The Making and Unmaking of a Supercontinent: Rodinia Revisited
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Tectonophysics 375 (2003) 261–288 www.elsevier.com/locate/tecto The making and unmaking of a supercontinent: Rodinia revisited Joseph G. Meerta,*, Trond H. Torsvikb a Department of Geological Sciences, University of Florida, 241 Williamson Hall, PO Box 11210 Gainesville, FL 32611, USA b Academy of Sciences (VISTA), c/o Geodynamics Center, Geological Survey of Norway, Leif Eirikssons vei 39, Trondheim 7491, Norway Received 11 April 2002; received in revised form 7 January 2003; accepted 5 June 2003 Abstract During the Neoproterozoic, a supercontinent commonly referred to as Rodinia, supposedly formed at ca. 1100 Ma and broke apart at around 800–700 Ma. However, continental fits (e.g., Laurentia vs. Australia–Antarctica, Greater India vs. Australia– Antarctica, Amazonian craton [AC] vs. Laurentia, etc.) and the timing of break-up as postulated in a number of influential papers in the early–mid-1990s are at odds with palaeomagnetic data. The new data necessitate an entirely different fit of East Gondwana elements and western Gondwana and call into question the validity of SWEAT, AUSWUS models and other variants. At the same time, the geologic record indicates that Neoproterozoic and early Paleozoic rift margins surrounded Laurentia, while similar-aged collisional belts dissected Gondwana. Collectively, these geologic observations indicate the breakup of one supercontinent followed rapidly by the assembly of another smaller supercontinent (Gondwana). At issue, and what we outline in this paper, is the difficulty in determining the exact geometry of the earlier supercontinent. We discuss the various models that have been proposed and highlight key areas of contention. These include the relationships between the various ‘external’ Rodinian cratons to Laurentia (e.g., Baltica, Siberia and Amazonia), the notion of true polar wander (TPW), the lack of reliable paleomagnetic data and the enigmatic interpretations of the geologic data. Thus, we acknowledge the existence of a Rodinia supercontinent, but we can place only loose constraints on its exact disposition at any point in time. D 2003 Elsevier B.V. All rights reserved. Keywords: Rodinia; Paleomagnetism; Gondwana; Supercontinent; True polar wander 1. Introduction data from all the continents could be fitted to a common apparent polar wander path (APWP). This The notion of a late Proterozoic supercontinent led him to suggest that a supercontinent composed of was postulated in the 1970s as geologists noted the most of the continental crust remained in a quasi- existence of a number of 1100–1000 Ma ‘mobile rigid configuration throughout the bulk of Precam- belts’ (Dewey and Burke, 1973). J.D.A. Piper pos- brian time (Piper, 1976, 2000). Paleomagnetic poles tulated, early on, that the available paleomagnetic published in the late 1970s and early 1980s generally had poor age control and allowed sufficient flexibil- ity to fit almost any pole on the rather tortuous * Corresponding author. Tel.: +1-352-846-2414; fax: +1-352- APWP proposed by Piper (see review in Van der 392-9294. Voo and Meert, 1991). Subsequent thoughts about E-mail address: [email protected] (J.G. Meert). Precambrian supercontinents by Bond et al. (1984) 0040-1951/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0040-1951(03)00342-1 262 J.G. Meert, T.H. Torsvik / Tectonophysics 375 (2003) 261–288 relied on evidence from passive margin sequences Rodinia reconstructions (e.g., Dalziel, 1997), Lauren- around the globe. They argued that the global pres- tia formed the core of the supercontinent with East ence of these margins heralded the breakup of a Gondwana situated along its present-day western supercontinent at the end of the Proterozoic. In margin (SWEAT, Southwest U.S.–East Antarctic), the early 1990s, Dalziel (1991), Moores (1991) and and with Amazonia and Baltica positioned along its Hoffman (1991)—following the suggestion of present-day eastern margin (Fig. 1). McMenamin and McMenamin (1990)—adopted the Early paleomagnetic tests of the Rodinia configu- name Rodinia for this Meso–Neoproterozoic super- ration were broadly supportive of the idea (e.g., continent (see also Meert and Powell, 2001). The Powell et al., 1993; Torsvik et al., 1996), but the geometry of the Rodinia supercontinent has remained quality of the available data were insufficient to flexible, but generally most models have sought to provide any rigorous test of the Rodinia paleogeog- develop the configuration around Grenvillian–Sve- raphy. In part, this was due to the fact that the older conorwegian–Kibaran aged metamorphic belts paleomagnetic studies on Proterozoic rocks were not ( f 1350–1000 Ma) and link geologic provinces tied to a specific radiometric age or were not com- across cratonic margins (Fig. 1). In the ‘archetypal’ pletely analyzed for the possibility of younger over- Fig. 1. The ‘traditional’ model of Rodinia adopted from Dalziel (1997) and Torsvik et al. (1996). The model posits two rifting events, one along the present-day western margin of Laurentia sometime between 800 and 700 Ma, and a second along the present-day eastern margin of Laurentia between 600 and 550 Ma. J.G. Meert, T.H. Torsvik / Tectonophysics 375 (2003) 261–288 263 printing. Of more recent vintage are paleomagnetic period are lacking for many of the cratonic elements studies that are conducted in conjunction with radio- thought to comprise Rodinia. Therefore, any recon- metric studies in order to develop temporally and struction developed for this time period must be spatially defined APWPs. Nevertheless, current Pro- extremely fluid as new data may create significant terozoic paleomagnetic studies are sufficient only to changes in the overall distribution of cratonic ele- test paleogeographic relationships between two or ments surrounding Laurentia. At the same time, any three continents at discrete (and widely separated) reconstruction for this period must also fit the paleo- intervals (Meert and Powell, 2001). geographic constraints imposed by younger (pre- Despite a lack of paleomagnetic evidence in favour breakup) paleomagnetic data and they must be of the Rodinia supercontinent, geologic links between geologically reasonable. Table 1 lists available paleo- the various cratonic nuclei were held forth in support magnetic results from the cratonic elements compris- of widely varying reconstructions (Young, 1995; ing Rodinia and, where possible, these poles are fitted Pelechaty, 1996; Dalziel, 1997; Rainbird et al., to an APWP. All poles in Table 1 are calculated on the 1998; Karlstrom et al., 1999; Sears and Price, 2000; assumption that the time-averaged geomagnetic field Dalziel et al., 2000). Our review paper, a tribute to the is that of a geocentric axial dipole (GAD) field. Some pioneering work of Chris Powell, focuses on key recent studies cast doubts on this fundamental as- intervals during the formation and breakup of the sumption (e.g., Kent and Smethurst, 1998; Van der supercontinent and highlights several controversial Voo and Torsvik, 2001; Torsvik et al., 2001a), and this aspects of those reconstructions. We wish to note, at raises some concern about the detailed resolution the outset, that each reconstruction discussed below is power in palaeomagnetic reconstructions. For exam- based on a particular set of paleomagnetic poles and ple, zonal non-dipole octupole contributions of 10– polarity options. At times, there are not so subtle 20% will introduce latitudinal discrepancies on the differences between our choice of poles and those order of 750–1500 km at intermediate latitudes. chosen by earlier authors. We do not have the space to develop a pole-by-pole comparison in this review 2.1. 1100–900 Ma: Laurentia and Baltica paper and therefore our goal is to generate a set of paleogeographic maps for critical intervals of Neo- The paleomagnetic database for Laurentia (Fig. proterozoic history and show how they differ from 2a) and Baltica (e.g., Fig. 2b and c) during this previous interpretations. At the same time, we recog- interval has been reviewed by a number of different nize that the limited dataset creates a host of problems authors (see Weil et al., 1998; Walderhaug et al., for previous interpretations. Because of the limitations 1999; Buchan et al., 2001; Powell et al., 2001). For a of the paleomagnetic data, we highlight these prob- complete discussion of the individual poles and the lems for further consideration rather than attempting shape/direction of the Laurentian apparent polar wan- to rescue any particular reconstruction. der path please refer to the above-mentioned articles. Weil et al. (1998) and Walderhaug et al. (1999) discussed the enigma of clockwise, anticlockwise 2. Paleomagnetic constraints on the Neoproterozoic and no loops for the Grenvillian poles in North supercontinent America and Baltica. Our path differs from that of Weil et al. (1998) because we draw the path through The interval from f 1100 to 900 Ma marks a the ‘eastern’ group of Keewanawan paleomagnetic geologically important period during the formation of poles rather than through the ‘western’ group (see the Neoproterozoic supercontinent. The Grenvillian, Fig. 2a). Hartz and Torsvik (2002) discuss the rela- Sveonorwegian and slightly older Kibaran orogenic tionship of younger Baltica poles with Laurentia belts are thought to mark the sutures between the (f 750 Ma) and suggest that Baltica might be various elements of the Rodinian supercontinent (see inverted with respect to the more ‘traditional’ Rodinia Dalziel, 1997; Meert