Whiter the Supercontinent Cycle

Australia: The Land Where Time Began

Whither the Supercontinent Cycle

In this article the author¹ says belief in the periodicity in the processes of tectonics predates the acceptance of the plate tectonics paradigm (e.g. Umbgrove, 1940; Holmes, 1951; Sutton, 1963). When the case was made for the repeated opening and closing of marine basins it was specifically advocated (Wilson, 1966), the process now being known as "Wilson Cycles" (Dewey & Burke, 1974). It was originally proposed that there was a relationship between tectonic periodicity and the supercontinent cycle (Worsley et al., 1982, 1984), the argument being that supercontinent amalgamation was reflected in number of peaks in the episodic collisions between continents and the breakup of these supercontinents is reflected in the episodes of rift-related mafic dike swarms. Trends in tectonic activity, development of platforms, climate, life and stable isotopes that accompanied the amalgamation, breakup and dispersal of supercontinents, were recognised (Nance et al., 2013). Over the past 30 years a broad consensus has emerged that since the Late Archaean that repeated cycles of amalgamation and dispersion of supercontinents, which had a profound effect on the evolution on the geosphere, hydrosphere, atmosphere and biosphere, though there are dissenters (e.g., Stern, 2008). Age distributions of orogenic granites and detrital zircons display statistical peaks, as well as negative ε_Hf excursions in zircons that the author¹ suggests may match supercontinent amalgamation timing. Lu/Hf is lower in the crust than in the depleted mantle relative to the Earth's bulk, as Hf is the more incompatible element, and therefore the crust evolves towards εHf values. Whether episodicity is implied by these εHf data, a preservational bias, or a combination of both phenomena is debated (e.g., Roberts, 2012; Cawood et al., 2013; Nance et al., 2013).

The mechanisms that are potentially responsible for the supercontinent cycle have remained controversial, with different interpretations of the reconstructions of palaeocontinents. For example, the reconstruction of continents for the breakup of Rodinia about 800-650 Ma (e.g., Hoffman, 1991; Li et al., 2008) imply that Gondwana (part of Pannotia; see Dalziel, 2013) was assembled as a result of preferential subduction of oceanic lithosphere, that was relatively old, around Rodinia (the exterior ocean of Murphy and Nance, 2003), though Palaeozoic reconstructions of continents, about 545-245 Ma (e.g., Stampfli & Borel, 2002; Scotese, 2007), implied that Pangaea was formed as a result of preferential subduction of oceanic lithosphere, that was relatively young, that had formed by the breakup of Pannotia (the interior oceans of Murphy & Nance, 2003).

There are 2 papers in the same issue of Geology as this paper¹ (Spencer et al., 2013, p.795; Van Kranendonk & Kirkland, 2013, p. 735) that add to the increasing evidence that each supercontinent forms in a different manner from the others, and that the processes involved in their formation have changed over time (e.g., Bradley, 2011; Condie, 2011; Nance et al., 2013). The aim of both these studies was to attempt to understand continental margin behaviour during the amalgamation of a supercontinent by use of the isotopic proxy method, one compared the amalgamation of Rodinia and Gondwana (Spencer et al., 2013) and the other studied the amalgamation of Rodinia and the Grenville Orogenesis (Van Kranendonk & Kirkland, 2013).

The marked differences between seawater Sr and zircon Hf isotope signatures during the amalgamation of Gondwana at 750-550 Ma and at 1250-980 Ma, the amalgamation of Rodinia (Cawood et al., 2013). A record of enhanced weathering over time is provided by variations in the initial ⁸⁷Sr/⁸⁶Sr that accompanies orogenesis (high initial Sr) relative to the spreading of ocean ridges and an increase of hydrothermal activity (low initial Sr), which are therefore sensitive to the amalgamation of supercontinents and their breakup, respectively. During the Late Neoproterozoic-Early Cambrian initial Sr ratios were higher than at any time in the past billion years (Veizer et al., 1999) which has been related to increased continental weathering during and following Gondwana collisions. The weathered material is suggested by a pronounced negative excursion in the ε_Hf (zircon) data to be ancient continental crust that was recycled (Belousova et al., 2010; Collins et al., 2011).

According to the author¹ it is difficult to decipher the amalgamation of Rodinia from the proxy record of either zircon or strontium. About 1.8 Ga the initial Sr values began to decrease, the decline continuing until about 750 Ma, while the ε_Hf (zircon) values remained close to those of Chondritic Uniform Reservoir (CHUR) from about 1.6-0.7 Ga, except for a modest excursion of about 20-50 My. The lack of changes in the Sr seawater and ε_Hf (zircon) suggests, given the unprecedented scale of the orogenesis that was associated with the assembly of Rodinia (Beaumont et al., 2010), that during the amalgamation of Rodinia relatively juvenile crust was recycled, which is compatible with evidence that the eastern flank (modern coordinates) of Laurentia was a margin of Pacific type for nearly 0.8 Gy prior to the collision, and between 1.7 and 1.3 Ga produced abundant juvenile crust (e.g., Åhäll & Gower, 1997; Dickin, 2000).

It has been interpreted that these contrasting signatures reflect the closing of an ocean by a single-sided, Gondwanan, subduction zone versus a double-sided subduction zone (Rodinia). A collision between a passive margin and a juvenile continental arc results from a Gondwana-type subduction zone, and 2 juvenile arc systems result in the Rodinia-type subduction. The author¹ suggests an important corollary of this study is that it further confirms the status of Gondwana, or Gondwana plus Laurentia = Pannotia (Dalziel, 2013) as a supercontinent. The amalgamation of Gondwana produced very strong global signals, as is clearly indicated by proxy data. The development of the Iapetus Ocean in the Early Palaeozoic has been assigned to the final splitting up of Rodinia (e.g., Li et al., 2008; Bradley, 2011), though it masks the importance of the assembly of Gondwana for global events in the Late Neoproterozoic, that includes a explosion of biological activity, as well as dramatic swings of climate (e.g., Hoffman et al., 1998; Knoll, 2013). The proxy records for the breakup in the Early Palaeozoic that led to the opening of the Iapetus and Rheic Oceans are characterised by initial Sr that is decreasing and ε_Hf (zircon) that is increasing, these being consistent with ocean ridge activity that is enhanced, as well as showing a signal that is very different from earlier phases in the breakup of Rodinia. Passive margin formation match these trends (Bond et al.. 1984), and a sharp rise of sea level (e.g., Miller et al., 2005) that occurred during the Early Palaeozoic when there was a progressively less contribution from terrestrial weathering, that led to a time when continental subsidence and ocean floor formation, that was youthful and more elevated.

Between Archaean and Phanerozoic terranes, secular differences, such as a greater abundance of komatiites, tonalite-trondhjemite-granodiorite, TTG intrusions, and in Archaean terranes, banded iron formations; only in Phanerozoic terranes etc.), and these were accompanied in the original concept (Worsley et al., 1984), generally being attributed to higher mantle temperatures and an atmosphere that was oxygen-poor in the Archaean (e.g., Brown, 2008; Campbell & Allen, 2008). Contrasting with this it was proposed by van Kranendonk & Kirkland that Rodinia was amalgamated during a limited time window when the Earth was characterised by a goldilocks combination of "thicker plates on a warmer Earth, with more rapid drift relative to modern Earth." The slab pull of an oceanic lithosphere that was thicker, contributed to rapid continental drift. The unprecedented level of cycling of the crust can, in their view, be explained by the Goldilocks scenario, as is indicated by the global δ¹⁸O database, as well as by the enormous scale of the Grenvillian Orogens. The scenario envisaged for the time before the amalgamation of Rodinia is one in which tectonic switching (Collins, 2002) between the formation of juvenile crust and occurring during episodes of roll-back, and during compressive episodes, crustal recycling and accretion. The author¹ suggests this scenario is broadly compatible with the scenario envisaged by Spencer et al., who indicate the subducted material was predominantly juvenile.

In recent times there has been much progress in understanding the configuration of supercontinents, and the timing of their amalgamation and breakup, and also in the relation of the supercontinents to the evolution of the biosphere, hydrosphere, and atmosphere. The author¹ suggests the 2 papers in the same issue of Geology¹ contribute further evidence that shows how there were no 2 supercontinents that formed in the same way, and the elusiveness of the mechanisms involved in their amalgamation and breakup.

The author¹ suggests integration of geological constraints derived from proxy records to numeric models would lead to a way forward. There have been significant advances in numerical modelling to simulate the amalgamation and breakup of supercontinents on realistic timescales (e.g., Zhang et al., 2009; Yoshida & Santosh, 2011) since the pioneering work of Gurnis (1988). He also suggests it would be interesting to know how these models could be tweaked to allow the formation of supercontinents to occur in the manner that has been implied by the proxy data.

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