactions of the Royal Society of Edinburgh 63, 413–431.

454

R.D. Nance et al. / Journal of Geodynamics 37 (2004) 437–455

Rast, N., 1961a. Mid-Ordovician structures in southwestern Snowdonia. Geological Journal (Liverpool and Manchester) 2, 645–651. Rast, N., 1961b. Textural evidence for the origin of ignimbrites. Geological Journal (Liverpool and Manchester) 2, 600–611. Rast, N., 1963. Structure and metamorphism of the Dalradian rocks of Scotland. In: Johnson, M.R.W., Stewart, F.C. (Eds.), British Caledonides. Oliver and Boyd, Edinburgh, pp. 123–142. Rast, N., 1964. Morphology and interpretation of folds, a critical essay. Geological Journal 4, 177–188. Rast, N., 1965. Nucleation and growth of metamorphic minerals. In: Pitcher, W.S., Flinn, G. (Eds.), Controls of Metamorphism. Oliver and Boyd, Edinburgh, pp. 73–102. Rast, N., 1966. Recrystallization of mica in crenulated schists. Nature 210, 727–728. Rast, N., 1969. The relationship between Ordovician structures and vulcanicity in Wales. In: Wood, A. (Ed.), The Precambrian and Lower Palaeozoic Rocks of Wales. University of Wales Press, Cardiff, pp. 305–335. Rast, N., 1970. The origin, ascent and emplacement of igneous magmas. In: Newall, G., Rast, N. (Eds.), Mechanism of Igneous Intrusion. Letterpress, Liverpool, pp. 339–362. Rast, N., 1983. Variscan Orogeny. In: Hancock, P. (Ed.), Variscan Fold Belt. Adam Hilger, London, pp. 1–19. Rast, N., 1984. The Alleghanian orogeny in eastern North America. In: Hutton D.H.W., Sanderson, D.J., (Eds.), Variscan Tectonics of the North Atlantic Region. Geological Society (London) Special Publication 14, 197–217. Rast, N., Crimes, T.P., 1969. Caledonian orogenic episodes in the British Isles and northwestern France and their tectonic and chronological interpretation. Tectonophysics 7, 277–307. Rast, N., Currie, K.L., 1976. On the position of the Variscan front in southern New Brunswick and its relation to Precambrian basement. Canadian Journal of Earth Sciences 13, 194–196. Rast, N., Grant, R.H., 1973. Transatlantic correlation of the Variscan-Appalachian Orogeny. American Journal of Science 273, 572–579. Rast, N., Kohles, K.M., 1986. The origin of the Ocoee Supergroup. American Journal of Science 286, 593–616. Rast, N., Litherman, M.P., 1970. The correlation of the Ballachulish and Perthshire Dalradian successions in Scotland. Geological Magazine 107, 259–272. Rast, N., Platt, J.I., 1957. Cross-folds. Geological Magazine 94, 159–167. Rast, N., Skehan, J.W., ?, S.J., 1983. The evolution of the Avalonian plate. Tectonophysics 100, 257–286. Rast, N., Sturt, B.A., 1957. Crystallographic and geological factors in the growth of garnets from central Perthshire. Nature 179, 215. Rast, N., Beavon, R.V., Fitch, F.J., 1958. Sub-aerial vulcanicity in Snowdonia. Nature 181, 508. Rast, N., Sturt, B.A., Harris, A.L., 1962. Inclusions in garnets. Nature 195, 274–275. Rast, N., O’Brien, B.H., Wardle, R.J., 1976. Relationships between Precambrian and Paleozoic rocks of the ‘Avalon Platform’ in New Brunswick, the northeast Appalachians and the British Isles. Tectonophysics 30, 315–338. Samson, S.D., Secor, D.T., Stern, R., 1999. Provenance and paleogeography of Neoproterozoic circum-Atlantic arc terranes: constraints from U-Pb ages of detrital zircons. Geological Society of America, Abstracts with Programs 31 (7), A–429. Samson, S.L., Secor, D.T., Snoke Jr., A.W., Palmer, A.R., 1982. Geological implications of recently discovered Middle Cambrian trilobites in the Carolina Slate Belt. Geological Society of America, Abstracts with Programs 14 (1), 607. Schenk, P.E., 1971. Southeastern Atlantic Canada, northwestern Africa, and continental drift. Canadian Journal of Earth Sciences 8, 1218–1251. Shackleton, R.M., 1969. The Precambrian of North Wales. In: Wood, A. (Ed.), The Precambrian and Lower Palaeozoic Rocks of Wales, a Symposium. University of Wales Press, Cardiff, pp. 1–18. Shackleton, R.M., 1974. Late Precambrian rocks of Wales, Brittany and the Iberian Peninsula. Geological Association of Canada-Mineralogical Association of Canada Annual Meeting, Program Abstracts, p. 85. Smith, A.J., Rast, N., 1958. Sedimentary dykes in the Dalradian of Scotland. Geological Magazine 95, 234–240. Smith, D.C., 1982. Review of the tectonic history of the Florida basement. Tectonophysics 88, 1–22. Strachan, R.A., D’Lemos, R.S., Dallmeyer, R.D., 1996. Late Precambrian evolution of an active plate margin: North Armorican Massif, France. In: Nance, R.D., Thompson, M.D. (Eds.), Avalonian and Related Peri-Gondwanan Terranes of the Circum-North Atlantic. Geological Society of America Special Paper 304, 319–332.

R.D. Nance et al. / Journal of Geodynamics 37 (2004) 437–455

455

Tait, J.A., Bachtadse, V., Franke, W., Soffel, H.C., 1997. Geodynamic evolution of the European Variscan fold belt: palaeomagnetic and geologic constraints. Geologische Rundschau 86, 585–598. Thorpe, R.S., 1972. The geochemistry and correlation of the Warren House, the Uriconian and the Charnian volcanic rocks from the English Pre-cambrian. Proceedings of the Geologists’ Association 83, 269–284. Thorpe, R.S., 1974. Aspects of magmatism and plate tectonics in the Precambrian of England and Wales. Geological Journal 9, 115–136. Thorpe, R.S., Beckinsale, R.D., Patchett, P.J., Piper, J.D.A., Davies, G.R., Evans, J.A., 1984. Crustal growth and late Precambrian-early Palaeozoic plate tectonic evolution of England and Wales. Journal of the Geological Society (London) 141, 521–536. Torsvik, T.H., Rehnstro¨m, E.F., 2003. The Tornquist Sea and Baltica-Avalonia docking. Tectonophysics 362, 67–82. Torsvik, T.H., Smethurst, M.A., Meert, J.G., Van der Voo, R., McKerrow, W.S., Brasier, M.D., Sturt, B.A., Walderhaug, H.J., 1996. Continental break up and collision in the Neoproterozoic and Palaeozoic—A tale of Baltica and Laurentia. Earth-Science Reviews 40, 229–258. Trench, A., Torsvik, T.H., 1992. The closure of the Iapetus Ocean and Tornquist Sea: new palaeomagnetic constraints. Journal of the Geological Society (London) 149, 867–870. Tucker, R.D., Pharoah, T.C., 1991. U-Pb zircon ages of late Precambrian rocks in southern Britain. Journal of the Geological Society (London) 148, 435–443. Unrug, R., Haran´czyk, C., Chocyk-Jamin´ska, M., 1999. Easternmost Avalonian and Armorican-Cadomian terranes of central Europe and Caledonian-Variscan evolution of the polydeformed Krako´w mobile belt: geological constraints. Tectonophysics 302, 133–157. Van der Voo, R., 1982. Pre-Mesozoic paleomagnetism and plate tectonics. Annual Review of Earth and Planetary Sciences 10, 191–220. von Raumer, J.F., Stampfli, G.M., Borel, G., Bussy, F., 2002. Organization of pre-Variscan basement areas at the north-Gondwanan margin. International Journal of Earth Sciences (Geologische Rundschau) 91, 35–52. White, C.E., Barr, S.M., 1996. Geology of the Brookville terrane, southern New Brunswick, Canada. In: Nance, R.D., Thompson, M.D. (Eds.), Avalonian and Related Peri-Gondwanan Terranes of the Circum-North Atlantic. Geological Society of America Special Paper 304, 133–147. Williams, H., 1964. The Appalachians in northeastern Newfoundland—a two-sided symmetrical system. American Journal of Science 262, 1137–1158. Williams, H., Hatcher Jr., R.D., 1982. Suspect terranes and accretionary history of the Appalachian orogen. Geology 10, 530–536. Wilson, J.T., 1966. Did the Atlantic close and then re-open? Nature 211, 676–681. Zeh, A., Bra¨tz, H., Millar, I.L., Williams, I.S., 2001. A combined zircon SHRIMP and Sm-Nd isotope study of highgrade paragneisses from the Mid-German Crystalline Rise: evidence for northern Gondwanan and Grenvillian provenance. Journal of the Geological Society (London) 158, 983–994.