The Neoproterozoic Supercontinent: Rodinia or Palaeopangaea?
Introduction
It is widely accepted that continental crust was aggregated into a supercontinent during Neoproterozoic times (see summaries of evidence in [1], [2]). For the past decade the popular reconstruction has, at its core, been based on correlation of sedimentary successions between eastern Australia and western North America [3], [4]; this is the SWEAT (Southwest US–east Antarctica) hypothesis [5]. It has been developed by siting elements of cratonic Africa at peripheral positions around Laurentia linked by ∼1100 Ma (Grenvillian) orogenic belts [6], South China adjacent to Australia [7], and Precambrian nuclei of east Asia outboard of Australia and India [8]. Dalziel [9] further places the western margin of South America adjacent to eastern North America and speculative geological models have explained Neoproterozoic–Early Cambrian interactions between these two blocks [10], [11]. The combined reconstruction (Fig. 1) is referred to as ‘Rodinia’ from the Russian infinite ‘to beget’ on the premise that it was the precursor of all subsequent continents [12]. Discussions of diverse aspects of Neoproterozoic geology are now taking place within these general geometrical constraints.
In spite of wide acceptance, Rodinia poses two major problems. Firstly palaeomagnetic poles from the core Australia–North America link do not accord with the hypothesis after ∼730 Ma [13], [14], [15]. Whilst this has been taken to signify the time of a putative breakout of Laurentia [6], [13], there are two major temporal difficulties with this solution: (1) the stratigraphic correlation between North America and Australia is derived from correlation of post-800 Ma sedimentary successions [3], [4] and (2) supercontinent fragmentation is required ∼150 Ma before this event is recognised in the geological record: although break-up has been linked to glaciation at ∼750–700 Ma [16], the key evidence (rifting, alkaline magmatism, marginal subsidence, isotope signatures and faunal diversity) is concentrated near the Neoproterozoic–Cambrian boundary [1], [2].
A second major difficulty is geometrical. The transition from Rodinia to Gondwana by early Palaeozoic times requires large and differential continental motions with Australia overtaking African blocks which in turn overtake South America. The simplest model retains Australia, India and Antarctica in their Gondwana configurations and postulates radial opening of a large Palaeopacific ocean [9]. This requires extraordinary plate movements which are becoming increasingly difficult to reconcile with the Gondwana time frame [17], [18].
This kinematic model is further integrated with the hypothesis that East and West Gondwana continents had remote origins and collided in early Phanerozoic times following closure of a Mozambique Ocean to form the East African segment of the Pan-African Orogen [19]. Two difficulties with this proposition are: (1) there is no evidence for closure of a major ocean in southeastern Africa [20]; subduction, arc accretion and terrane emplacement were concentrated in a zone from Tanzania to Arabia in Neoproterozoic times [21]. (2) It violates an Archaean link between the Kaapvaal craton of southern Africa and the Archaean terranes of India, Antarctica and Australia [22]. Thus the Rodinia reconstruction has grown piecemeal from a range of geological evidence which is non-unique. Palaeomagnetic evidence has been peripheral to the case although it can potentially eliminate untenable solutions and constrain viable models.
An alternative Neoproterozoic configuration derived primarily from analysis of palaeomagnetic data a decade before Rodinia is Palaeopangaea [2], [23], a hemispheric and symmetrical crescent-shaped crust analogous to the later supercontinent Pangaea (Fig. 2). This solution requires relatively small, and predominantly strike slip, motions between the continental blocks and invokes their dispersal only near to the Neoproterozoic–Cambrian boundary. It allows for no proximity between Australia and western North America but does provide an equally plausible explanation for the distribution of ∼1100 Ma belts by recognising them as coaxial lineaments inherited from a Palaeo–Mesoproterozoic precursor (Fig. 2). Palaeopangaea preserves the essential integrity of Archaean continental nuclei [22] (Fig. 2) and, by surmounting temporal and geometrical problems with Rodinia, merits re-evaluation in the context of an expanded database. In this paper palaeomagnetic poles assigned to the interval ∼1150–500 Ma are used to assess merits of these two contrasting models for the Neoproterozoic supercontinent.
Section snippets
The palaeomagnetic test
To derive representative apparent polar wander paths (APWPs) palaeomagnetic poles based on n≥24 thermally and/or magnetically cleaned samples defined by precision parameters k>10 are retrieved from the Global Palaeomagnetic Database. Listed mean magnetisation ages estimated to <±150 Ma are used with minor updating. Suspected remagnetisations have only been included when they can be linked to thermo-tectonic events (dyke swarms adjoining the Grenville Front for example). Whilst these criteria
Palaeomagnetic evaluation of Rodinia and Palaeopangaea
The Laurentia APWP at ∼1150 Ma had moved northwest from a location near the centre of the projections (180°E, 0°N) occupied for much of the interval 1300–1180 Ma [23]. The subsequent path is embraced by Gardar and Keweenawan Tracks with the apex of a loop defined by 1130–1110 Ma poles from alkaline igneous episodes in Greenland and the Canadian Shield (Fig. 3) [29]. The ensuing Grenville Track is predominantly an uplift-related cooling magnetisation record dated from isotopic closure in
Discussion
This analysis acknowledges a dynamic tectonic setting with blocks in quasi-continuous motion relative to the rotation axis [17] and, within limits of age uncertainties, the comparisons of Fig. 3, Fig. 4, Fig. 5 have aimed to optimise recovery of APW. The polar distributions can be evaluated at two levels by questioning (a) whether pole positions accord with a single APWP and (b) whether any such agreement is compatible with their age estimates.
The first question can be answered in the
Conclusions
Although discussion of the Neoproterozoic supercontinent has mostly considered Rodinia to the exclusion of alternative possibilities, this is shown to be unwarranted. Palaeopangaea provides a more satisfactory solution to both geological and palaeomagnetic evidence with the qualification that there is no doubt scope for fine tuning the reconstruction. Since palaeomagnetic data are consistent with limited relative motions between the major continental blocks, untestable movements implicit in the
Acknowledgements
I am grateful to Kay Lancaster for drafting the figures, to Dr Buang Huang, Professor Zhang Huimin and Dr T. Radhakrishna for advice on Chinese and Indian data respectively, and to the two anonymous reviewers whose advice materially improved the paper.[FA]
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