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The Maniitsoq structure was a proposed 3 billion-year-old (3 Ga)
impact structure An impact structure is a generally circular or craterlike geologic structure of deformed bedrock or sediment produced by impact on a planetary surface, whatever the stage of erosion of the structure. In contrast, an impact crater is the surface ...
located in the
Akia terrane The Akia terrane is a tectonostratigraphic terrane located in the North Atlantic Craton in southern West Greenland. The Akia terrane is bounded to the Southeast by the Eo- to Neo-archaean tectonostratigraphic terranes of the Nuuk region, and to th ...
of the North Atlantic Craton, centred about south-east of the town of
Maniitsoq Maniitsoq (), formerly Sukkertoppen, is a town in Maniitsoq Island, western Greenland located in the Qeqqata municipality. With 2,534 inhabitants , it is the sixth-largest town in Greenland. History Archaeological finds indicate that the a ...
, Greenland, at . However, the Maniitsoq structure has not been widely recognised as an impact structure, and the proposal was criticised for not meeting established criteria for recognising
impact craters An impact crater is a circular depression in the surface of a solid astronomical object formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal collapse, impact craters ...
. Furthermore, subsequent studies in the region have demonstrated that there is no evidence for an impact structure, and in fact a number of observations directly contradict the earlier impact structure proposals. The Maniitsoq structure is not recognised as an impact structure by the
Earth Impact Database The Earth Impact Database is a database of confirmed impact structures or craters on Earth. It was initiated in 1955 by the Dominion Observatory, Ottawa, under the direction of Carlyle S. Beals. Since 2001, it has been maintained as a not-for-pr ...
.


Impact structure proposal

Garde ''et al''. suggested the presence of a ~ scale impact structure, formed by the impact of a large comet or meteorite, in the Maniitsoq region. They argued that consensus accepted diagnostic criteria for recognising impacts should be relaxed when searching for particularly large, ancient, and eroded impacts, and instead suggested the presence of an impact structure on the basis of the following observations: 1) the presence of an irregular aeromagnetic anomaly; 2) curved ~100 km scale deformation patterns; 3) intense fracturing; 4) sheets of crushed rock without the presence of faults; 5) a central domain of homogenised rocks (the Finnefjeld Orthogneiss Complex); 6) remelting of rocks around the central domain; 7) formation of breccias; 8) proposed evidence of direct
K-feldspar Orthoclase, or orthoclase feldspar ( endmember formula K Al Si3 O8), is an important tectosilicate mineral which forms igneous rock. The name is from the Ancient Greek for "straight fracture," because its two cleavage planes are at right angles ...
melting; 9) planar elements within minerals; 10) presence of shear zones; 11) presence of
ultramafic Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are compos ...
sills (the Maniitsoq Norite Belt); 12) proposed widespread
hydrothermal alteration Metasomatism (from the Greek μετά ''metá'' "change" and σῶμα ''sôma'' "body") is the chemical alteration of a rock by hydrothermal and other fluids. It is the replacement of one rock by another of different mineralogical and chemical co ...
; 13) a coincidence of a
zircon Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of th ...
U-Pb ages at approximately 2975 million years ago (Ma). The impact was argued to post-date the end of deformation in the Maniitsoq region. The age was subsequently refined to 3000.9 ± 1.9 Ma based on mean age of five
orthogneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures an ...
samples suggested to represent rocks melted and hydrothermally altered by the impact. The proposal was criticised by Reimold ''et al''. for devising new criteria for recognising an impact, because it failed to meet existing criteria. Furthermore, they argued that the structure was not circular, that there was no evidence for
shock metamorphism Shock metamorphism or impact metamorphism describes the effects of shock-wave related deformation and heating during impact events. The formation of similar features during explosive volcanism is generally discounted due to the lack of metamorp ...
, and no geochemical evidence for an impact. In particular, they demonstrated that Garde ''et al''. had mistaken features commonly found in deformed and
metamorphosed Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...
terranes In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust fragment formed on a tectonic plate (or broken off from it) and accreted or " sutured" to crust lying on another plate. The crustal block or fragment preserves its ow ...
, such as
migmatite Migmatite is a composite rock found in medium and high-grade metamorphic environments, commonly within Precambrian cratonic blocks. It consists of two or more constituents often layered repetitively: one layer is an older metamorphic rock th ...
s and inclusion trails in quartz, for shock features, such as microbreccias and planar deformation features.


Evidence against an impact

Subsequent studies in the Maniitsoq region demonstrated that deformation in the region continued after the proposed impact age, with major metamorphic and deformation events at ∼2.86–2.70 Ga and ~2.55 Ga. Extensive deformation was noted both near the proposed impact centre and in ultramafic rocks previously suggested to be post-tectonic. Kirkland ''et al''. noted that it was difficult to reconcile the preservation of a circular impact structure and other proposed impact related features with the severe deformation that followed, and instead interpreted the 'impact' features as the result of multiple phases of high-grade
metamorphism Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of ...
and partial melting. Further
zircon Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of th ...
U-Pb dating also contradicts an impact model. The ages of rocks interpreted as impact melts within the impact structure are indistinguishable from the ages of the unaffected rocks from outside the impact structure. This requires that the impact coincidentally occurred at the same time as major (non impact-related) crustal formation in the region, which Gardiner ''et al''. consider unlikely. Furthermore, Gardiner ''et al''. note the presence of a second homogeneous body of orthogneiss further east within the Akia terrane, the Taserssuaq Orthogneiss Complex, which formed at 2982 Ma and contains homogeneous gneisses and magnetic anomalies that are very similar to the Finnefjeld Orthogneiss Complex, interpreted to be the centre of the impact structure. This orthogneiss complex is too young to have formed in response to the proposed impact, and demonstrates that similar orthogneiss complexes and magnetic anomalies can be generated without an impact event. Dating of metamorphic zircon and rocks formed during high temperature metamorphism at ~3 Ga, indicate that the metamorphic event lasted for >40 million years, which is too long to have been caused by a single impact. Instead, the metamorphism and deformation is better explained by endogenic (terrestrial) processes, such as
stagnant lid Lid tectonics, commonly thought of as stagnant lid tectonics, is the type of tectonics that is believed to exist on several planets and moons in the Solar System, and possibly existed on Earth during the early part of its history. The lid is the eq ...
processes or an ultra-hot orogenic event. Finally, new dating of the ultramafic intrusions of the Maniitsoq Norite Belt shows that these formed at 3013 Ma, and are therefore too old to have been generated by the impact event. Further evidence against an impact origin comes from analyses of
oxygen isotopes There are three known stable isotopes of oxygen (8O): Oxygen-16, , Oxygen-17, , and Oxygen-18, . Radioactive isotope, Radioactive isotopes ranging from to have also been characterized, all short-lived. The longest-lived radioisotope is with a ...
within the ultramafic intrusions of the Maniitsoq Norite Belt, which show no evidence of the widespread hydrothermal alteration asserted to have been caused by the impact. This is supported by geochemical and petrographical observations from the same rocks, which show that most rocks were largely dry, with only limited local hydrothermal alteration occurring adjacent to intrusions of much younger granitic rocks. Due to the reasons outlined above, the Maniitsoq Structure is widely believed not to have formed due to a giant impact, and is instead interpreted to reflect terrestrial tectonic processes.


See also

*
List of possible impact structures on Earth This is a list of possible impact structures on Earth. More than 130 geophysical features on the surface of the Earth have been proposed as candidate sites for impact events by appearing several times in the literature and/or being endorsed by the ...


References

{{reflist, 2 Geology of Greenland Possible impact craters on Earth Archean geology