Long-lived ( Ga) convergent orogen in southern Laurentia, its extensions to Australia and Baltica, and implications for refining Rodinia

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1 I ; :, d c ELSEVIER Precambrian Research 1 11 (2001) 5-30 Precambrian Research Long-lived ( Ga) convergent orogen in southern Laurentia, its extensions to Australia and Baltica, and implications for refining Rodinia Karl E. Karlstrom ".*, Karl-Inge Ahall b, Stephen S. Harlan ", Michael L. Williams d, James McLelland ", John W. Geissman a a Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA Department of Earth Sciences, University of Karlstad, SE Karlstad, Sweden Department of Geography and Earth Systems Science, George Mason University, Fairfax, VA 22030, USA * Department of Geosciences, University of Massachusetts, Amherst, MA , USA 'Department of Geology, Colgate University, Hamilton, NY 13346, USA Received 10 January 2000; accepted 29 March 2000 Abstract Between 1.8 and 1.0 Ga (Grenville-age), a series of subparallel accretionary orogens were added progressively to the southern edge of Laurentia. These belts now extend from GreenlandILabrador to southern California and are truncated at late Precambrian passive margins, suggesting that they once extended farther. We propose that Australia and Baltica contain their continuations. Together they comprise a long-lived orogenic system, > km long, that preserves a record of 800 million years of convergent margin tectonism. This tectonism culminated during Grenvillian continent-continent collisions in the assembly of the supercontinent Rodinia. Our reconstruction of the Australiawestern US part of this assembly (AUSWUS) differs from the SWEAT reconstruction in that Australia is adjacent to the southwestern US rather than to northern Canada. The AUSWUS reconstruction is supported by a distinctive 'fingerprint' of geologic similarities between Australia and the southwestern US from 1.8 to 1.0 Ga, by numerous possible piercing points, and by an arguably better agreement between 1.45 and 1.0 Ga paleomagnetic poles between Australia and Laurentia. Geologic and paleomagnetic data suggest that separation between Laurentia and Australia took place Ma and between Laurentia and Baltica Ma. The proposed association of Australia, Laurentia, and Baltica, and the long-lived convergent margin they expose, provide a set of testable implications for the tectonic evolution of these cratons, and an important constraint for Proterozoic plate reconstructions. O 2001 Elsevier Science B.V. All rights reserved. Keywords: Paleogeography; Paleomagnetism; Laurentia; Baltica; Australia * Corresponding author. Tel.: ; fax: address: kekl@unm.edu (K.E. Karlstrom) /01/$ - see front matter O 2001 Elsevier Science B.V. All rights reserved. PII: SO (01)

2 K.E. Karlstrom et al. /Precambrian Research 11 1 (2001) Introduction Since the initial proposal by Valentine and Moores (1970), there has been increasing support for the hypothesis that continental masses were assembled at about 1.0 Ga (Grenville-age) into a Proterozoic supercontinent, called Rodinia (Mc- Menamin and Schulte-McMenamin, 1990). The breakup of Rodinia, beginning about 800 Ma, seems to have coincided with dramatic changes in Earth systems, such as diversification of life, unique climatic conditions, and global changes in ocean chemistry (Dalziel, 1997; Hoffman et al., 1998) as well as unusual, long-lived mantle convection patterns (Evans, 1998). However, the duration and configuration, and even the existence (Piper and Zhang, 1999), of a late Precambrian supercontinent (or supercontinents; Dalziel, 1992; Young, 1995) remain controversial. Detailed reconstructions are hindered by the absence of a seafloor record, lack of high quality apparent polar wander (APW) paths, and subsequent modifications to Precambrian plate margins. One approach to define the original geometry of a dispersed supercontinent is to recognize and match formerly adjacent rifted margins. Reconstructions of the Mesozoic breakup of Pangea must account for the shape and length of rifted margins and also satisfy seafloor magnetic data for Mesozoic/Cenozoic ocean basins. Rodinia reconstructions (Dalziel, 1997) have also tried to match late Precambrian rift margins that developed during the breakup of Rodinia. One key to this approach is the observation that Laurentia was nearly circumscribed by late Precambrian rift margins and thus may have had a central position within Rodinia. Stratigraphic data and subsidence curves suggest that thermal subsidence following the rift-to-drift transition took place in the latest Precambrian on many continental margins, and this has led to models for Rodinia breakup at about Ma (Bond et al., 1984; Levy and Christie-Blick, 1991). However, both geologic (Stewart, 1976; Ross, 1991; Karlstrom et al., 2000) and paleomagnetic (Powell et al., 1993; Wingate and Giddings, 2000) data argue for an earlier stage of rifting between western Laurentia and Australia ( x Ma; seen below). This evidence for multiple rift events, plus the possible presence of continental fragments between larger continents (e.g., South China; Li et al., 1995), make it difficult to apply the 'rift-budget' approach to Proterozoic supercontinent reconstructions because of uncertainties regarding both the original geometry and the diachronous nature of rift margins. Another approach to reconstructions is to use piercing points and geologic correiation between continents. For Pangea, it has been possible to match orogenic belts, fossil assemblages, and glaciogenic sequences. For Rodinia, the most widely used piercing points are segments of the 1.0 Ga orogenic belts.that record continent-continent collisions during assembly of Rodinia. However, many of the 'Grenville-age' belts themselves remain poorly understood. Most of these belts partially overprint older (Paleo- and Mesoproterozoic) orogens and/or are partially overprinted by younger (Pan-African and Caledonide) histories. Thus, the proposed geometry of x 1.0 Ga orogens during assembly of Rodinia (Unrug, 1997) remains uncertain. The approach taken here is to obtain time-integrated piercing points by understanding the tectonic evolution of orogenic systems rather than a single events or belts. We examine three key cratons within Rodinia: Australia, Laurentia, and Baltica. For Australia and Laurentia, we evaluate the most popular Rodinia reconstruction (SWEAT), but support an alternate reconstruction (AUSWUS; Fig. 1; Karlstrom et al., 1999), similar to that proposed by Brookfield (1993). For Laurentia and Baltica, we support the modified reconstruction of Park (1995), based on the paleomagnetically determined fit of Patchett et al. (1978). Recent detailed geologic syntheses of the Proterozoic tectonic evolution in Australia (Myers et al., 1996), Baltica (Torsvik et al., 1996; Ahall and Connelly, 1998; hiill and Larson, 2000), and Laurentia (Van Schmus et al., 1993; Rivers, 1997) facilitate comparisons. Our approach has global implications in terms of continental reconstructions and Proterozoic tectonic processes. In addition to further supporting the AUSWUS reconstruction, perhaps our most provocative conclusion is that there was a

3 K.E. Karlstrom et al. /Precambrian Research 111 (2001) 5-30

4 Fig. 1. (Continued)

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7 K.E. Karlstrom et al. / Precambrian Research 11 1 (2001) 5-30 globally significant, contiguous orogenic system that extended from Australia, across southern Laurentia, to Baltica during the Paleo- and Mesoproterozoic. These belts then became variably overprinted by Ga continent-continent collisions during the Grenville orogeny. 2. The SWEAT reconstruction of Rodinia The most influential continental reconstruction for Rodinia has been the SWEAT hypothesis, initially suggested by Jefferson (1978), then named and advanced by Dalziel (1991), Hoffman (1991), and Moores (1991). As part of this reconstruction, the western US was matched with Antarctica, western Canada with Australia, and the truncated Ga Grenville orogen of Texas was matched with East Antarctica (Dalziel, 1997; Fig. 11). According to this hypothesis, the supercontinent Rodinia was assembled by continental collisions at 1.0 Ga, rifted apart in the Neoproterozoic ( Ma) and 'turned inside out' (Hoffman, 1991) as Laurentia went from near the center of the supercontinent to an external position, and as East and West Gondwana were amalgamated during Pan-African collisions ( Ma). According to many reconstructions (Dalziel, 1997; Unrug, 1997), Laurentia at was bordered by Baltica to the northeast, Siberia to the North (Condie and Rosen, 1994, but cf. Sears and Price, 1978, 2000), and the African and South American cratons to the southeast (in present Laurentian co-ordinates). Newer geologic and paleomagnetic data raise doubts about the main piercing points used for the ties between Australia and Laurentia in the original SWEAT reconstruction. Dalziel (1991) and Moores (1991) suggested that the truncated Grenville front of West Texas could be matched to the Weddell Sea area of Antarctica. However,, paleomagnetic data indicate that this area of Antarctica was part of West Gondwana (Kalahari), not East Gondwana, at 1.1 Ga (Gose et al., 1997), and therefore cannot be used as a piercing point between East Gondwana and Laurentia (Dalziel, 1997; p. 33). Other proposed correlations are also uncertain. Lithologic and isotopic similarities between the Shackleton Range of Antarctica and the Yavapai Province of Laurentia are weakened by isotopic data showing Archean crustal components in the Shackleton Range Fig. 1. AUSWUS reconstruction for four Proterozoic time slices. Easternmost Australia was removed along the Tasman line (TA, Myers et al., 1996), which marks a major line of truncation of magnetic anomalies in Australia and the western edge of the Paleozoic accreted terranes of the Tasman orogen (Coney, 1990). The conjugate rift margin of Laurentia is defined by the Sr line (0.706), the inferred western edge of Precambrian basement (restored to its pre-mesozoic position; Levy and Christie-Blick, 1989) and by the Mojave-Sonora megashear (Anderson and Silver, 1979). Continents were rotated to this configuration about an Euler pole located at 51.46", E, rotation angle = ". Continents appear in equal area projection in North American co-ordinates. Geology is based mainly on Myers et al. (1996) for Australia, and Karlstrom and Bowring (1993), Van Schmus et al. (1993) and Karlstrom and Humpheys (1998) for the southwestern US. The position of Australia in the SWEAT reconstruction is shown for comparison (Moores, 1991). The pre-grenvillian Laurentia-Baltica configuration in (a) and (b) is according to Park (1995) where minor crustal slices between Greenland and Baltica mark the reworked Archean crust in northern Scotland and the Ga crust in Scotland, Ireland and the Rockall Bank. This Baltica fit is supported by paleomagnetic data from rocks as young as 1.27 Ga (Buchan et al., 2000); subsequent break-up, relative clockwise rotation of Baltica, and re-amalgamation with Laurentia during the Rodinia assembly are still not reliably constrained by paleomagnetic data. (a) Cpstal provinces inferred from U to Pb data; = Sr line; CB = Cheyenne Belt; DL = Diamantina lineament; GF = Great Falls tectonic zone; KL = Koonenberry fault zone; MS = Mojave-Sonora megashear; TA = Tasman Line. (b) Ga orogenic belts and A-type granites and anorthosites; G-R = granite rhyolite provinces; histogram shows wide variation in ages of A-type magmatism along the orogen in Laurentia (Hoffman, 1989). (c) Grenville orogenic belts (showing foliation trends) and temporally-coincident NE intracratonic extension (mafic dikes and normal faults); an estimated late-sveconorwegian ( = 1.0 Ga) position of Baltica is shorn after its postulated collision with a South American craton (Park, 1992). (d) Ga extension and sedimentation just prior to supercontinent fragmentation. Centralian Superbasin includes the Officer basin (0), Amadeus basin (AM), Georgina basin (G), and Adelaidian basin (AD); Neoproterozoic rocks in Laurentia outcrop in Caborca (C), Death Valley (DV), and Uinta Mountains (U). The final Baltica position within Rodinia was possibly not accomplished until 0.80 Ga (Vance et al., 1998), and we show two alternate Baltica positions (Park, 1992; Torsvik et al., 1996) as well as an area of 0.96 Ga metamorphism in northeastern Greenland (Strachan et al., 1995) indicating a northern arm of Grenvillian overprinting.

8 K.E. Karlstrorn et al. /Precambl,ian Research 11 1 (2001) 5-30 (Helper et al., 1996). Further, ophiolite belts cited by Moores (1991) are discontinuous and of different ages and therefore do not provide convincing correlations. Proposed connections between Australia and northern Canada are also questionable. There are striking lithostratigraphic similarities between the Neoproterozoic sequences of the Adelaide geosyncline of Australia and the Mackenzie Mountains- Windermere sections of Canada (Young, 1992; Rainbird et al., 1996), but these sequences can also be correlated southward along the Cordilleran margin (Link et al., 1993), and perhaps globally (Hoffman et al., 1998), and thus do not provide reliable piercing points for reconstructions. Similarly, % 780 Ma mafic dikes in the Mackenzie Mountains of northwest Canada, in Montana, and in Wyoming were postulated to be part of a plume-generated radiating dike swarm that extended from Australia to Laurentia (Park et al., 1995). However, the 827 Ma U-Pb date for the Australian Gairdner mafic dikes indicates that they were not all part of a single radiating dike swarm (Wingate et al., 1998) and therefore cannot be used as piercing points. Also, Proterozoic rocks of southern Australia record a prolonged history of tectonism from 1.8 to 1.0 Ga, more similar to that of the southwestern US (see below) than to the rather short-lived % 1.8 Ga orogenic event recorded in the Wopmay orogen (Hoffman and Bowring, 1984). 3. The AUSWUS reconstruction Several studies have suggested modifications of the SWEAT reconstruction, placing Australia farther south relative to North America (Fig. 1). For example, Ross et al. (1992) suggested that Ga detrital zircons in the Belt-Purcell Supergroup were derived from the Gawler Range Volcanics of South Australia, and they placed Australia halfway between the SWEAT and AUSWUS positions (Fig. 1). However, Blewett et al. (1998) suggested North Queensland as a possible source for this detritus, and the identification of 1.58 Ga zircons in Idaho (Doughty et al., 1998) also provides a possible local (North American) source. Borg et al. (1994) speculated that the Ross et al. (1992) reconstruction might explain Nd isotopic provinces in Antarctica if terranes had been translated southwards as small allochthonous strike-slip blocks. However, their data may be more simply explained by the AUSWUS model, which moves the entire continent southwards. Unfortunately, neither detrital provenance studies nor Nd model ages of provinces provide unique piercing points. Brookfield (1993) placed Australia adjacent to western US by matching inferred rift-transform segments of Proterozoic margins. Notably, the promontory of the Sr line in Laurentia was matched with the re-entrant in the Tasman line of central Australia. This reconstruction was also used by Marshak and Paulsen (1996) to help explain fault systems in the central US. A similar fit was proposed by Karlstrom et al. (1999) and Burrett and Berry (2000) based on matching of geologic provinces and lineaments. In all of these studies, major lineaments on both continents are viewed as part of the rift-transform fault system that was active during supercontinent breakup. For example, the NW-trending Mojave-Sonora lineament of Laurentia, which may initially have formed at w 1.1 Ga (Timmons et al., 2000), continues into Australia as the Koonenberry fault zone of the Tasman line, and the Great Falls tectonic zone is matched to the Diamantina Lineament (Fig. l(a); Burrett and Berry, 2000). This paper further evaluates a modified version of the Brookfield (1993) reconstruction for the Australia-Laurentia connection (Fig. 1) using geologic and paleomagnetic data. This reconstruction aligns the 1.8 Ga southern edge of the Archean-early Paleoproterozoic nucleus of western Laurentia, defined by the Cheyenne Belt, with its possible continuation in Australia as the southern edge of the North Australian craton (Fig. l(a); Burrett and Berry, 2000). This alignment is also supported by similarity of tectonic evolution of the juvenile belts that were added to this nucleus, in addition to the matching of lineaments and inferred shapes of the margins mentioned above. Our hypothesis is that parts of Australia were adjacent to the southwestern US for much of the Proterozoic (Fig. 1). To help evaluate the

9 K.E. Karlstrom et al. /Precambrian Research 111 (2001) 5-30 geologic and paleomagnetic 'fits', we rotate proto- Australia into North American co-ordinates using an Euler pole at 51.46"N, E (angle of rotation = "). In the North Atlantic region, Paleoproterozoic belts and Archean segments have long been used to match Baltica with Greenland and eastern Laurentia (see review in Gower et al., 1990; Gorbatshev and Bogdanova, 1993). Although later modified (Gower and Tucker, 1994; Park, 1995) to accommodate the intervening Rockall Bank and northwestern British Isles, these reconstructions remain generally accepted, in spite of a paucity of high quality paleomagnetic poles. The reconstruction used here (Fig. l(a); Park, 1995) aligns the Ga southeastern edge of Laurentia ( Ga KetilidianIMakkovikian and Ga Labradorian belts) with the northern part of the coeval belts in western Baltica ( Ga TIB 1 unit of the Transscandinavian Igneous Belt and the Ga Gothian units). This fit requires subsequent rifting between NW Europe and Greenland as well as an approximate 80" clockwise rotation of Baltica, generally regarded as pre-grenvillian (Gower et al., 1990; Park, 1992), but still not well constrained temporally. In the following sections, we evaluate our reconstruction for each of four Proterozoic time periods (Fig. I), then review available paleomagnetic data. 4. Proterozoic evolution of Laurentian margins 4.1. Paleoproterozoic assembly and major addition of new crust, Ga The nucleus of Laurentia consists of a mosaic of Archean cratons welded together by Ga orogenic belts of the Trans-Hudson 'type' (Hoffman, 1988). This pattern is similar to that of western and northern Australia (Myers et al., 1996) and Baltica (Gorbatshev and Bogdanova, 1993). These early Paleoproterozoic ( Ga) belts record collisions between and substantial reworking of the Archean cratons. In contrast, the post-1.8 Ga orogenic evolution in all three conti- nents marks a shift in tectonic style, though not precisely coeval, as the Ga orogenic belts formed from accretionary growth along one side of these newly assembled continental masses (Windley, 1992). South of its Archean-early Paleoproterozoic core, Laurentia is characterized by wide orogens that were juvenile at Ga (Yavapai province) and Ga (Mazatzal province, Karlstrom and Bowring, 1993; Labradorian orogen, Gower, 1996). These belts may have counterparts in the Arunta and Musgrave blocks of Australia (McCulloch, 1987; Ross et al., 1992) and the Transscandinavian Igneous Belt and Gothian terranes of Baltica (Gorbatshev and Bogdanova, 1993; Ahall and Larson, 2000), as indi- ' cated by similar Nd model ages and tectonic characteristics. On each continent, these broadly coeval belts include dominantly juvenile volcanogenic sequences that likely represent oceanic arcs and arcs built upon slightly older crust. Timing of deformation was generally within tens of millions of years of crust formation (based on Nd model ages) and these belts thus record a complex system of plate margin and island arc activity that led to episodic continental growth, and southward-progressing cratonic stabilization. When viewed from a broad perspective, this Ga tectonism was almost continuous at one place or another along the margin, appears to have been subduction-dominated during this interval, and seems analogous to Cordilleran-type plate margins. Details of these similarities are presented below, first for Laurentia-Baltica and then for Laurentia-Australia Northeastern Laurentia- Baltica The Archean cratons in the North Atlantic region were assembled during Ga events that produced juvenile belts as well as belts with appreciable reworked Archean crust (Gorbatshev and Bogdanova, 1993; Van Kranendonk et al., 1993; Park, 1995). These include the Svecofennian (Baltica), Laxfordian (Dl-D2, British Isles), Nagssugtoquidian (Greenland), and Torngat (Canada) orogens. A Laurentia-Baltica assembly by 1.81 Ga is supported by the fact that the subsequent growth resulted in a continuous

10 K.E. Karlstrom et al. /Precambrian Research 111 (2001) 5-30 array of new belts that are external and generally oblique to the preceding crustal architecture. These include the Ketilidian-Makkovikian- Penokean-Yavapai belts in Greenland and Laurentia (Fig. l(a); Van Schmus et al., 1993; Kerr et al., 1996), and the Ga TIB 1 unit of the Transscandinavian Igneous Belt in Baltica (Ahall and Larson, 2000). Coeval crustal growth is also known from the British Isles ( x Ga Malin segment; Park, 1995). The pre-1.75 Ga events were followed by recurring accretionary growth along the evolving southern margin. Accreted belts include the Ga Gothian belts in Baltica, the Rockall Bank (x 1.75 Ga), and the Ga Labradorian orogen in eastern Canada (Gower 1996; Aha11 and Gower, 1997). These are similar in age and character to the Ga Yavapai and Ga Mazatzal belts of the southwestern US (Karlstrom and Bowring, 1993). Together, they make an apparently contiguous belt of juvenile crust throughout a more than 9000 km long south-facing Laurentia-Baltica margin (Fig. l(a)). In the northeast, the accreted belts were partially reworked and telescoped during the Grenvillian/Sveconorwegian orogeny (see below), but are preserved as km wide segments (Fig. l(a)). In southwestern Laurentia, the accreted belts widen to a 1000 km wide segment of juvenile Ga crust where deformation stepped southwards from 1.75 to 1.65 Ma during progressive cratonization (Karlstrom and Humphreys, 1998) Southwestern Laurentia-Australia As in the case of Laurentia-Baltica, the Archean blocks in Australia were amalgamated during Ga orogenies to form three, probably separate, cratons: North, West, and South Australian cratons (Fig. 1; Myers et al., 1996). As mentioned above, the southern margin of the North Australian craton is interpreted to be the continuation of the Cheyenne belt of Laurentia (Fig. l), and the juvenile terranes of the Yavapai-Mazatzal Province in Laurentia are similar to the Arunta-Musgrave blocks of central Australia. Both western Laurentia (Mojave province) and eastern Australia (including the eastern parts of the South and North Australian cratons) are underlain by crust with a distinctive Nd 'Mojave' signature involving a mixture of Ga juvenile crust and older crustal material. Nd model ages for rocks of the Mojave province are Ga (Ramo and Calzia, 1998), significantly older than their Ga crystallization ages (Bennett and DePaolo, 1987). Similarly, Nd model ages for central and northeastern Australia are Ga (see compilation in Ross et al., 1992). In both areas, the model ages are interpreted to record Archean detrital zircons, and/or Archean and early Paleoproterozoic crust in the subsurface, incorporated into developing Ga arcs (Ross et al., 1992; Wooden et al., 1994; Zhao and McCulloch, 1995; Hawkins et al., 1996; Ramo and Calzia, 1998). Pb isotopic data from ores and whole rocks are also similar between southwestern Laurentia and Australia. Economically important volcanogenic massive sulfide deposits occur in the Broken Hill and Mount Isa areas of Australia and similar ore deposits occur at Jerome and Bagdad in central Arizona, and the Carlin district of Nevada, respectively (Fig. l(a)). Galena deposits and whole rock analyses show similar Pb isotope signatures that, like the Nd model ages, indicate juvenile rocks to the East (Jerome; Wooden and DeWitt, 1991) and an increasing component of older crust to the West (Burrett and Berry, 2000). Thus, major metallogenic districts in Australia have possible counterparts in the western US. The South Australian craton, which is Archean in part, lies South of the juvenile Paleoproterozoic terranes in Australia and appears to be an exception to the pattern of southward-younging juvenile Proterozoic belts seen in LaurentiaIBaltica. However, it was not stabilized adjacent to the other Australian blocks until Ga (Myers et al., 1996) or Ga (Teasdale, 1997) and hence its position during Paleoproterozoic accretion of arc terranes was likely different than its present position. A recent model (Betts and Giles, 2000; Giles and Betts, 2000) suggests that the South Australian craton at Ga (minus the Archean Gawler block, which was outboard) was rotated - 55" clockwise and attached to the Strangeways orogen of the Arunta area to form a

11 K.E. Karlstrom et al. /Precambrian Research 11 1 (2001) 5-30 continuous south-facing, north-dipping subduction margin along the southern margin of the North Australian craton, thus forming a continuation of the Yavapai-Mazatzal system of Laurentia. Timing of Paleoproterozoic deformation is similar on both continents. Accretion of juvenile arc terranes began Ga along the Cheyenne Belt (Chamberlain, 1998), with north-vergent thrusting along this suture zone representing a continuation of south-dipping subduction (Karlstrom and Houston, 1984). Contractional Ga deformation of the Strangeways orogeny in central Australian terranes was also south- and west-vergent and is interpreted to have involved oblique convergence and addition of magmatic arcs (Sivell and McCullough, 1991; Collins and Shaw, 1995; Fig. 1). In the South Australian craton, the Kimban ( Ga) orogen may represent accretion of the Gawler block at Ga (Betts and Giles, 2000). Continued accretion in central Australia led to the Argilke event ( Ga), and compression continued during the intracratonic Chewings orogeny ( Ga). By 1.70 Ga, Giles and Betts (2000) postulate opening of N-S rifts between Australia and western North America. These rift basins, with associated clastic sedimentation and rhyolitic volcanism, developed from 1.69 to 1.60 Ga, recording interaction of extensional and contractional intracratonic tectonism (Myers et al., 1996). In the South Australian craton, these include clastic and bimodal volcanic packages of the 1.69 Ga Willyama Supergroup and 1.65 Ga Tarcoola Formation. These began to close along a subduction system that dipped westward beneath the Georgetown-Mount Isa-Broken Hill-Willyama-Gawler areas, which were part of a coherent terrane, based on similar ages and histories. Closure resulted in the Ga Isan-Olarian-Kararan orogenies. Similarly, deposition of Ga cover sequences took place on top of newly stabilized Ga crust of the Yavapai province in southwestern Laurentia. This was synchronous with and followed by renewed or continued deformation associated with addition of the Ga Mazatzal province, which included volumi- nous 1.65 Ga arc granitoids as well as thick supracrustal successions (Karlstrom and Bowring, 1993). Apparently interspersed with these periods of crustal growth, extrusion of high-silica rhyolites followed by deposition of mature quartz arenite cover successions, took place in numerous places. These rhyolite-quartzite successions appear to mark a distinctive, yet diachronous, part of the progressive addition and stabilization of Proterozoic crust to the growing continental margin. In the central US, such sequences were deposited between and 1.63 Ga (Holm et al., 1998), following a major 1.76 Ga magmatic and tectonic event. In Arizona and New Mexico, the rhyolite-quartz arenite cover sequences (Mazatzal and Ortega quartzites) range in age from 1.70 to ' 1.65 Ga, and overlap in time with progressive shortening at middle crustal levels (Karlstrom et al., 1987). Hence, they are interpreted to be mature continental margin successions (possibly riftrelated) that were deposited on newly stabilized crust, then imbricated with that crust during continued shortening (Mazatzal orogeny). In addition to similar timing, style of deformation, plutonism, and metamorphism are very similar in Proterozoic rocks in Laurentia and Australia. Both regions record heterogeneous mid-crustal shortening from 1.8 to 1.6 Ga, compatible with progressive thickening and stabilization of juvenile arc terranes into new continental lithosphere (Dirks and Wilson, 1990; Gascombe, 1991; Collins and Shaw, 1995; Karlstrom and Williams, 1998). Distinctive styles of metarnorphism in the southwestern US involved high-t, low-p metamorphism and looping P-T paths involving isobaric mid-crustal cooling (Williams and Karlstrom, 1996). These paths seem to be compatible with low-p, high-t anticlockwise P-T paths described from central Australia (Vernon et al., 1990; Collins and Vernon, 1991; Gascombe, 1991; Clarke et al., 1995) and suggest similar tectonic styles involving pluton-enhanced metamorphism and syndeformational plutonism in both regions (Vernon et al., 1990; Williams 1991; Karlstrom and Williams, 1995; Zhao and Bennett, 1995; Collins and Sawyer, 1996; Williams and Karlstrom, 1996; Vry and Cartwright, 1998). All these cratons contain voluminous granitoid mag-

12 K.E. $arlstrom et al. / Precambria m Research 11 1 (2001) 5-30 matism, including Ga calc-alkaline plutons, Ga syntectonic granites (Olariafi and Yavapai-Ivanpah orogenies) and Ga late-orogenic granitoids (Wooden and Miller, 1990; Shaw et al., 1999). Metamorphism and tectohism in both areas was followed by isobaric cooling and long-term residence in the middle crust prior to exhumation during later events (Vernon et al., 1990; Collins et al., 1991; Hodges et al., 1994; Collins and Shaw, 1995). Cooling histories of the Gawler and Mojave blocks are remarkably similar; 40Ar/39Ar data show differential but slow cooling of blocks from 1.7 to 1.5 Ga, local rapid cooling at 1.43 Ga, reactivation of shear zones at 1.1 Ga, and cooling or reheating at Ga (Foster and Ehlers, 1998). We argue that this history of pluton-enhanced tectonism and subsequent unroofing of mid-crustal rocks was the signature of a specific sequence of events that followed the addition and stabilization of juvenile crust 'to a contiguous Australia-Laurentia margin Continued plate margin tectonism and cratonization Ga A hiatus of magmatism and tectonic activity from 1.6 to 1.5 Ga in the southwestern US marks a period of tectonic stability that apparently reflects cratonization of new lithosphere, perhaps due to growth of lithospheric mantle (Karlstrom and Humphreys, 1998). A similar hiatus is also noted from other areas of Laurentia (Gower, 1996; Rivers, 1997) suggesting that this period marked an important change in style of tectonism. However, there is evidence of recurring tectonic activity in some places along the margin by the end of this time interval. Areas in central and northeastern Laurentia have Ga Nd model ages (Bowring et al., 1988; Dickin and Higgins, 1992; Hanmer et al., 2000), 1.51 Ga metamorphism (Scott et al., 1993) and Ga tectonism (Pinwarian orogeny), the latter interpreted to have involved subduction beneath Laurentia (Gower, 1996; Rivers, 1997; Wasteneys et al., 1997; Cotrigan et al., 2000). Similar, though not identical, events are recorded in Baltica. For example, the Gothian orogen, traditionally viewed as lasting from 1.75 to 1.55 Ga, appears to involve separate orogenic events including Ga calc-alkaline magmatism and deformation due to arc accretion followed by continued growth-related magmatism at least until 1.55 Ga (Aha11 and Gower, 1997; Brewer et al., 1998; ~h511 et al., 2000). Coeval to this stepwise growth was episodic, bimodal rapakivi magmatism in distal inboard settings in the Ga interval (Laitakari et al., 1996; Riimo et al., 1996; Amelin et al., 1997; Persson, 1999). At variance to previous models, recent studies have suggested that the recurring Gothian subduction along oceanward-stepping zones was the first-order control on episodic mantle melting and the consequent emplacement of Ga rapakivi suites well within Baltica (Ah211 et al., 2000). In Australia, Ga A-type tectonism, including transpressional deformation and formation of large sedimentary basins, while perhaps different in style than the Ga accretionary events, suggests a near continuum of tectonism behind and along an active margin (see below) Intracratonic A-type magmatism and related tectonism, Ga A prominent feature of the Ga time slice (Fig. l(b)) is the bimodal A-type magmatism involving granites and associated anorthosites generally dispersed through the Paleoproterozoic crust of Laurentia-Baltica (Windley, 1993; Karlstrom and Humphreys, 1998). The origin of these rocks remains controversial, but they form distinctive magmatic suites, some of which can be correlated across the Atlantic, for instance at 1.46 Ga (Gower and Tucker, 1994; Ahall and Connelly, 1998). Although commonly termed 'anorogenic', there is increasing evidence for an orogenic linkage (McLelland et al., 1996; Corrigan and Hanmer, 1997), involving both continental arc magmatism and collision of a Ga juvenile crustal block along the south-facing margin of Laurentia (Fig. l(b)). In Laurentia, these rocks have been overprinted by the Grenville orogeny and concealed by Paleozoic mid-continent cover, but juvenile volcanic and intrusive rocks are

13 K.E. Karlstrom et al. /Precaml ~rian Research 111 (2001) 5-30 present in eastern Canada (Rivers, 1997), eastern Missouri (Van Schmus et al., 1993), and western Texas (Mosher, 1998). Evidence for contractional deformation and regional metamorphism between 1.45 and 1.35 Ga is widespread in this belt and suggests a transpressive plate margin (Nyman et al., 1994). The A-type compositions suggest that the granitoids formed from lower crustal melting due to basalt underplating (Frost and Frost, 1997), rather than subduction. The extensive record of A-type magmatism, with sporadic activity over a huge area and a wide time interval ( Ga) suggests that there may have been various tectonic settings favorable for this type of magmatism. However, field relationships indicate that many of these intracratonic events echoed subduction-related and transcurrent tectonism taking place along the southern plate margin of Laurentia (Rivers, 1997). In Baltica, the intracratonic Ga magmatism was episodic, generally bimodal, and especially abundant in the newly stabilized Gothian orogen (Ahall and Connelly, 1998). One pre-1.26 Ga episode was associated with extension leading to widespread deposition of intracontinental redbeds (Jotnian sandstones; Laitakari et al., 1996), but its poor temporal constraints ( Ga) do not allow direct correlation across the Atlantic. The onset of broadly simultaneous and bimodal intracratonic magmatism at some distance from the margins of both Baltica and northeastern Laurentia suggests that continental back-arc processes controlled the morphology of the margin and provided the thermal energy required to produce magmas. In particular, the large-scale x 1.46 Ga episode of abortive rifting provides a strong linkage across the proto-atlantic (Gower and Tucker, 1994; Ahall and Connelly, 1998), further supported by new age constraints on the Laurentian side (Corrigan et al., 2000). In Australia, A-type granites of the Gawler craton ( Ga St. Peter and Hiltaba Suite) are somewhat older but similar in composition and character to those in southwestern Laurentia ( Ga). In the southwestern US, the belt of plutons is part of a wide zone of regional metamorphism (and partitioned deformation) that seems to have highest temperatures in New Mexico ( C; Shaw et al., 1999). There is mounting evidence for similarly extensive metamorphism, contractional deformation, and granitic plutonism in Australia at Ga (Isan orogeny, Connors and Page, 1995; Fowler Domain, Teasdale, 1997), southwestern Laurentia (Karlstrom and Humphreys, 1998) and in the Grenville Province at Ga (Ketchum et al., 1994). In East Antarctica, there is crust with Nd model ages reaching 1.5 Ga (Borg et al., 1994) that may also be part of this belt. Major sedimentary sequences were deposited in both Australia and Laurentia at Ga. The Belt-Purcell Supergroup of western Laurentia accumulated tens of kilometers of sediment between 1.47 and 1.35 Ga (Aleinikoff et al., 1997). Detritus was derived from North American sources but also from the West (Ross et al., 1992). In Australia, the broadly coeval Roper Group, Birrinduda Basin, and Bangemall Basin form a zone parallel to the inferred transpressive orogen (Myers et al., 1996). Betts and Giles (2000) and Giles and Betts (2000) suggested that the 1.43 Ga Roper basin developed when the Gawler block separated at Ga. Combined with the Belt- Purcell basin, these Mesoproterozoic basins imply a continued interaction of contraction and extension behind a large-scale Andean mountain belt along southern Laurentia and Australia by 1.47 Ga (i.e., coeval with the Pinwarian orogeny farther East in Laurentia). The inferred shortening directions for this time period in Laurentia (WNW - Nyman et al., 1994) and Australia (SE - Myers et al., 1996) resulted in dextral transpressive deformation along pre-existing structures within the (now intracratonic) Paleoproterozoic orogenic belts Grenville-age tectonism, Ga Tectonism over a protracted period from x 1.3 to 0.9 Ga took place in Laurentia, Baltica, Australia, and on many other continents, and culminated in continent-continent collisions that resulted in the assembly of the supercontinent Rodinia (Dalziel, 1997). In Australia-Laurentia-

14 K.E. Karlstrom et al. /Precamb, rian Research 11 1 (2001) 5-30 Baltica, this ended a 800 Ma history of accretionary orogeny along a south-facing margin. The culminating orogenic event has been referred to as the Grenvillian orogeny (Rivers, 1997) and spans the interval Ga. It was preceded by a series of accretionary/collisional events from to 1.2 Ga that represent a continuation of the Paleoproterozic-Mesoproterozic accretionary orogens (see also Davidson, 1995) Grenville of Laurentia The period from 1.3 to 1.2 Ga comprised outboard-and back-arc magmatism of the Elzevirian orogeny (Moore and Thompson, 1980), during which composite terranes were amalgamated against the southeastern margin of Laurentia with consequent collision, deformation, and high-grade metamorphism. Subsequently, overthickened crust underwent delamination, extensional collapse, and the emplacement of large anorthosite plutons from to 1.12 Ga (McLelland et al., 1996; Corrigan and Hahmer 1997). Although some areas may have remained under compression, there is little evidence of widespread deformation until Ga when the major collisions appear to have taken place with one or more large continental masses to the southeast (Ammonia; Hoffman 1991). During this stage, referred to as the Ottawan pulse ( Ga) of the Grenvillian orogeny, amphibolite to granulite facies metamorphism took place in almost all areas of the orogen. Metamorphism in most of the province peaked at Ga, but high-grade conditions continued locally until 0.98 Ga (Scharer et al., 1986), which was also the time at which movement ceased along the Grenville Front (Haggart et al., 1993). The crustal thickening was followed by abundant post-kinematic granite plutonism between 0.99 and 0.96 Ga. The collision resulted in imbrication of the province, with large thrust slices driven to the northwest. In more ductile regimes, such as the Adirondack Mountains of New York, extensive folding helped accommodate contraction. In the Llano uplift of Texas, Ga (Elzevirian-age) arc accretion and orogeny imbricated diverse tectonic elements, including a possible fragment of 1.37 Ga North American basement and three distinct juvenile arc fragments ranging in age from 1.33 to 1.23 Ga (Mosher, 1998). These packages were stitched by Ga late-tectonic granitoids. In West Texas, the Ga Hazel orogeny records continued transpressional events (coeval with Ottawan orogenesis; Soegaard and Callahan, 1994; Bickford et al., 2000). Tectonism in Texas was generally imprinted on Ga crust, but juvenile ( Ga) crust is also present (Roback, 1996) Sveconorwegian of Baltica Most models for the Ga Sveconorwegian (Grenvillian) orogeny in Baltica have invoked a pre-sveconorwegian break-up between Baltica and Laurentia followed by - 80" clockwise rotation of Baltica and collision, likely with a third continent involved (Hoffman, 1991; Park, 1992; Gower, 1996; Aha11 and Connelly, 1998). Migrating thrusting and metamorphism have been matched with those of the Grenville province (Berthelson, 1980; Gower et al., 1990; Romer and Smeds, 1996). However, emerging data reveal a partly asynchronous development where the convergence leading to 1.10 Ga high-grade metamorphism in western Baltica (Kullerud and Machado, 1991) was followed by subduction beneath Baltica as late as Ga (Bingen and van Breeman, 1998). The associated consumption of oceanic crust appears to record a culminating plate assembly, with progressively younger thrusts and metamorphism (to 0.98 Ga; Johansson et al., 1998; Moller, 1998) towards the (present-day) east. This contraction was followed by both mafic and felsic magmatism between 0.96 and 0.92 Ga (ha11 and SchGberg, 1999); the later pulses including the voluminous Rogaland anorthosite complex in the West ( Ma, Scharer et al., 1996) and the Bohus granite in the East (920 Ma, Eliasson and Schoberg, 199 1). Conclusive evidence is still lacking, however, whether rifting and rotation between the two continents commenced at z 1.25 Ga or later during the Sveconorwegian event. It also remains to be established whether the juxtaposition of Baltica and Laurentia occurred with or without an earlier oblique collision between Baltica and South American cratons (likely Ammonia and possibly Rio de la Plata; Bingen et al., 1998), as discussed by Park (1992).

15 ICE. Karlstrom et al. /Precambrian Research 11 1 (2001) Grenville-age tectonism of Australia In the Albany-Fraser and Musgrave belts, there are several events that correspond in style and age to those of the Grenville Province of Laurentia (Clarke et al., 1995; Myers et al., 1996) and tectonism of this age also affected the Arunta block and South Australia craton. These belts are/ almost exclusively underlain by older ( > 1.6 Ga) crust (Ross et al., 1992), and no wholly juvenile blocks appear to exist. Both the Albany-Fraser and Musgrave belts contain rocks that include Ga plutonism and regional high-grade metamorphism ( Ga Albany-Fraser and Ga Musgravian orogenies; Myers et al., 1996). These events are thought to reflect accretion of the West Australia ( Ga) and the Gawler (- 1.2 Ga) cratons to the North Australia craton (Myers et al., 1996; Betts and Giles, 2000; Clark et al., 2000). Accretion was followed by Ga 'enriched' (intraplate) granitic magmatism that extends northward into the Arunta Block, where alkaline intrusive rocks are also present. In terms of composition, timing, and tectonic setting, this magmatism is similar to the Ga granitic facies of northeastern Laurentia. Post-1.15 Ga events are present in the Musgrave belt where large-scale mafic magmatism (e.g., Giles complex) was accompanied by bimodal volcanism and dike swarms at Ga, and tectonism, together with granulite facies metamorphism continued to Ga (Clarke et al., 1995). As a possible extension of the Albany- Fraser belt into the northern Gawler craton, a wide network of shear zones form a 700 km long mobile belt that was active Ga and resulted in the collage of Archean and Proterozoic blocks that now make up the South Australian craton. Mylonitic shear zones dated at 1.06 Ga are also present in the Musgrave block, and presumably are related to the same event (Clarke et al., 1995). These high-grade metamorphic rocks and shear zones appear to record the final Proterozoic assembly of Australia. A possibly related granulite facies event at Ma is also recognized along the southwestern coast of Australia (Myers et al., 1996), but its relationship to the Albany-Fraser belt remains uncertain. All of these events are similar in age and style to Ga accretionary and Ga collisional events in Laurentia. This similarity has been noted by Clark et al. (2000), who divide the Albany-Fraser orogeny into two stages. According to these authors, the earliest of these ( Ma) corresponds to the Elzevirian orogeny and the younger, intracratonic and collisional, event ( Ma) corresponds to the culminating Grenvillian orogeny. In the Musgrave belt, magmatism, high-grade metamorphism, and deformation continued until 1060 Ma and are thought to represent a late phase of this global-scale collisional system (Betts and Giles, 2000; Clark et al., 2000; Giles and Betts, 2000) that terminated the long-lived Australian-Lau-, rentian-baltic accretionary margins. Diachronism, which is to be expected, exists along the belt, but is, in fact, remarkably limited in scale. In our proposed reconstruction (Fig. l(c)), there is a large gap between the truncated Grenville belts in southwestern Laurentia and central Australia. However, the Oaxaca terrane of Mexico (Fig. l(c); Oaxacquia of Ortega-Gutierrez et al. (1995)) contains rocks with a similar tectonic history: basement as old as 1.3 Ga, an igneous sequence ( Ga) with charnockites, syenites, anorthosites, and granites, and high-grade metamorphism that includes 1.1 Ga migmatization and 0.98 Ga granulite overprinting (Solari et al., 1999). Speculatively, the Oaxaca terrane in the Proterozoic may have occupied an intervening position along a more continuous Grenvillian orogen. Oaxaca fossil affinities suggest that Oaxaca was attached to South American cratons, not Laurentia, during the early Paleozoic (Ortega- Gutierrez et al., 1995). The Oaxaca terrane may thus have originated between Laurentia and Australia (Fig. l(c)) and traveled in proximity to South American cratons after Ma breakup of Rodinia (Fig. l(d)), before it was finally reassembled to Laurentia in the late Paleozoic. During the Grenvillian orogeny, northwest-directed contraction at the southern margin of Laurentia was accompanied by northeast intracratonic extension and voluminous mafic magmatism (e.g., the 1.27 MacKenzie and Sud-

16 K.E. Karlstrom et al. /Precambrian Research 111 (2001) 5-30 bury mafic dike swarms, the 1.1 Ga Midcontinent Rift, and 1.1 Ga Central Basin platform and diabase sheets in Arizona), and deposition and normal faulting of the Ga Unkar and Apache Groups in Arizona. A set of NW-striking extensional faults formed over much of Laurentia. These had important influence in creating and/or reactivating lineaments such as the Mojave- Sonora megashear, the Texas-Mogollon-Walker lineament, the Uncompahgre lineament, the Oklahoma aulacogen, and the Lewis and Clark-Oklahoma-Alabama lineament (Thomas, 199 1; Marshak and Paulsen, 1996; Marshak et al., 2000; Timrnons et al., 2000). In Australia, the corresponding intracratonic activity was similar in style (Fig. l(c)). NW-trending extensional faults were active along the Torrens Hinge between 1.3 and 1.0 Ga (Myers et al., 1996) and the mafic Stuart and Kulgera dikes of central Australia (Camacho et al., 1991) are similar in age and orientation (in the AUSWUS configuration) to the Arizona 1.1 Ga diabases (Fig. l(c); Hammond, 1986; Howard, 1991) Break-up of Rodinia, Ma Fragmentation of Rodinia was apparently diachronous and involved rifting events at Ma and Ma. Initial separation between western Laurentia and Australia prior to 755 Ma is supported by the deposition of rift sequences (Fig. l(d)) and coeval mafic magmatism between 800 and 750 Ma on Australia (Centralian Superbasin, Walter and Veevers, 1997) and Laurentia (Windermere Supergroup and equivalents, Ross, 1991; Chuar Group, Karlstrom et al., 2000). This timing is also consistent with paleomagnetic data (Powell et al., 1993, 1994), including data from the 755 Ma Mundine Well dike swarm of Australia (Wingate and Giddings, 2000). Continued or renewed rifting took place from 600 to 550 Ma, based on sedimentary data and tectonic subsidence modeling (Bond and Kominz, 1984) that suggest that both Laurentia and Australia underwent regional subsidence and marine transgression presumably due to thermal subsidence of rift margins during this interval (Lindsay et al., 1987; Levy and Christie-Blick, 1991). To reconcile these data, a multistage rifting history is suggested, perhaps involving a third plate (or microplates), for example, South China (Li et al., 1995; Powell, 1998), situated adjacent to western Canada, or between northern Australia and Laurentia. Recent pre-600 Ma reconstructions place western Baltica facing eastern Greenland and Amazonia as the conjugate margin (Torsvik et al., 1996; Dalziel, 1997). Initial, simultaneous, rifting has been documented at 615 Ma in both eastern Laurentia (Long Range dike swarm, Kamo and Gower, 1994) and western Baltica (Egersund dike swarm, Bingen et al., 1998). Similarly, rifting between Baltica and eastern Greenland is recorded by extensive injection of 608 Ma MORB dikes, including sheeted dikes now present in the Caledonian nappes in Baltica (Bingen and Demaiffe, 1999). In a model by Bingen et al. (1998), large volume mafic magmatism at 608 Ma reflects the breakup between Laurentia and Baltica, whereas the subsequent Ma magmatic events along the assumed border zone with Amazonia- Rio de la Plata suggest a somewhat later separation between Baltica and these South American continents. 5. Paleomagnetic data bearing on proterozoic continental reconstructions APW paths consisting of high quality poles are required to test competing plate reconstructions. However, for all continents there is a paucity of Proterozoic paleomagnetic data for which precise ages and primary magnetizations can be demonstrated (Meert, 1999), such that an error factor of at least 15" is warranted in all reconstructions (Weil et al., 1998). A simple, non-quantitative, way to consider this is to assume that the APW path for each craton is at least 15O wide. Also, the lack of longitudinal control from paleomagnetic data adds uncertainty. Considerable Neoproterozoic paleomagnetic data from Laurentia indicate that its western margin was approximately northfacing for most of the Neoproterozoic. The difference between the SWEAT and AUSWUS configurations prior to Ma involves a large shift in paleolongitude, with less change in pale-

17 K.E. Karlstrom et al. /Precambrian Research 11 1 (2001) 5-30 AUSWUS Reconstruction SWEAT Reconstruction Fig. 2. Orthogonal global projections centered on 30 N, 180 E showing comparison of the paleomagnetic poles from Australia with those from North America for the AUSWUS (a) and SWEAT (b) reconstructions. For each projection, Australia and the Australian paleomagnetic poles have been rotated into present-day North American co-ordinates using the Euler poles listed in Table 1. Paleomagnetic poles for North America and their 95% confidence limits are denoted by open circles and thin lines, whereas Australian poles and 95% confidence limits are denoted by black squares and heavy lines. The thick shaded line shows the overall track of the x Ga APW path for North America. Pole locations for the Ma part of the North American APW path are tabulated in Harlan et al. (1994), sources for the w 1450 Ma poles are from Harlan and Geissman (1998). The 780 Ma North American poles are from Park et al. (1995) as slightly modified by Harlan et al. (1997). The 723 North American pole is from the Global Paleomagnetic Online Database; the Australian MDS pole is from Wingate and Giddings (2000). Sources and rotated co-ordinates for the Australian poles are given in Table 1. olatitude, and thus high precision data with adequate age control are required to paleomagnetically resolve the configurations (Fig. 1). High quality Paleoproterozoic data are especially sparse for North America, as many of the magnetizations reported are not well dated nor reliably referenced to a paleohorizontal. Nevertheless, using the reconstruction of Ross et al. (1992), where Australia is in an intermediate position relative to SWEAT and AUSWUS, Idnurm and Giddings (1995) suggested a broad agreement between the Australian and North American APW paths for the entire interval Ga. Their conclusion is compatible with the similarities in tectonic histories over this time interval, but the paleomagnetic data themselves must be viewed with caution, for reasons further discussed below. Mesoproterozoic poles from Australia and North America arguably provide the best paleomagnetic test for the reconstructions. Although some uncertainties exist, the APW path for North America between and 1.40 Ga and between and 1.08 Ga is fairly well defined (Harlan et al., 1994; Harlan and Geissman, 1998). Available Australian poles are reasonably consistent with the North American APW path in the AUSWUS reconstruction (Fig. 2(a)). The only notable exception is the z 1.36 Ga Australian pole from the Morrowa lavas (ML), but the data from these rocks have yet to be published and their age is poorly known (Table 1). Thus, comparison between Mesoproterozoic poles for Laurentia and Australia provides reasonable support for the AUSWUS configuration. In contrast, the overall fit of the Ga Australian poles in the SWEAT configuration is

18 Table 1 Australian Paleomagnetic poles used in AUSWUS and SWEAT rewnstructions Pole symbol Rock unit Age (Ma) Present day A,, Reference AUSWUS" SWEAT^ Reliability Q Plat. Plong. Rlat. Rlong. Rlat. Rlong IM GRV G A IB ML SDS KDS IAR MDS YB Mt. Isa dikes (metamorphosed) Gawler Range Volcanics GA dikes Mount Isa dikes Morawa Lavas Stuart Dike Swarm Kulgera Dike Swarm Mt. Isa Dikes Mundine Dike Swarm Yilgarn Dikes Tanaka and Idnurm (1994) Idnurm and Giddings (1988) Idnurm and Giddings (1988) Idnurm and Giddings (1988) Idnurm and Giddings (1988) Idnunn and Giddings (1988), Zhao and McCulloch (1995) Camacho et al. (1991) Tanaka and Idnurm (1994) Wingate and Giddings (2000) Giddings (1976). x x x o x o x 5 x x x o x o x 5 X X X 3? 0 o x x x x o 4 Notes: Plat. and Plong. are the latitude and longitude of the paleomagnetic pole in present-dayco-ordinates; A,, is the semi-angle of the 95% cone of confidence about the pole; references gives the reference for the pole and isotopic age used in this study; Rlat. and Rlong. are the latitude and longitude of the palbomagnetic pole after rotation about the specified Euler pole to bring the proto-australian continent into North American co-ordinates; Reliability is the reliability criteria of Van der Woo (1990) and include (1) well detebined rock age and a presumption that the magnetization is the same age; (2) sufficient number of samples (N124, precision parameter k (or K)> 10 and cone of wnfidence a,, (or Ag,) $ lo"; (3) adequate demagnetization' that demonstrably includes vector subtraction (or equivalent method); (4) field tests that constrain the age of magnetization; (5) structural control, and tectonic coherence with craton or block involved, (6), presence of reversals; (7) no resemblance to paleopoles of younger age (by more than a period); x indicates that it meets the stated criteria; o means that it fails the criteria; a blank indicates that it is not determinable from the published data; Q is the numerical sum of the accepted reliability criteria. AUSWUS configuration (this study), Euler pole of rotation is: latitude = 51.46"N, longitude = E, angle = ". SWEAT configuration of Dalziel (1997) (keeping North America fixed in present day co-ordinates), Euler pole of rotation is: latitude = 28.90, longitude = ", angl

19 K.E. Karlstrom et al. /Precambrian Research 11 1 (2001) 5-30 not compelling. Only one Australian pole (IAR, Fig. 2(b)) is consistent with the North American APW path. Dalziel's (1997) SWEAT reconstruction for 1.1 Ga was based on three paleomagnetic poles, one from Laurentia, one from India, and one from Coats Land, Antarctica. Notably, recent data suggest that the Coats Land pole does not support the tie between this part of Antarctica and Laurentia and instead suggests that this part of Antarctica was adjacent to the Kalahari craton (Gose et al., 1997). Furthermore, the Indian pole cannot be used as a proxy for the position of Australia until the timing and geometry of assembly of these two cratons is better known (Fitzsimmons, 2000). The Ma poles from Laurentia and Australia do not discriminate between the two reconstructions. The SWEAT model shows better consistency of the Australian Ma Yilgarn B pole with available Neoproterozoic poles from North America (780 and 730 Ma; Fig. 2(b), as noted by Powell et al. (1993)). However, the Yilgarn B pole has an unacceptably large a,, (28") and a Rb-Sr date of questionable reliability. A new paleomagnetic pole for the 755 Ma Mundine Well dike swarm of western Australia (Wingate and Giddings, 2000), which includes positive paleomagnetic contact tests, does not coincide with the Laurentian Neoproterozoic poles in either the SWEAT or AUSWUS reconstructions (Fig. 2). This result suggests that if western Laurentia and Australia were joined to form a Neoproterozoic supercontinent, then the two continents were in relative motion and widely separated by 755 Ma. The new paleomagnetic pole from the 'cap dolomite' of the Walsh Tillite (Li, 2000) is statistically indistinguishable from the Yilgarn B pole (Table I), but there is no isotopic age determination for this unit. If the unit is Ma, as appears to be the case for many other tillites associated with the Sturtuan glaciation (Walter et al., 2000), then neither this pole, nor the Yilgarn B pole, should be compared with North American data to validate either the SWEAT or AUSWUS reconstructions, given the evidence for pre-755 Ma rifting. 6. Conclusions and implications Our lirst conclusion is that the southern margin of Laurentia was a long-lived ( Ga) convergent margin. This interpretation links events along this margin and leads to our attempt to find the western and eastern continuation of a series of orogenic belts. Our second conclusion is that these Ga orogens extended to Australia and Baltica, which both have strikingly similar tectonic histories until the post-grenvillian break up (Australia Ga; Baltica Ga). Overall, our hypothesis envisions an episodic southward accretional growth of Cordilleran-type orogens related to oceanward-migrating subduction in all three continents. This proposed margin was similar in scale to the modern Cordilleran system of North and South America. Widespread bimodal A-type magmatism and related deformation in all three cratons are interpreted to be diachronous inboard expressions of the convergent margin tectonism. These events probably involved basaltic underplating and crustal differentiation, which resulted in further stabilization of new continental lithosphere. In agreement with Rodinia models, a culminating set of continent-continent collisions at Ga shut off tectonism along the > km length of the Australia-Laurentia-Baltica margin. These Himalayan-style collisions were superimposed on a plate margin that had undergone active margin tectonism for about 800 million years, which may help explain the great width of the Grenvillianl Svecononvegian orogen in Laurentia ( > 600 km) and Baltica ( > 550 km). If correct, this hypothesis has important implications for the positions of other cratons within Rodinia. The more southerly position for Australia proposed by the AUSWUS reconstruction potentially leaves a long section of the Cordilleran miogeocline in Canada without a known conjugate margin. However, Li et al. (1995, 1999) proposed that South China contains very similar Neoproterozoic sedimentary successions and tectonic history and may have adjoined northwestern Laurentia before Ma, compatible with the paleomagnetic data of Evans et al. (2000). Sears and Price (1978, 2000) suggested Siberia may have

20 K.E. Karlstrom et al. /Precaw ~brian Research 11 1 (2001) 5-30 bordered western Laurentia until the Cambrian (but cf. Condie and Rosen, 1994, who place it North of Laurentia). North China may also have been in close proximity (Piper and Rui, 1997). Our model also leaves much of eastern Antarctica unpaired. However, Neoproterozoic rift-margin sediments are only well documented for part of this margin, and Pan-African-age tectonism has affected much of this area (Fitzsimmons, 2000) such that it may not be a continuous late Precambrian rift margin. These uncertainties mirror those of proposed correlations between all other pairs of cratons within Rodinia (Meert, 1999) and makes it especially important to continue to test the proposed Australia-Laurentia-Baltica connections as a potential lynchpin for Rodinia reconstructions. From a paleomagnetic perspective, robust reconstructions require more than comparison between single paleomagnetic poles from individual continents but rather comparison between well-defined APW segments, of sufficient time duration, for individual cratons. The concept of a supercontinent cycle remains central to understanding the long-term evolution of tectonism on Earth. For example, acceptance of the supercontinent of Pangea was an essential component of the validation of the plate tectonic paradigm. Accurate reconstruction of the preceding Precambrian supercontinents, such as Rodinia, may likewise be important in developing paradigms for the longer term evolution of continents and possible secular variation in Earth systems. Acknowledgements We thank numerous colleagues for discussions that helped improve this paper; in particular, we thank Toby Rivers and Randy Van Schmus for their reviews. References hill, K.-I., Connelly, J., Intermittent Ga magmatism in western Baltica; age constraints and correlations within a postulated supercontinent. Precamb. Res. 92, Ahill, K.-I., Gower, C.F., The Gothian and Labradorian orogens: variations in accretionary tectonism along a late Paleoproterozoic Laurentia-Baltic margin. GFF 119, Ahill, K.-I., Larson, s.a., Growth-related Ga magmatism in the Baltic Shield: a review addressing the tectonic characteristics of Svecofennian, TIB 1-related and Gothian events. GFF 122, hill, K.-I., Schiiberg, H., The 963 Ma Vinga intrusion and post-compressional deformation in the Sveconorwegian orogen, SW Sweden. GFF 121, Ahiill, K.-I., Connelly, J., Brewer, T.S., Episodic rapakivi magmatism due to distal orogenesis? Correlation of Ga orogenic and inboard "anorogenic" events in the Baltic Shield. Geology 28, Aleinikoff, J.N., Evans, K., Fanning, C.M., Obradovich, J.D., Ruppel, E.T., Zieg, J.A., Steinmetz, J.C., Shrimp U-Pb ages of felsic igneous rocks, belt supergroup, western Montana. Geol. Soc. Am. Abstr. Prog. 28, A376. Amelin, Y.V., Larin, A.M., Tucker, R.D., Chronology of multiphase emplacement of the Salmi rapakivi graniteanorthosite complex, Baltic Shield: implications for magmatic evolution. Contrib. Mineral. Petrol. 127, Anderson, T.H., Silver, L.T., The role of the Mojave- Sonora megashear in the tectonic evolution of northern Sonora. In: Anderson, T.H., Roldan-Quintana, J. (Eds.), Geology of Northern Sonora. Geol. Soc. Am. Field Guide 27, Bennett, V.C., DePaolo, D.J., Proterozoic crustal history of the western United States as determined by Neodymium isotopic mapping. Geol. Soc. Am. Bull. 99, Berthelson, A., Towards a palinspastic analysis of the Baltic Shield. Int. Geol. Cong. 6, C Betts, P., Giles, D., Ga to 1.1 Ga evolution of the Australian continent: a northern, central, and eastern Australian perspective. Geol. Soc. Austral. 59, 34 15th Geological Convention. Bickford, M.E., Soegaard, K., Nielson, K.C., McLelland, J.M., Geology and geochronology of Grenville-age rocks in the Van Horn' and Franklin Mountains, West Texas: implications for the tectonic evolution of Laurentia during the Grenville. Geol. Soc. Am. Bull. 112, Bingen, B., Demaiffe, D., Geochemical signature of the Egersund basaltic dyke swarm, SW Norway, in the context of late Neoproterozoic opening of the Iapetus Ocean. Norsk Geologisk Tidsskrift 79, Bingen, B., van Breeman, O., Tectonic regimes and terrane boundaries in the high grade Sveconorwegian belt of SW Norway, inferred from U-Pb zircon geochronology and geochemical signatures of augen gneiss suites. J. Geol. Soc. Lond. 155, Bingen, B., DeMaiffe, D., Van Breeman, O., The 616 Ma old Egersund basaltic dike swarm, SW Norway, and late Neoproterozoic opening of the Iapetus Ocean. J. Geol. 106, Blewett, R.S., Black, L.P., Sun, S.S., Knutson, J., Hutton, L.J., Bain, J.H.C., U-Pb zircon and Sm-Nd

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