Kenorland

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Kenorland was one of the earliest supercontinents on Earth. It was formed during the Neoarchaean Era ~2.7 billion years ago by the accretion of Neoarchaean sanukitoid cratons and the formation of new continental crust. It comprised the Laurentia, Baltica, Australia, and Kalahari cratons. Tectonic magma-plume rifting began to occur between 2.48 to 2.45 Ga. The protracted breakup of Kenorland during the Late Neoarchaean and early Paleoproterozoic Era 2.48 to 2.10 Ga, during the Siderian and Rhyacian periods, is manifested by mafic dykes and sedimentary rift-basins and rift-margins on many continents. On early Earth, this type of bimodal deep mantle plume rifting was common in Archaean and Neoarchaean crust and continent formation.

The geological time period surrounding the breakup of Kenorland is thought by many geologists to be the beginning of the transition point from the Hadean to Early Archean deep-mantle-plume method of continent formation (before the final formation of the Earth's inner core), to the subsequent two-layer core-mantle plate tectonics convection theory that is known today to form continents by subduction, convergence and accretion (see plate tectonics). However, with the findings of the earlier continent Ur and the ca. 3.1 Ga supercontinent Vaalbara, this transition period may have occurred much earlier.

The core of Kenorland, the Baltic/Fennoscandian Shield, traces its origins back to over 3.1 Ga. The Yilgarn Craton (present-day Western Australia) contains zircon elements in its crust that date back to 4.4 Ga.

Paleomagnetic studies show Kenorland was in generally low latitudes until the rifting breakup began 2.48 Ga. Paleomagnetic evidence shows that at 2.45 Ga the Baltic Shield (now parts of Norway, Finland and Sweden) was joined to Laurentia (the Canadian Shield) and formed a unity with both the Kola and Karelia cratons, and that the Baltic Shield was located over the equator. The Kola and Karelia cratons began to drift apart ~2.45 Ga, and by 2.4 Ga the Kola craton was located at ~15 degrees latitude and the Karelia craton was located at ~30 degrees latitude. Paleomagnetic evidence shows that at 2.45 Ga the Yilgarn craton (now the bulk of Western Australia) was not connected to Fennoscandia-Laurentia and was located at ~70 degrees latitude. This implies that at 2.45 Ga there was no longer a supercontinent and by 2.4 Ga an ocean existed between the Kola and Karelia cratons. Also, there is speculation based on the rift margin spatial arrangements of Laurentia, that at some point during the breakup, the Slave and Superior cratons were not part of the supercontinent Kenorland, but, at this point in time, may have been two different Neoarchaean landmasses (supercratons) that were on opposite ends of a very large Kenorland. This is based on how drifting assemblies of various constituent pieces should flow reasonably together toward the amalgamation of the new subsequent continent. The Slave and Superior cratons now constitute the NW and SE portions of the Canadian Shield, respectively.

The breakup of Kenorland was contemporary with the Huronian glaciation which lasted for over 60 million years. The banded iron formations (BIF) show their greatest extent at this period, thus indicating a massive increase in oxygen build-up from an estimated 0.1% of the atmosphere to 1%. The rise in oxygen levels caused the virtual disappearance of the greenhouse gas methane (oxidized into carbon dioxide and water). The simultaneous breakup of Kenorland generally increased continental rainfall everywhere, thus increasing erosion and further reducing the other greenhouse gas carbon dioxide. With the reduction in greenhouse gases, and with solar output being less than 85% its current power, this led to a runaway Snowball Earth scenario, where average temperatures planet-wide plummeted to below freezing. Despite the anoxia indicated by the BIF, photosynthesis continued, stabilizing climates at new levels during the second part of the Proterozoic Era.

[edit] References

Arestova, N.A., Lobach-Zhuchenko, S.B., Chekulaev, V.P., and Gus'kova, E.G. (2003). "Early Precambrian mafic rocks of the Fennoscandian shield as a reflection of plume magmatism: Geochemical types and formation stages." Russian Journal of Earth Sciences, Vol. 5, No. 3. Online Abstract:[1]
Aspler, Lawrence B., Chiarenzilli, Jeffrey R., Cousens, Brian L., Davis, William J., McNicoll, Vicki J., Rainbird, R.H. (1999). "Intracratonic basin processes from breakup of Kenorland to assembly of Laurentia: new geochronology and models for Hurwitz Basin, Western Churchill Province." Contributions to the Western Churchill NATMAP Project; Canada-Nunavut Geoscience Office.
Mertanen, Satu (2004). "Paleomagnetic Evidences for the Evolution of the Earth during Early Paleoproterozoic." Symposium EV04: Interaction of Endogenic, Exogenic and Biological Terrestrial Systems.[2]
Pesonen, L.J., Elming, S.-Å., Mertanen, S., Pisarevsky, S., D’Agrella-Filho, M.S., Meert, J.G., Schmidt, P.W., Abrahamsen, N. & Bylund, G. (2003). "Palaeomagnetic configuration of continents during the Proterozoic." Tectonophysics 375, 289-324.
Ramo, O.T., Halla, J., Nironen, M., Lauri, L.S., Kurhila, M.I., Kapyaho, A., Sorjonen-Ward, P., & Aikas, O. (2005). "Eurogranites 2005 - Proterozoic and Archean Granites and Related Rocks of the Finnish Precambrian."[3]