Supercontinents before Pangaea

Australia: The Land Where Time Began

Supercontinents Before Pangaea

A compilation of GDS map area and age data [24] has been completed, the authors¹ suggesting that data suggests GDS are related to mantle plumes and are feeder dikes for LIPs, the crude approximation that GDS map area (in km²) is proportional in volume to the volume of erupted flood basalt, provided the description of Ernst et al. [8] and Heaman [30] is correct. Based on the above cumulative plots of GDS map area should be analogous to LIP volume, though the authors¹ suggest it should be noted that not all dike swarms may have produced LIPs. It has been pointed out that more than 20 dike swarms from the Proterozoic have been documented in North America, not all of which are associated with LIPs. Beginning about 350 Ma it can be seen clearly in the occurrence of dike swarms dating from before the formation of Pangaea and that LIPs didn't occur until the voluminous Siberian Traps erupted 250 Ma (Fig.1). The authors¹ used the well-preserved and well-understood relations associated with the assembly of Pangaea to guide their interpretation of the timing of assembly of supercontinents. An assumption they made is that following the assembly of a supercontinent there is a significant increase in the rate of occurrence of GDS. The authors¹ suggest an estimate of the clustering of these occurrences should result from a plot of the time separating successive GDS vs. age of GDS (Fig.1). Based on these date, as well as similar data from LIPs, plume centres, and information obtained from the literature that place constraints on the age estimates that have been made of supercontinent assembly and fragmentation (Fig.1).

The authors¹ have constructed cumulative curves for 7 GDS groupings (Fig.3b), based on groupings of GDS ages as a guide (Fig.1), and the relation between the assembly of Pangaea and LIP production (Fig.3a). The difference in time between the initial points for each group decreases with increasing age from about 500 My to about 300 My, (Fig.3b), this periodicity is similar to that of supercontinent and mantle cycling times [3, 12]. In this scheme it is implied that there is an assembled supercontinent for each GDS group, therefore there is a need for 7 supercontinents, the named supercontinents Rodinia, Greater Gondwana B [34,35], and Pangaea (Figs.1 & 3b), plus 4 that are presently unknown. This periodicity implies that supercontinent assembly and breakup has been occurring at roughly regular intervals for the past 3 Gy, assuming this interpretation is correct.

Dramatic geochemical and geophysical changes took place between the breakup of Rodinia and the assembly of Pangaea [46,47], though the configuration of the continents are not well constrained. The term Greater Gondwana, coined by Stern [35], about 700-500 Ma, is used by the authors¹ to describe supercontinent(s) occurring during this interval, comparable to Pangaea B, about 720-560 Ma of Veevers et al. [37]. The authors¹ modified the timing for the assembly of Greater Gondwana, about 850-500 Ma (Fig.1). They have broken with conventional wisdom, defining a separate supercontinent during this time interval, the conventional position being that Rodinia, the supercontinent of the Late Proterozoic, assembled about 1100 years ago (Grenville orogeny) rifting apart between 750 and 700 Ma, and again between 550and 500 Ma [48-54]. They base this on the temporal distributions of LIPs and GDS and the assembly of Pangaea, the relations associated with the assembly of Pangaea that are understood relatively well (Figs. 1 & 3a). It is suggested that Rodinia was partially disassembled at this time, based on the paucity of GDS occurrences at about 1.0 Ga and 800 Ma (Fig.1), and the pattern for GDS occurrences between 1100 and 300 Ma differs from that of any other period in the history of the Earth (Fig.1). The authors¹ suggest it is likely these patterns are the result of the assembly of various continents and supercontinents that were significantly smaller than Rodinia and Pangaea. They suggest that if Rodinia assembled between about 1200 and 500 Ma, the mechanism driving mantle convention could assemble a supercontinent for about 700 My [1,3,5]. The authors¹ suggest it is unlikely a supercontinent was assembled for about 1.0 Gy, though cumulative GDS area data suggest that Rodinia strongly influences GDS production between 1267 and 250 (?) Ma (Fig.5).

There is a resemblance between the cumulative curves for each supercontinent group and the general shape of the cumulative LIP volume curve (Fig.3a). There is generally a period of rapid increase in the centre of each curve that is followed by a decrease at the youngest portion (Fig.3b). The authors¹ suggest cumulative areas for each supercontinent group may respond with the size of each supercontinent, Pangaea > Rodinia > Greater Gondwana > Early Proterozoic supercontinents). The authors¹ suggest the relatively small cumulative areas of GDS in the Early Proterozoic may be accounted for by small thin continents in that period of time, juvenile cratons being unlikely to have appreciable amounts of terraines accreted to them and basaltic underplating. They suggest the small individual cumulative areas cannot be accounted for only by poor preservation (Fig.2a). The dike swarm areas of the Early Proterozoic do not approach the values reported for Rodinia and Pangaea if the cumulative areas of GDS are reconstructed to allow for the preservation as modeled (Fig.3b).

The Mackenzie Dike Swarm, that is regarded as one of the most significant volcanic/igneous events in geologic history, resulted in a dramatic increase in the area of GDS at 1267 Ma. If there is no dramatic increase in area the Rodinia-related GDS group (Fig.3b) would have a cumulative area that is intermediate between the 4th unnamed supercontinent, about 1850-1650, Ma and Greater Gondwana, resulting in a progressive increase in cumulative areas for each supercontinent. A gradual increase in the starting positions of each successive supercontinent grouping, about 100,000-about 800,000 Km² (Fig.3b), is another feature of these cumulative curves. The authors¹ speculate that the cause of these features (size of initial GDS area and cumulative area) results from increases in continent (and thus supercontinent) area and thickness over time.

It has been speculated that before 1.8 Ga the total crustal mass may have not been enough to for a 'true supercontinent' thereby diminishing the insulating effect on the mantle [Hoffman, 4]. Based on evidence presented here it is suggested that the small supercontinents of the Early Proterozoic were capable of producing GDS and LIPs (Fig.3b).

See Source 1 for the references mentioned in the text.

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