Kenorland

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Kenorland was one of the earliest supercontinents on Earth. It is believed to have formed during the Neoarchaean Era ~2.7 billion years ago (2.7 Ga) by the accretion of Neoarchaean cratons and the formation of new continental crust. Kenorland comprised what later became Laurentia (the core of today's North America and Greenland), Baltica (today's Scandinavia and Baltic), Western Australia and Kalahari. Swarms of volcanic dikes and their paleomagnetic orientation as well as the existence of similar stratigraphic sequences permit this reconstruction. 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 crystals in its crust that date back to 4.4 Ga.

[edit] Formation of Kenorland

Kenorland was formed around 2.7 billion years ago (2.7 Ga) as a result of a series of accretion events and the formation of new continental crust(Halla, 2005).

According to an indepth analyses by Barley et al. (2005), 2.78 billion years ago submarine magmatism culminated with the eruption of extensive suites of mantle plume derived komatiites at 2.72 to 2.70 Ga. Extensive hydrothermal activity, produced volcanic massive sulfide mineralization and banded iron formation (BIF) deposition in anoxic arc-related basins. Arc and plume magmatism were followed by orogeny deformation, granitoid emplacement (by 2.68 Ga), stabilization of continental lithosphere and collision with the other cratons to form the Kenorland continent. The formation of Kenorland and possible collision of the Zimbabwe and Kaapvaal cratons at 2.6 Ga provides evidence that Late Archean cratons started to aggregate into larger continents at that time. Importantly granitoid–greenstone terranes and high-grade gneiss belts in the Gawler Craton, Antarctica, India, and China provide evidence for a second cycle of convergent margin tectonics and collision of cratons between 2.6 and 2.42 Ga. The Gawler Craton contains 2.56 to 2.5 Ga ultramafic to felsic volcanic rocks (including 2.51 Ga plume-derived komatiites), metasedimentary rocks, and granitoids with compositions that are typical of Archean granitoid–greenstone terranes interpreted to have formed at convergent continental margins. Central India and possibly eastern North China have similar histories from 2.6 Ga culminating with orogeny between 2.5 and 2.42 Ga corresponding to the aggregation and stabilization of Indian cratons within a larger continent. The Pilbara and Kaapvaal cratons are the only cratons with relatively complete and well-dated 2.6 to 2.4 Ga supracrustal rock records.

The accretion events are recorded in the greenstone belts of the Yilgarn Craton as metamorphosed basalt belts and granitic domes accreted around the high grade metamorphic core of the Western Gneiss Terrane, which includes elements of up to 3.2 Ga in age and some older portions, for example the Narryer Gneiss Terrane.

[edit] Breakup of Kenorland

Paleomagnetic studies show Kenorland was in generally low latitudes until tectonic magma-plume rifting began to occur between 2.48 Ga and 2.45 Ga. At 2.45 Ga the Baltic Shield was over the equator and was joined to Laurentia (the Canadian Shield), and formed a unity with both the Kola and Karelia craton. 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 dikes 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 . 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 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 persisted for up to 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.
Barley, Mark E., Andrey Bekker, and Bryan Krapez. (2005) “Late Archean to Early Paleoproterozoic global tectonics, environmental change and the rise od atmospheric oxygen.” Earth and Planetary Science Letters Vol. 238. pp. 156-171. [2]
Mertanen, Satu (2004). "Paleomagnetic Evidences for the Evolution of the Earth during Early Paleoproterozoic." Symposium EV04: Interaction of Endogenic, Exogenic and Biological Terrestrial Systems.[3]
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.
Halla, J., M.I., Kapyaho, Kurhila, M.I., A.,Lauri, L.S., Nironen M., Ramo, O.T., Sorjonen-Ward, P., & Aikas, O. (2005). "Eurogranites 2005 - Proterozoic and Archean Granites and Related Rocks of the Finnish Precambrian."[4]