Australia: The Land Where Time Began

A biography of the Australian continent 

Supercontinents & Superplume Events

The author concluded that increased production of juvenile crust correlates with the formation of supercontinents, but not with their fragmentation, based on U/Pb zircon ages. He also concluded that peaks in the production of juvenile crust are associated with alleged superplume events, at least with the superplume events that occurred at 2.7 Ga and 1.9 Ga. The author asks whether there are 2 types of superplume because they are associated with both formation and breakup of supercontinents  (Condie, 1998, 2000). Data from the opening of the Atlantic Ocean, that has occurred in the last 200 million years, leads many to believe that as superplumes are associated with mantle upwellings they are responsible for the breakup of supercontinents (Condie, 2001). Shielding of supercontinents requires an average of about 200-400 My to cause the mantle beneath it to begin upwelling, according to computer models (Lowman & Jarvis, 1996). Following the initiation of upwelling in the mantle, superplumes form within the upwelling, after which the supercontinent fragments over about 200 million years. A supercontinent may not completely breakup if the landmass is not large enough to shield sufficiently to produce a superplume. Condie suggests this appears to be the case with the supercontinent at about 1.9 Ga (Condie, 2002c). According to Condie, evidence is lacking for this type of superplume event a (shielding superplume event) to have large volumes of juvenile crust associated with it. Flood basalts and associated mafic underplating are the main type of juvenile crust associated with a shielding event. Evidence of the volume of Phanerozoic flood basalts and mafic underplates has been found by reflection seismology indicates that though the plume head can be up to 2500 km across, the volume of juvenile mafic crust associated with it is probably  relatively small.

In the unlikely case that all of the high seismic velocity layer at the base of continental crust from the Proterozoic (Durrheim & Mooney, 1991) was composed of later plume underplate, it comprises 10-25 % of the Proterozoic crust. The volumes of oceanic plateaus that are associated with a shielding superplume event are an important uncertainty, as well as the fraction of these plateaus that eventually collide with, and accrete to, the continents. Condie suggests that if his zircon ages of continental crust are representative, the indications are that, at least for the Archaean, a relatively small volume of oceanic plateaus accrete to the continent (Condie, 2001).

Condie asks what is different about the superplume events that maybe associated with juvenile crust production peaks at 2.7 Ga and 1.9 Ga. He suggests some process, other than plate shielding, must be responsible for triggering these events, catastrophic superplume events. Lasting for <100 million years, they are short-lived compared to shielding superplume events that last for more then 200 My. They are suggested to be more intense, and possibly more widespread than shielding events, as large volumes of continental crust are associated with these superplume events. Supercontinent breakup has been suggested to trigger slab avalanches at the discontinuity in the mantle at 660 km, the result being catastrophic superplume events (Peltier et al., 1997; Condie, 1998). It has been suggested that such a model may explain the event at 1.9 Ga, and possibly for the 3 events that occurred in the Phanerozoic, but does not explain the 2.7 Ga event, because of the lack of evidence for an earlier fragmenting supercontinent. The interval between a slab avalanche and the production of juvenile crust is quite short, <100 million years, according to the slab avalanche model (Condie, 1998). This is the result of the slabs being able to sink to the bottom of the mantle in 100 million years of less (Larson & Kincaid, 1996), and mantle plumes can rise to the lithosphere base in a few million years (Larson & Yuen, 1997) as mantle viscosity increases with depth.

The accretion of arc components to the continents is the main process of the growth of crust associated with catastrophic superplume events, accretion occurring along the margins of the continents, or by the collision of oceanic arcs with continents (Condie, 2001). Condie suggests that a relatively minor role is played in the growth of continents by oceanic plateau accretion, providing their geological sampling is representative in the last 2.5 Gy. It is suggested it may not be coincidental that supercontinent formation and the 1.9 Ga superplume event occurred simultaneously. The supercontinent breakup in the Late Archaean, 2.2-2.0 Ga, possibly triggered the 1.9 Ga superplume event, and in this sense, positive feedback for the growth of crust that began during a shielding superplume event, results from a catastrophic event. The trapping of juvenile crust in accretionary  and collisional orogens by a growing supercontinent may contribute to the preservation of the juvenile crust.

Many other mantle plume events were recognised using plume proxies (Abbott & Isley, 2002). Most of these events are not associated with increased juvenile crust production rates, and many are not associated with either the formation or fragmentation of supercontinents. Many of the proxy events may not be superplume events, being mantle plume events that are more localised, that affect only some parts of the crust. There appear to be 2-3 scales of mantle plume event periodicity (Abbott & Isley, 2002), some possibly being associated with impact events. Another question is how to distinguish between superplume and supercontinent events, assuming both are recorded in the geologic record. Precise U/Pb zircon dating is critical because of the importance of timing. Catastrophic superplume events last for <100 My, supercontinent events for more than 200 My. Shielding superplume events and supercontinent events are of comparable duration. 

According to Condie, as more precise ages become available it will be possible to look for asymmetric decay of catastrophic events over time. An example is the rapid addition of CO2 to the atmosphere associated with superplume events that are short-lived, and may show an abrupt increase in the deposition of black shale and global warming, and as the mantle plume cools over time a gradual decline would occur. A gradual onset and a duration of more than 200 My should be shown by supercontinent events and shielding superplume events. A global effect may be minor or lacking if the 2 events cancel each other, as occurs with the opposing effects of the formation of supercontinents and a superplume event on the deposition of black shale and sea level. The magnitude of the excursions of global change should be enhanced by positive feedback, as the production rate of juvenile crust is by the effects of processes such as the formation of a supercontinent and a superplume event.

See Source 1 for more detailed information

Sources & Further reading

  1. Condie, 2004, Supercontinents and Superplume events: Distinguishing signals in the geological record
Author: M. H. Monroe
Email:  admin@austhrutime.com
Last Updated 07/05/2011

 
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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading