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Murphy, J. B., Pisarevsky, S. A., Nance, R. D. and Keppie, J. D. 2001. Animated History of in Neoproterozoic-Early Palaeozoic. In: Jessell, M. J. 2001. General Contributions: 2001. Journal of the Virtual Explorer , 3, 45 - 58.

Animated History of Avalonia in Neoproterozoic-Early Palaeozoic

J. B. MURPHY1, S. A. PISAREVSKY2, R. D. NANCE3 AND J. D. KEPPIE4

1Department of , St. Francis Xavier University, Antigonish, Nova Scotia B2G 2W5, Canada email: [email protected] 2Tectonics Special Research Centre, University of Western Australia, Nedlands, WA, Australia 3Department of Geological Sciences, Ohio University, Athens, Ohio 45701 U.S.A 4Instituto de Geología, Universidad Nacional Autónoma de México, México D.F., 04510 México.

Abstract: Current debate regarding the configuration and breakup history of the late Precambrian supercontinent has focused on the development of Grenville-aged orogenic belts and the evolution of Neoproterozoic passive margin sequences. However, supercontinent amalgamation and breakup also have profound tectonic effects on the evolution of continental margins that continuously faced oceans as the supercontinent assembled and dispersed. This evolution is commonly recorded by exotic along the continental margin. For Rodinia, these would include exotic terranes within the Appalachian, Caledonide and Variscan orogens that are interpreted to have evolved along an active margin of Neoproterozoic . Isotopic data indicate that some of these peri- Gondwanan terranes originated from ca. 1.2 to 1.0 Ga juvenile crust within a Panthalassa-type ocean that surrounded Rodinia and became accreted to the Gondwanan margin by 650 Ma. Other terranes, however, formed along this margin by recycling ancient Gondwanan crust. These interpretations require specific relationships with Gondwana that can be tested against the paleomagnetically constrained movements of and Gondwana from ca. 800-500 Ma. Problematically, Amazonia is unconstrained paleomagnetically, and there is a lack of reliable paleomagnetic data from Laurentia between 720 and 615 Ma. In addition, some models favour a high latitude for Laurentia around 570 Ma whereas others favour a low latitude. To these two models we apply two approaches, which we present in four animations. The first approach assigns the minimum movement to Laurentia and Gondwana required to satisfy the paleomagnetic data and examines the relationship between this motion and the contemporaneous tectonothermal evolution of the peri- Gondwanan terranes. The second approach again satisfies the paleomagnetic data but in time periods where there is no data Laurentia and Gondwana are permitted to migrate in a fashion that would make them compatible with the tectonothermal history of peri-Gondwanan terranes. The available paleomagnetic data from both West and East Avalonia, although not of high quality, show systematically lower paleolatitudes than predicted by these models. Hence, a suggestion from both approaches is that either Laurentia had a more complicated movement history between 720 and 615 Ma than is currently constrained by the available data, or the configuration of Laurentia-W. Gondwana-Avalonia on many reconstructions is incorrect. Although the Laurentia-W.Gondwana fit shown in this paper is constrained by the abundant Neoproterozoic paleomagnetic poles for Laurentia and , several tests of this configuration, including the correlation between Dalradian Scotland and the Peruvian Arequipa massif and the discovery of the Neoproterozoic Marañon belt in the northern , have failed to provide conclusive proof.

Introduction supercontinent assembly and dispersal. As a result, efforts to The amalgamation and breakup of the supercontinent constrain its history and configuration have focused on the Rodinia has influenced global-scale tectonothermal events distribution of Grenville-aged orogenic belts and the in the Neoproterozoic in much the same way that the evolution of Neoproterozoic passive margin sequences. amalgamation and breakup of Pangea influenced the However, continental margins that continuously faced Phanerozoic. Although the configuration and breakup oceans as the supercontinent assembled and dispersed history of Rodinia is controversial (e.g., Hoffman, 1991; preserve tectonothermal histories that provide additional Dalziel, 1992, 1997; C. Powell et al, 1993; Karstrom et al., geologic constraints on the timing of supercontinent 1999; Loewy et al., 2000; Wingate and Giddings, 2000; Li assembly and fragmentation. For Rodinia, the history of and Powell, 2001), the consensus is that Rodinia was a such margins is collectively recorded in those peri- relatively long-lived (c. 1100-755 Ma) supercontinent Gondwanan terranes, such as Avalonia, Carolina and produced by collisional events of broadly Grenvillian age Cadomia, that are believed to have occupied peripheral (McMenamin and McMenamin, 1990; offman, 1991; positions with respect to the supercontinent. Hence, the Wingate and Giddings, 2000). Neoproterozoic tectonothermal evolution of these terranes Studies of the amalgamation and breakup of Rodinia have may provide further insights into the configuration of centred on the evolution of its continental margins during Rodinia and the timing of its breakup.

45 Murphy et al., 2001 Journal of the Virtual Explorer

In this paper, we test the relationship between the peri-Gondwanan terranes and Gondwana implied by a wealth of diverse geologic data against paleomagnetically constrained movements of Laurentia and Gondwana from ca. 800-500 Ma. Using this approach, we can show that if a tectonothermal linkage existed between the peri-Gondwana terranes and Gondwana during the late Neoproterozoic, then a linkage between Laurentia and Gondwana is unlikely.

Geologic Setting

The peri-Gondwanan terranes are exotic terranes within the Appalachian, Caledonide, and Variscan orogens (Fig. 1A) that are interpreted to have evolved along an active continental margin of Neoproterozoic Gondwana (Fig. 1B, e.g., O’Brien et al., 1983; Rast and Skehan, 1983; Quesada, 1990; Nance et al., 1991). Early Paleozoic shallow-marine successions within these terranes contain peri-Gondwanan faunas (e.g., Theokritoff, 1979; Keppie, 1985; Nance and Thompson,1996). Avalonia, the largest peri-Gondwanan , extends from New England and Atlantic Canada (West Avalonia) into southern Britain and Brabant (East Avalonia). Other peri-Gondwanan arc terranes include Carolina and related terranes in the southern Appalachians and the subsurface of Florida, and Cadomia, a group of related terranes in northwestern France and Bohemia, the age and evolution of which resembles those of parts of Iberia and the Pan-African belts of West Africa (Fig. 1B). In Figure 1. A: Early Mesozoic reconstruction showing the locations of Middle America, peri-Gondwanan terranes include Precambrian peri-Gondwanan terranes. B. Late Neoproterozoic (635-590 Oaxaquia and the Yucatan block of Mexico, and the Chortis Ma) reconstruction showing the location of Avalonia and related peri- block of Honduras and Guatamala. The geology of these Gondwanan terranes (modified after Nance and Murphy, 1996; Linneman terranes has been reviewed in several recent publications et al., 2000; Nance et al. in press) relative to the continental reconstruction (e.g. Nance et al., in press; Keppie and Ramos 1999, Nance of Dalziel (1997). (Ch = Chortis Block, Ox = Oaxaquia, Y = Yucatan Block, and Thompson, 1996) and is briefly outlined here. F = Florida). Figure modified after Nance and Murphy (1994, 1996) and Murphy et al. (1999) to accommodate (1) polarities proposed Avalonia for West Avalonia (Dostal et al., 1996; Keppie and Dostal, 1998; Murphy et al., 1999), Cadomia (e.g., Chantraine et al., 1994), and Iberia (Quesada, The evolution of Avalonia has seven main elements (e.g. 1990; Eguíluz et al., 2000), (2) the present position of Carolina relative to Nance et al., in press); (1) the development of juvenile crust West Avalonia and the preponderance of early Proterozoic and Archean at ca. 1.2 to 1.0 Ga, (2) an early arc phase (pre 650 Ma) , (3) ages among its detrital zircon population (Samson et al., 1999), (3) accretion to Gondwana at ca. 650 Ma, (4) a main arc phase Neoproterozoic correlations of Florida and West Africa (Dallmeyer, 1989), (640-570 Ma), (5) its transition to a platform (570-540 Ma), (4) late Neoproterozoic paleomagnetic evidence for the relative positions of (6) the rifting of Avalonia from Gondwana (ca. 515 Ma), and Baltica-Laurentia (Meert et al., 1996) and West Avalonia-Gondwana (7) its accretion to Laurentia (ca. 440 Ma). (MacNamara et al., 2001), (5) the Neoproterozic Cadomian/Baikalian belt Although Avalonia developed along a continental margin of eastern Baltica (Roberts and Siedlecka 1999), and (6) the peri- subduction zone, the basement upon which the main Gondwanan basement terranes of Middle America (Keppie and Ramos, Avalonian arc was developed is nowhere unequivocally 1999). Not included are peri-Gondwanan fragments of Amazonian affinity exposed. Fragments of this basement may occur in recently identified within the European Variscan belt of Iberia (Fernández- northwestern Cape Breton Island (Keppie and Dostal, 1991) Suárez et al., 2000) and Bohemia (Friedl et al., 2000). and in the Goochland terrane of the southern Appalachians (Hibbard and Samson, 1995), but the provenance of these ranging in age from 740 to 370 Ma. These igneous rocks regions is controversal (e.g., Farrar, 1984; Barr et al., 1998). have elemental Sm/Nd ratios typical of intracrustal melts As a result, the nature of Avalonian basement has been (Sm/Nd ~0.19; Allègre and Ben Othman, 1980) and similar characterized indirectly from the neodymium isotopic initial eNd values that range between Ð2.5 and +5.0 composition of crustally derived felsic igneous rocks (Thorogood, 1990; Barr and Hegner, 1992; Whalen et al.,

46 Animated History of Avalonia in Neoproterozoic-Early PalaeozoicJournal of the Virtual Explorer

1994; Kerr et al., 1995; Murphy et al., 1996a; Keppie et al., Blasband et al., 2000), and in the ca. 950-900 Ma calc- 1997; Murphy et al., 2000; Samson et al., 2000). alkalic granitoid orthogneisses and metarhyolites of the Extrapolated to the depleted mantle curve, εNd growth lines Tocantins province in central Brazil which yield a similar consistently yield overlapping model ages (TDM) of 0.8-1.1 envelope of eNd growth lines and almost identical (ca. 0.9- Ga in Atlantic Canada and 1.0-1.3 Ga in southern Britain 1.2 Ga) depleted mantle model ages to those of Avalonia (Thorogood, 1990; Murphy et al., 2000). (Pimental and Fuck, 1992). On our reconstructions These depleted mantle model ages closely coincide therefore, this juvenile Avalonian crust, called proto- with the timing of Grenville (c. 1.25-0.95 Ga) orogenesis Avalonia, is positioned within the Panthalassa-type peri- considered to be responsible for the amalgamation of Rodinian ocean. Rodinia (e.g., Hoffman, 1991). However, igneous rocks in Fragmentary evidence for initial subduction in Avalonia Grenville-aged orogens are characterized by higher model dates from at least 730 Ma to 650 Ma and is termed the early ages, implying significant recycling of older crust. In arc phase. In Atlantic Canada, examples of this activity contrast, the quite strongly positive distribution of Avalonian include the ca. 734 Ma calc-alkalic Economy River eNd values suggests a largely juvenile basement dominated in mainland Nova Scotia (Doig et al., 1993), the ca. 681 Ma by c. 1.0 Ga mantle-derived material with only minor arc-related Stirling Belt (Bevier et al., 1993) in Cape Breton elements of older Proterozoic crust. Island, and the calc alkalic ca. 683 Ma Tickle Point The interpretation of depleted mantle model ages is Formation and ca. 673 Ma Furby’s Cove Intrusive Suite controversial (Arndt and Goldstein, 1987). Detailed (Swinden and Hunt, 1991; O’Brien et al., 1996) in southern arguments for the interpretation of the Sm-Nd data are Newfoundland. The ophiolite volcanics of the Burin presented elsewhere (e.g. Nance and Murphy, 1994, 1996; Group in Newfoundland (Strong et al., 1978) may extend Murphy et al., 2000). For magma produced by recycling of this early Avalonian magmatic activity to ca. 763 Ma (Krogh a single crustal source, the model age represents the time at et al, 1988). which the crustal basement was itself extracted from the In Britain, evidence for early arc-related activity is mantle. More commonly, however, magmas contain represented by the c. 700 Ma calc-alkalic Stanner-Hanter mixtures of juvenile, mantle-derived material and older Complex of central Wales (Patchett et al., 1980) and the c. crustal components. In these situations, the model age has 677 Ma calc-alkalic Malverns Plutonic Complex of the no geologic significance. That the model ages for Avalonia British Midlands (Tucker and Pharoah, 1991). 40Ar/39Ar represent mantle extraction ages is suggested by the very mineral ages of ca. 650 Ma in the Malverns Complex are similar model ages in Avalonian magmatism ranging from interpreted to date cooling following upper greenschist to 740 Ma to 370 Ma (Murphy et al., 1996b; Murphy et al., facies (Strachan et al., 1996). 2000). The generation of such similar model ages over a Early arc activity may also be represented in the undated time interval of this length is unlikely to be the outcome of of the Rosslare Complex in southeastern Ireland mixing, but instead, indicates the predominant influence of a and the Coedana Complex in North Wales (Gibbons and single basement source. Horák, 1996). Hence, the neodymium isotopic data imply that A short period of high grade metamorphism is recorded at successive generations of Avalonian felsic magma were ca. 650 Ma in various parts of Avalonia, including coastal produced largely as a result of recycling ca. 1.0-1.2 crust. Maine (Stewart and Tucker, 1998) and the Malvern Plutonic Lead isotopic data (Ayuso et al., 1996) similarly suggest that Complex (Strachan et al., 1996). Amphibolite facies the igneous rocks represent mixtures of juvenile, c. 1 Ga metamorphism of pre-630 Ma age may also be present in basement and typical Avalonian crust. The depleted mantle central Cape Breton Island (Keppie et al., 1998) and model ages are therefore thought to record a genuine southern Newfoundland (O’Brien et al., 1996), and some tectonothermal event during which the bulk of Avalonian form of accretion must likewise be recorded in the basement was itself extracted from the mantle. emplacement of ophiolitic rocks of the ca. 760 Ma Burin The formation of Avalonian basement is therefore Group (Keppie et al., 1991). This metamorphism is considered to have been broadly coeval with Grenvillian interpreted to reflect the accretion of Avalonia to the orogenesis. However, the primitive isotopic signature of Gondwanan continental margin prior to the beginning of the Avalonia relative to that of the Grenville Belt suggests that main phase of Avalonian magmatism at ca. 635 Ma and the basement formed, not as part of a collisional , but coincides with a temporary cessation (ca. 650-635 Ma) in in one or more largely juvenile oceanic island arcs (Murphy subduction-related magmatism. In our reconstructions, et al., 2000). These data can be reconciled if the basement therefore, we show the outboard arc terranes of Avalonia developed within the Panthalassa-type ocean that would colliding with the northern Gondwanan margin at 650 Ma. have surrounded Rodinia following its amalgamation. Other The main phase of Avalonian magmatism is recorded in remnants of primitive island arcs developed within this peri- voluminous late Neoproterozoic magmatic arc-related Rodinian ocean may be preserved in the ca. 900-700 Ma volcanic and cogenetic plutonic rocks with crystallization rocks of the Arabian-Nubian Shield (e.g., ages of 635 to 570 Ma (e.g., Nance et al., 1991). Coeval

47 Murphy et al., 2001 Journal of the Virtual Explorer

sedimentary successions that are dominated by volcanogenic diachronous cessation of arc volcanism and the apparent turbidites are locally associated with these arc-related reversal of kinematics on major basin-bounding faults. magmatic rocks, and have been attributed to deposition in a Avalonia likely remaining linked to Gondwana until the variety of intra-arc, interarc and back arc basins (e.g., Pe- Early Ordovician. During the Ordovician, however, faunal Piper and Murphy, 1989; Pe-Piper and Piper, 1989; Pauley, provinciality and paleomagnetic data indicate increasing 1990; Smith and Socci, 1990; O’Brien et al., 1996; Murphy separation from Gondwana concurrent with a decrease in the et al., 1999). This magmatic activity and the generation of separation between Avalonia and Laurentia (Cowie, 1974; arc-related basins is interpreted to reflect oblique subduction Boucot, 1975; Pickering et al., 1988; Cocks and Fortey, beneath the northern Gondwanan margin. 1990; Trench and Torsvik, 1992; Dalziel et al., 1994; Cocks, The timing of the onset of this main phase activity was 2000). The Arenig Stiperstone Quartzite in Britain and the broadly similar throughout much of Avalonia. However, its correlative Armorican Quartzite of Cadomia and Iberia (e.g., cessation was diachronous, terminating at ca. 590 Ma in Noblet and Lefort, 1990), is thought to reflect the subsidence New England (Kaye and Zartman, 1980; Hermes and associated with this separation. Minor bimodal rift Zartman, 1985, 1992; Thompson et al., 1996; Thompson and volcanism in Avalonia is predominantly of Cambrian age Bowring, 2000), 600 Ma in southern New Brunswick and may reflect rifting prior to separation (e.g., Murphy et (Bevier and Barr, 1990; Barr et al., 1994; Currie and al., 1985; Greenough and Papezik, 1986). McNicoll, 1999), 605 Ma in mainland Nova Scotia (Doig et al., 1991; Murphy et al., 1997; Keppie et al., 1998), 575 Ma Cadomia-Iberia in southern Cape Breton (Barr et al., 1990; Bevier et al., 1993), 585 Ma in Newfoundland (Krogh et al., 1988; The Trégor-La Hague terrane of Cadomia (Strachan et al., O’Brien et al., 1996) and 600 Ma in the British Isles (e.g., 1996) contains the only undisputed basement exposed in any Tucker and Pharaoh, 1991; Horák, 1993; Noble et al., 1993). of the peri-Gondwanan arc terranes. The age (Auvray et al, Cessation of main-phase subduction is accompanied by a 1980; Piton, 1985; Samson and D’Lemos, 1998) and transition to intracontinental extension, marked by the onset isotopic signature (D’Lemos and Brown, 1993; Samson and of bimodal magmatism. As with the onset of main phase arc D’Lemos, 1998) of this basement (the c. 2.2-1.8 Ga Icart arctivity, the transition occurred at different times along the Gneiss) resembles that of the 2.1 Ga Eburnian basement of belt; at c. 595 Ma in New England (Mancusco et al., 1996), the West African craton (Allègre and Ben Othman, 1980). at c. 560 Ma in southern New Brunswick (Bevier and Barr, Early arc-related magmatism is recorded by the c. 746 Ma 1990; Barr et al., 1994; Currie and McNicoll, 1999), at c. orthogneiss of the Pentevrian Complex (Egal et al., 1996), 605 Ma in mainland Nova Scotia (Murphy et al., 1997), and by deformed granodioritic conglomerate cobbles with between 575 Ma and 560 Ma in southern Cape Breton Island protolith ages of 670-650 Ma (Guerrot and Peucat, 1990) in (Bevier et al., 1993), at c. 570 Ma in Newfoundland the Armorican massif of northwestern France. Deformation (O’Brien et al., 1996), and in the interval 570-560 Ma in and metamorphism of the early Cadomian arc occurred in Britain (Tucker and Pharoah, 1991). Although this stage is the interval (c. 650-615 Ma) separating the early and main locally accompanied by deformation and metamorphism, no phases of arc magmatism (Egal et al., 1996; Strachan et al., evidence exists for the regional orogenesis, crustal 1996). shortening, and crustal thickening and uplift characteristic of Cadomian magmatism between 616-570 Ma is widespread continental collision zones. Instead, deformation is usually in the Armorican massif and Iberia (e.g., Quesada, 1990; localized and resulted in the inversion of the earlier volcanic Strachan et al., 1996; Miller et al., 1999) and produced arc basin successions. voluminous late Neoproterozoic magmatic arc-related To account for such a tectonic transition in the apparent volcanic and cogenetic plutonic rocks. Coeval sedimentary absence of a major collisional event, Murphy and Nance successions are dominated by volcanogenic turbidites that (1989) proposed that Avalonian subduction was terminated are thought to have been deposited in arc-related basins as a result of transform activity. In their model, the main (e.g., Dennis and Dabard, 1988; Chantraine et al., 1994; phase of Avalonian magmatism at c. 635-570 Ma occurred Egal et al., 1996; Strachan et al., 1996). as the result of oblique subduction, leading to the The main phase of arc magmatism continued to c. 570 Ma development of an extensional magmatic arc and a variety of but is progressively replaced by sinistral strike-slip tectonics volcanic arc basins. Subsequently, the interaction of a in the interval c. 570-540 Ma (e.g., Strachan et al., 1996), continental margin transform system with the subduction and is superseded by widespread intracrustal melting, zone resulted in the termination of subduction, the structural migmatization and bimodal magmatism including post- inversion of a number of volcanic arc basins, and the tectonic granitoid emplacement at c. 550-540 Ma (e.g., Rabu formation of new rift and wrench-related basins in the et al., 1990; Quesada, 1990; Chantraine et al., 1994; Egal et interval c. 590-540 Ma. Murphy et al. (1999) and Keppie et al., 1996). al. (2000) later postulated ridge-trench collision as a The basement isotopic Sm-Nd signatures of Cadomia mechanism for the transition in order to account for the together with U-Pb detrital zircon data from within its late

48 Animated History of Avalonia in Neoproterozoic-Early PalaeozoicJournal of the Virtual Explorer

Neoproterozoic (Brioverian) sedimentary succession (Glover, 1989; Rankin et al., 1989) that collided with the (Samson et al., 1999) suggest a position near the West Carolina terrane at about 590 Ma. Piedmont terrane African craton. Thus, in contrast to Avalonia, Cadomia and assemblages are dominated by a Cambro-Ordovician Iberia appear to have originated above complex of arc, fore-arc and accretionary complexes crust along the continental margin of West Africa, rather (Hibbard and Samson, 1995) that may be a continuation of than within the peri-Rodinian ocean. As a result, Avalonia the Pampean orogeny of western South America (Keppie and Cadomia-Iberia did not form a coherent orogenic belt and Ramos, 1999). until the collision of Avalonia with northern Gondwana at ca. 650 Ma. Middle American terranes Voluminous bimodal rift volcanism of predominantly Middle Cambrian age (Quesada, 1990, Giese and Buehn, Although their paleogeography is perhaps the least 1994) records an important extensional event in Iberia. understood, the distribution of Early Paleozoic Gondwanan Widespread Arenig subsidence, recorded in the broad fauna, indicates that several terranes in Middle America distribution of the Armorican Quartzite across Cadomia and have peri-Gondwanan affinities. However, they do not Iberia (e.g., Noblet and Lefort, 1990) suggests that rifting preserve evidence of Neoproterozoic arc activity, suggesting extended into the Early Ordovician. Potential correlatives in they were located inboard of the magmatic arc. These Britain (the Stiperstones Quartzite) suggest that rifting terranes expose basement of Pan African (Yucatan block) occurred at about the same time in East Avalonia. Faunal and Grenville (Oaxaquia and Chortis block) age (Keppie and data (eg. Cocks and Fortey, 1990; Cocks, 2000) indicate that Ortega-Gutiérrez, 1999). The Yucatan block is thought to by the Early Ordovician, Avalonia had separated from have been contiguous with the Florida basement until the Gondwana, resulting in the birth of the , opening of the Gulf of Mexico in the Mesozoic (e.g. Pindell whereas Cadomia and Iberia remained along the West et al., 1990; Dickinson and Lawton, 2001). The Grenville African portion of this margin. basement of Oaxaquia and the Chortis block is isotopically transitional between that of the Grenville Belt and the Carolina/Goochland/Piedmont terranes basement massifs of Grenvillian age in Columbia (Ruiz et al., 1999). Following Keppie and Ramos (1999), we The oldest rocks in the Carolina terrane are ca. 670 Ma position these along the Columbian margin in accordance granitoid bodies of the Roanoke Rapids terrane (Hibbard et with the paleomagnetic data of Ballard et al., (1989). al., in press). They are interpreted as evidence of early arc magmatism broadly coeval with that in Avalonia. The Reconstructions: basement to the Carolina terrane is not exposed. However, initial eNd values of +0.5 to +5.9 and TDM model ages of Interpretations of the tectonothermal evolution of the peri- 0.7-1.1 Ga from c. 635-610 Ma volcanic rocks of the Gondwanan terranes imply a genetic relationship with the Virgilina sequence (Samson et al., 1995; Wortman et al., northern margin of Gondwana (present coordinates) for the 2000) suggest that the Carolina terrane, like Avalonia, was entire Neoproterozoic. For example, most models for the located outboard from the northern Gondwanan margin until main phase (ca. 640-570 Ma) of arc-related peri-Gondwanan at least 700 Ma. magmatism require that these terranes were distributed The Carolina terrane is dominated by a ca. 633-607 Ma along the northern Gondwanan margin (present juvenile arc assemblage, overlain unconformably by a 580- coordinates), and were therefore moving with Gondwana 540 Ma mature arc sequence, followed by middle Cambrian during this time period. Hence, models for the platformal sedimentary strata that contain cool-water tectonothermal evolution of the peri-Gondwanan terranes trilobites similar to those of Cadomia and Baltica (Samson et should be testable against the paleomagnetically constrained al., 1990; Hibbard and Samson, 1995; Wortman et al., 2000). movements of the Amazonian and West Africa cratons. Possible episodes of arc rifting have been documented at c. Unfortunately, reliable data from these cratons is sparse, 590-570 Ma and c. 560-535 Ma (e.g., Dennis and Shervais, so that these connections cannot be tested directly. Most 1991, 1996; Shervais et al., 1996). The earlier event is global-scale reconstructions imply a connection between probably related to a transition from arc to strike-slip Amazonia, Laurentia and Baltica throughout the tectonics and may be responsible for the unconformity Neoproterozoic, for which some paleomagnetic constraints between the older and younger volcanic successions. The exist. However, there is considerable disagreement later event may have been coeval with widespread concerning the paleolatitude of Laurentia during a critical deformation and metamorphism (Dennis and Wright, 1997; time interval between 625 and 550 Ma. There are two Barker et al., 1998). reliable paleomagnetic poles from Laurentia for this time The neighboring Goochland terrane has a ca. 1.0 Ga interval Ð one is from the 577 Ma Callander Complex granulite facies basement that has been interpreted as either (Symons and Chaisson, 1991), another from the Sept Iles part of the Laurentian Grenville Belt, or as an exotic terrane intrusion (Tanczyk et al, 1987), dated at 565 Ma (Higgins

49 Murphy et al., 2001 Journal of the Virtual Explorer

and van Breeman, (1998). As it is impossible to incorporate By ca. 635 Ma, the occurrence of abundant ensialic arc- them into the same tectonic model, two models (with high- related magmatism in all peri-Gondwanan terranes, together and low-latitude position of Laurentia, respectively) were with the presence of Gondwanan detrital zircons, indicates used (Pisarevsky et al., 2000, 2001). that a subduction zone had been established outboard of the The following reconstructions are an initial attempt to peri-Gondwanan terranes and was angled beneath these evaluate such connections and to identify critical areas of accreted terranes and the cratonic margin of Gondwana. uncertainty that require resolution. We apply two approaches to each model. In the first, we assign the minimum 625-570 Ma movement to Laurentia and Gondwana required to satisfy The critical interval of 625 to 570 Ma is the time of the paleomagnetic data and then examine the relationship greatest uncertainty because of the paucity and controversial between this motion and the contemporary tectonothermal nature of the paleomagnetic data base. As a result, there is evolution of the peri-Gondwanan terranes. In the second, we considerable uncertainty about the paleolatitude of again satisfy the paleomagnetic data but in those time Laurentia. Since many reconstructions imply a connection periods for which there is no data we allow Laurentia and between Laurentia and West Gondwana, resolution of this Gondwana to migrate in a fashion compatible with the issue has fundamental implications for the interpretation of tectonothermal history of the peri-Gondwanan terranes. For the geodynamic significance of peri-Gondwanan each approach, we provide alternative reconstructions for tectonothermal events. the high latitude and low latitude positions of Laurentia in Approach One: the 625-550 Ma time interval, making a total of four In a high latitude configuration, Laurentia, and by animations. (Figures 2, 3, 4, and 5) implication, Amazonia, drifts rapidly southward, between 615 to 580 Ma, and would imply oblique sinistral 800-625 Ma: convergence across the arc along the northern Gondwanan In the four animations there is little variation in our margin. This is consistent with field data in Avalonia (Nance reconstructions between 800 and 625 Ma, and so they are and Murphy, 1990, Murphy et al., 2001; Nance et al., in described together. Isotopic data suggest that Avalonia and press) where such a style of convergence has been ascribed Carolina originated as juvenile crust (proto-Avalonia - to the opening of intra-arc basins related to sinistral strike- Carolina) in the peri-Rodinian ocean. Accordingly, at 800 slip activity (Murphy et al., 2000) and provides a Ma, these terranes are positioned well outboard of the geodynamic explanation for the onset of the main phase Gondwanan margin. The distance, however, is magmatic episode that characterizes the terranes along the unconstrained. In contrast, isotopic data indicate that northern Gondwanan margin. This scenario would be Cadomia and Iberia have ancient, West African basement. analogous to the modern relationship between the westward These terranes are consequently positioned adjacent to the drift of North America and South America and the style of West Africa craton. Between 800 and 650 Ma, all tectonic activity along the eastern margin of the Pacific animations show the distance between Avalonia-Carolina Ocean. and Laurentia-West Gondwana progressively decreasing. If on the other hand, Laurentia remains at low latitudes This convergence is accommodated in large part by the during this time interval, the main phase tectonothermal south-southeastward drift of Laurentia-West Gondwana, and events along the northern Gondwanan margin would require is held to be responsible for the early arc stage of the peri- a different explanation. In this scenario, the most probable Gondwanan terranes. Given that there is little evidence of explanation is the re-establishment of subduction along the coeval arc-related activity along the cratonic Gondwanan margin following accretion of outboard terranes such as margin between 800 and 700 Ma, closure of the intervening Avalonia and Carolina, and the geodynamic relationship tract of requires either (a) a subduction zone between this event and global-scale plate motions is unclear. angled away from the Gondwanan margin or (b) the The potentially profound influence of the separation of development of a Western Pacific type margin in which Baltica from Laurentia at about 600 Ma (Meert et al., 1996) Gondwana is separated from the early Avalonian arc by a is apparent in both high and low-latitude models. This back arc basin. separation implies the existence of a spreading ridge The collision of Avalonia-Carolina with the Gondwanan between these two continents. According to Murphy and margin at ca. 650 Ma brings the so-called "Avalonian- Nance, (1989) and Murphy et al. (1999), it was the collision Cadomian belt" into alignment for the first time and broadly of a spreading ridge with the northern Gondwanan margin coincides with a brief hiatus in arc magmatism. The birth of that was responsible for the diachronous cessation of arc- the Avalonian-Cadomian belt at this time is analogous to the related magmatism and the onset of strike-slip tectonics. Mesozoic-Cenozoic evolution of western North America in Both reconstructions suggest that the colliding ridge may that proximal and exotic terranes were incorporated into a have been the spreading ridge between Baltica and single belt that shared a similar subsequent history. Laurentia. Such a collision would additionally explain the

50 Animated History of Avalonia in Neoproterozoic-Early PalaeozoicJournal of the Virtual Explorer

Figures 2. 800-490 Ma reconstructions of Laurentia-Gondwana-Baltica, Figure 4. 800-490 Ma reconstructions of Laurentia-Gondwana-Baltica, emphasizing the history of the peri-Gondwanan terranes. This reconstruc- emphasizing the history of the peri-Gondwanan terranes. This reconstruc- tion assigns the minimum movement to Laurentia and Gondwana required tion satisfies the paleomagnetic database but Laurentia and Gondwana are to satisfy the paleomagnetic data and examines the relationship between permitted to migrate so as to make them compatible with the tectonother- this motion and the contemporaneous tectonothermal evolution of the peri- mal history of peri-Gondwanan terranes. This reconstruction also incorpo- Gondwanan terranes and incorporates the high latitude option for rates the high latitude option for Laurentia between at about 570 Ma. Laurentia between at about 570 Ma. Click on image to view animation. Click on image to view animation.

Figure 3. 800-490 Ma reconstructions of Laurentia-Gondwana-Baltica, Figure 5. 800-490 Ma reconstructions of Laurentia-Gondwana-Baltica, emphasizing the history of the peri-Gondwanan terranes. This reconstruc- emphasizing the history of the peri-Gondwanan terranes. This reconstruc- tion assigns the minimum movement to Laurentia and Gondwana required tion satisfies the paleomagnetic database but Laurentia and Gondwana are to satisfy the paleomagnetic data and examines the relationship between permitted to migrate so as to make them compatible with the tectonother- this motion and the contemporaneous tectonothermal evolution of the peri- mal history of peri-Gondwanan terranes. This reconstruction also incorpo- Gondwanan terranes and incorporates the low latitude option for Laurentia rates the low latitude option for Laurentia between at about 570 Ma. between at about 570 Ma. Click on image to view animation. Click on image to view animation. change from sinistral to dextral motion along basin- Approach Two: bounding faults within Avalonia that occurs at about this In these animations, we force the peri-Gondwanan time (Nance and Murphy, 1990; Murphy et al. 2001). The terranes to remain in a general position along the orientation of the spreading ridge between Laurentia and Amazonian-West African margin. We feel this position best Amazonia would have been highly oblique to the peri- matches the geology recorded in these terranes, including Gondwanan subduction zone, resulting the termination of the evidence of detritus derived from the adjacent cratons. If arc-related magmatism (except in localized areas such as so, the style of subduction outboard of these terranes would Anglesey; Gibbons and Horak, 1996) and the eastward drift be profoundly influenced by the drift of these continental of Gondwana-peri-Gondwana relative to Laurentia. blocks.

51 Murphy et al., 2001 Journal of the Virtual Explorer

Once again, the drift of Laurentia-Amazonia to high South America, the peri-Gondwanan terranes are dominated latitudes during this time interval provides an explanation by stable platformal assemblages and localized rift-related for the eastern Pacific-style of subducion recorded in these magmatism at this time. terranes. In this scenario, the propagation into the Avalonian- Cadomian belt of a spreading ridge associated with the Paleomagnetic constraints from Avalonia separation of Baltica from Laurentia would again account for the diachronous termination of subduction. We used our models to construct the paleolatitudinal On the other hand, arc magmatism would be expected to positions for West and East Avalonia (representative points continue along the leading edge of Baltica (NE Norway and are 46¼N, 60¼W and 52¼N, 0¼E, respectively) (Fig. 6). NW Russia), which is consistent with recent Unfortunately there are no high-quality paleomagnetic data geochronological data from drill-holes beneath the Pechora for Avalonia with well constrained ages older than 600 Ma. Basin (Roberts and Siedlecka, 1999). We have shown available magnetic paleolatitudes (Fig. 6) using the results with reliability criteria of Q>2 (Van der 570-495 Ma Voo, 1990). These paleolatitudes are systematically lower In this time interval, there is very little difference between than those predicted by our models. This discrepancy could the continental configurations derived from approaches one be reduced slightly if the Amazonia-Laurentia fit of Dalziel and two. However, there are fundamental differences in (1997) is used. However, the apparent lack of correlation implication between the high and low latitude models for between Dalradian of Scotland and the Peruvian Arequipa Laurentia. A high latitude position for Laurentia at 570 Ma massif, together with the discovery of the Neoproterozoic implies a subsequent a northward drift relative to Amazonia Marañon belt in the northern Andes suggests this between 570 and 550 Ma, associated with the opening of this configuration requires substantial modification. In addition, portion of the . On the other hand, if Laurentia this fit contradicts the abundant Neoproterozoic maintains a low latitude position, the opening of the Iapetus paleomagnetic data from Laurentia and Baltica (e.g., Weil et Ocean between these blocks requires rapid southward drift al., 1998; Pisarevsky and Bylund, 1998). of Amazonia and, by implication, the attached peri- Hence, a suggestion from both approaches is that either Gondwanan terranes. This setting would be analogous to the Laurentia had a more complicated movement history modern relationship between spreading in the South between 720 and 615 Ma than is currently constrained by the Atlantic, the westward drift of South America, and the style available data, or the configuration of Laurentia-West of magmatism along the Andean margin. Gondwana-Avalonia on many reconstructions is incorrect. The peri-Gondwanan terranes are dominated by wrench- related tectonics during this interval. The relationship Discussion between this tectonothermal activity and the rapid movement of Laurentia is not immediately obvious, unless Although there is general consensus that the the vector of plate motion was at a low angle to the peri- amalgamation and subsequent dispersal of the Gondwanan portion of the continental margin. Such a supercontinent Rodinia profoundly influenced the evolution direction would have been at a high angle to East of Earth systems in the Neoproterozoic, the configuration is Gondwana, and consistent with near-orthogonal collision controversial. which resulted in the formation of Gondwanaland at ca. 530 We present four animations for the crucial time Ma (e.g. Hoffman, et al. 1998). interval between 800 and 495 Ma that examine the potential By 540 Ma, both models for Laurentia show the geodynamic linkages between the tectonothermal evolution essentially the same configuration, with Laurentia at low of peri-Gondwanan terranes and Laurentia-Amazonia- latitudes. However, they also suggest that the spreading Baltica continental configurations. Each animation has between Laurentia and Baltica after 600 Ma would be several simplifying assumptions and some critical coincident with subduction under the leading edge of uncertainties. However, using the Mesozoic-Cenozoic Baltica. The growing evidence for protracted deformation breakup of Pangea as a modern analogy, serve to do focus and metamorphism along the present northern margin of attention on potential geodynamic linkages between regional Baltica between 600 and 550 Ma (e.g. Roberts and tectonothermal events. The most critical uncertainty is the Siedlecka, 1999) suggests that this may well have been the paleolatitude of Laurentia (and by implication, Amazonia) at leading edge. This observation may shed light on ca. 570 Ma. For example, the high latitude and a low latitude controversies concerning the orientation of Baltica relative options for Laurentia at this time allow very different to Laurentia in the late Neoproterozoic and Early Paleozoic geodynamic interpretations for the evolution of peri- (e.g. Torsvik et al., 1996; Dalziel, 1997). Gondwanan terranes and the opening of the Iapetus Ocean. At 535 Ma, Baltic had reached its maximum latitudinal Such reconstructions also focus attention on uncertainties separation from northern Gondwana. Although subsequent in the database. For example, relatively reliable convergence may be reflected in the Pampean orogeny of paleomagnetic data (according to the criteria of Van der Voo,

52 Animated History of Avalonia in Neoproterozoic-Early PalaeozoicJournal of the Virtual Explorer

Australia (Office of Industry and Innovation) using the PLATES reconstruction program of the University of Texas at Austin, Texas, and Generic Mapping Tools of P.Wessel and W.H.F. Smith. This project was also supported by grants from the Program for North American Mobility in Higher Education, by a Programa de Apoyo a Proyectos de Investigación e Innovación Tecnológia (PAPIIT) grant (IN116999) to J.D.K., and by the James Chair of Pure and Applied Sciences at St. Francis Xavier University to R.D.N and J.D.K.. The paper is a contribution to International Geological Correlation Programme Project 453. Tectonics Special Research Centre Publication No. 190.

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