On a Nordic geological winter meeting in Oslo in January 2010 Adam Garde told about a possible impact structure in Greenland (see my post
). He and co-authors have now published further findings in a paper in Earth and Planetary Science Letters of 1 July 2012.
The so far oldest (i.e. 2975 million years) impact crater on Earth was found near Maniitsoq (in “my days” - and in Danish - known as Sukkertoppen
) in West Greenland. It is the fourth multi-ring crater discovered on Earth, besides Chicxulub, Sudbury, and Vredefort. It is a 100 km-scale, circular structure in the Archaean North Atlantic Craton centred at 65°15′N, 51°50′W near the Maniitsoq town in the Qeqqata municipality in West Greenland. The structure comprises a set of highly unusual geological features that were created during a single event involving intense crushing and heating and are incompatible with crustal orogenic processes. The presently exposed features of the Maniitsoq structure were buried 20–25 km below the surface when this event occurred at about 3000 million years ago, during waning convergent orogeny. These features include: a large aeromagnetic anomaly; a central 35×50 km2
large area of comminuted quartzo-feldspathic material; regional-scale circular deformation; widespread random fractures with featherlike textures; intense fracture cleavage; amphibolite–granite-matrix breccias unrelated to faulting or intrusions; formation and common fluidisation of microbreccias; abundant evidence of direct K-feldspar and plagioclase melting superimposed on already migmatised rocks; deformation of quartz by <c> slip; formation of planar elements in quartz and plagioclase; and, emplacement of crustally contaminated ultramafic intrusions and regional scale hydrothermal alteration under amphibolite-facies conditions. The diagnostic tools employed to identify impacting in the upper crust are inadequate for structures preserved deep within the continental crust. Nevertheless, the inferred scale, strain rates and temperatures necessary to create the Maniitsoq structure rule out a terrestrial origin of the structure.
Finding sufficient evidence was extremely hard because there is no obvious bowl-shaped crater left to find. Over the 3 billion years since the impact, the land has been eroded down to expose deeper crust 25 km below the original surface. All external parts of the impact structure have been removed, but the effects of the intense impact shock wave penetrated deep into the crust -- far deeper than at any other known crater -- and these remain visible. Because the effects of impact at these depths have never been observed before it has taken nearly three years of painstaking work to assemble all the key evidence. Only around 180 impact craters have ever been discovered on Earth and around 30% of them contain important natural resources of minerals or oil and gas. The largest and oldest known crater prior to this study, the 300 kilometre wide Vredefort crater in South Africa, is 2 billion years in age and heavily eroded. It has taken Adam Garde and his co-workers nearly three years to convince their peers in the scientific community that this is an impact structure, while the mining industry was far more receptive. A Canadian exploration company has been using the impact model to explore for deposits of nickel and platinum metals at Maniitsoq since the autumn of 2011.Reference
Adam A. Garde et al.Searching for giant, ancient impact structures on Earth: The Mesoarchaean Maniitsoq structure, West Greenland
Earth and Planetary Science Letters
Volumes 337–338, 1 July 2012, Pages 197–210http://dx.doi.org/10.1016/j.epsl.2012.04.026