Editing Arkenu structures (section)

{{Short description|Pair of geological features in Libya}}
{{Infobox terrestrial impact site
| name            = Arkenu structures
| other_name      = 
| photo           = Arkenu craters.jpg
| photo_size      = 
| photo_alt       = 
| photo_caption   = [[Landsat]] image of the Arkenu structures; screen capture from [[NASA World Wind]]
| map             = Libya
| map_alt         = 
| map_caption     = 
| map_size        = 
| location        = 
| label           = 
| label_position  = 
| coordinates     = 
| coordinates_ref = 
| confidence      = 
| diameter        = {{convert|10|km|mi}} and {{convert|6.8|km|mi}}
| depth           = 
| rise            = 
| imp_size        = 
| age             = <120 Ma
| exposed         = Yes
| drilled         = No
| bolide          = 
| translation     = 
| language        = 
| pronunciation   = 
| topo            = 
| access          = 
| country         = [[Libya]]
| state           = 
| province        = 
| district        = 
| municipality    = [[Jabal Arkanu]]
| module          = 
}}
The '''Arkenu structures''', also known as the '''Arkenu craters,''' are a pair of prominent circular geological structures in eastern [[Libya]]. The structures are approximately {{convert|10|km|mi}} and {{convert|6.8|km|mi}} in diameter, and lie about {{convert|70|km|mi}} west of [[Jabal Arkanu]] on the eastern margin of the al-Kufrah Basin.<ref name="PaillouOthers2003a">Paillou P., A. Rosenqvist A., J.M. Malezieux, B. Reynard, T. Farr, and E. Heggy (2003) ''Discovery of a double impact crater in Libya: The astrobleme of Arkenu.'' Comptes Rendus Geoscience. vol. 335, no. 15, pp. 1059–1069.</ref><ref name="CigoliniOthers2012a">Cigolini, C, C Laiolo, and M Rossetti (2012) ''Endogenous and nonimpact origin of the Arkenu circular structures (al-Kufrah basin-SE Libya)'' Meteoritics & Planetary Science. 47(11):1772–1788.</ref> 

It has been argued that both structures were formed by simultaneous [[meteorite impact]]s. Field investigations by Dr. P. Paillou, Dr. A. Rosenqvist, and others reported the presence of impact [[breccia]]s at the structures' bottoms, [[shatter cone]]s pointing toward the center of the structures, and microscopic [[planar deformation features]] found in [[quartz]] grains of [[sandstone]]s outcropping in the structures. Based on these reports and other observations, it was proposed that both structures are extraterrestrial [[impact crater]]s that were formed simultaneously as a double [[impact event]] less than 140 million years ago ([[Jurassic]] or younger).<ref name=" PaillouOthers2003a"/> 

More recently, on the basis of field, [[petrology|petrographic]], and textural observations, it has been argued that the Arkenu structures are not in fact extraterrestrial impact craters. Field studies found a lack of identifiable shatter cones at both structures. Instead, the striations which were previously reported as shatter cones were identified as [[ventifact]]s created by wind erosion in sandstones. These striations are surficial features that are unrelated to fracturing of the sandstone, are consistently oriented with the prevailing [[Holocene]] wind patterns, and occur within and outside of, even distal to, both of the Arkenu structures. In addition, detailed petrographic analyses of rock samples from both of the Arkenu structures found a lack of any microscopic effect of [[shock metamorphism]] including a lack of planar deformation features in quartz grains and evidence of impact melting, or presence of glass. A lack of any apparent differences between the [[sedimentary rock]]s outcropping inside and outside these circular structures was found. Finally, field observations found silicified sandstone [[Dike (geology)|dikes]] and [[igneous rock]]s, such as [[syenite]], [[Porphyry (geology)|porphyries]], [[tephrite]]s, [[phonolite]]s, and [[lamprophyre]]s ([[monchiquite]]s) directly associated with each circular feature. Based on these and other observations, it was concluded that the Arkenu structures are [[Stock (geology)|stocks]] of porphyritic syenitic that have intruded the Nubia Formation to form rather simple and eroded ring dike complexes. [[Hydrothermal circulation|Hydrothermal activity]] that followed the intrusion of these ring dike complexes resulted in the formation of massive [[magnetite]]–[[hematite]] deposits and dikes of silicified sandstone.<ref name="CigoliniOthers2012a"/><ref name="DiMartinoOthers2008a">Di Martino, M, C Cigolini, and L. Orti (2008) ''Non-impact origin of the Arkenu craters (Libya)'' Large Meteorite Impacts and Planetary Evolution IV, 17–21 August, Vredefort Dome South Africa. abstract no. 3012, Lunar and Planetary Institute, Houston, Texas.</ref> As a result of this research, the Arkenu structures were removed from and are currently not listed in the [[Earth Impact Database]].<ref name="Anonymousnda">Anonymous (nd) [http://www.passc.net/EarthImpactDatabase/Africa.html ''Africa'' (Impact Craters)] {{Webarchive|url=https://web.archive.org/web/20151222204644/http://www.passc.net/EarthImpactDatabase/Africa.html |date=2015-12-22 }}, [http://www.passc.net/EarthImpactDatabase/index.html Earth Impact Database] {{Webarchive|url=https://web.archive.org/web/20150207114032/http://www.passc.net/EarthImpactDatabase/index.html |date=2015-02-07 }}, [https://www.unb.ca/passc/ImpactDatabase/ Planetary and Space Science Centre], [[University of New Brunswick]], New Brunswick, Canada.</ref>