Endogenous and nonimpact origin of the Arkenu circular structures (al-Kufrah basin—SE Libya)

Authors

  • Corrado CIGOLINI,

    Corresponding author
    1. Dipartimento di Scienze della Terra, Università degli Studi di Torino, Via Valperga Caluso 35, 10125 Torino, Italy
    2. NatRisk, Centro Interdipartimentale sui Rischi Naturali in Ambiente Montano e Collinare, Università degli Studi di Torino, Torino, Italy
      Corresponding author. E-mail: corrado.cigolini@unito.it
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  • Mario DI MARTINO,

    1. INAF–Osservatorio Astronomico di Torino, 10025 Pino Torinese, Italy
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  • Marco LAIOLO,

    1. Dipartimento di Scienze della Terra, Università degli Studi di Torino, Via Valperga Caluso 35, 10125 Torino, Italy
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  • Diego COPPOLA,

    1. Dipartimento di Scienze della Terra, Università degli Studi di Torino, Via Valperga Caluso 35, 10125 Torino, Italy
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  • Piergiorgio ROSSETTI,

    1. Dipartimento di Scienze della Terra, Università degli Studi di Torino, Via Valperga Caluso 35, 10125 Torino, Italy
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  • Marco MORELLI

    1. Fondazione Prato Ricerche, Museo di Scienze Planetarie, 59100 Prato, Italy
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Corresponding author. E-mail: corrado.cigolini@unito.it

Abstract

Abstract– The twin Arkenu circular structures (ACS), located in the al-Kufrah basin in southeastern Libya, were previously considered as double impact craters (the “Arkenu craters”). The ACS consist of a NE (Arkenu 1) and a SW structure (Arkenu 2), with approximate diameters of about 10 km. They are characterized by two shallow depressions surrounded by concentric circular ridges and silica-impregnated sedimentary dikes cut by local faults. Our field, petrographic, and textural observations exclude that the ACS have an impact origin. In fact, we did not observe any evidence of shock metamorphism, such as planar deformation features in the quartz grains of the collected samples, and the previously reported “shatter cones” are wind-erosion features in sandstones (ventifacts). Conversely, the ACS should be regarded as a “paired” intrusion of porphyritic stocks of syenitic composition that inject the Nubia Formation and form a rather simple and eroded ring dike complex. Stock emplacement was followed by hydrothermal activity that involved the deposition of massive magnetite–hematite horizons (typical of iron oxide copper-gold deposits). Their origin was nearly coeval with the development of silicified dikes in the surroundings. Plugs of tephritic-phonolitic rocks and lamprophyres (monchiquites) inject the Nubian sandstone along conjugate fracture zones, trending NNW–SSE and NE–SW, that crosscut the structural axis of the basin.

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