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N.28 di martino-impact-and-non-impact-craters-in-eastern-sah


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Talk of the "International Workshop on Paolo Farinella (1953-2000): the Scientists, the man", Pisa, 14-16 June 2010

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N.28 di martino-impact-and-non-impact-craters-in-eastern-sah

  1. 1. IMPACT AND NON-IMPACT CRATERS IN EASTERN SAHARA Mario Di Martino INAF – Osservatorio Astronomico di Torino
  2. 2. “Tunguska 96” conference Bologna July 15-17, 1996
  3. 3. The Gilf Kebir region and surroundings Libyan Desert Glass Great Sand Sea BP EGYPT Kebira “impact crater” Oasis El-Baz volcanic crater LIBYA Gilf Kebir Plateau “Impact crater field” Arkenu ”impact craters” J. Arkenu J. Uweinat SUDAN
  4. 4. The “trigger”
  5. 5. “The largest impact crater field on the Earth” GKCF10
  6. 6. November 2005 expedition We carried out fieldwork on 7 of 13 craters identified as impact craters, namely GKCF 1, GKCF 6, GKCF 7, GKCF 8, GKCF 11, GKCF 12, GKCF 13, and we collected rock samples from GKCF 1, GKCF 7, GKCF 11, GKCF 13, on which petrographic studies has been in part performed. Also some other similar circular structures in the surroundings have been examined. GKCF 13 (950 m) GKCF 1 (630 m) GKCF 12 (500 m) GKCF 11 (1.200 m) GKCF 6 (20 m) GKCF 7 (40 m) GKCF 8 (75 m)
  7. 7. Pseudo-shatter cones (1) GKCF 1 GKCF 13 Sudbury GKCF 1 Gosses Bluff GKCF 1 GKCF 1 El-Baz volcanic crater
  8. 8. Pseudo-shatter cones (2) Dashed lines are early Holocene W and NW flows, solid lines are late Holocene N and NE flows. Brookes, I.A. Geomorphology 56, 155, 2003.
  9. 9. No Planar Deformation Features (PDF) Breccias found around most of the structures we visited could have been produced by fluidized sediments reaching the surface.
  10. 10. CONCLUSIONS (1) The crater-like structures in Gilf Kebir area we studied are not of impact origin, but likely related to endogenic processes typical of hydrothermal vent complexes in volcanic areas, which may reflect the emplacement of subvolcanic intrusives.
  11. 11. BP (2.8 km) 1 Arkenu 2 (10.3, 6.8 km) Libyan “impact” structures Oasis (11.5 km)
  12. 12. CONCLUSIONS (2) The Arkenu craters are not of impact origin, but very probably the result of the intrusion of a paired nearly cylindrical subvolcanic stocks coupled with ring dike injections in the surroundings, accompanied by hydrothermal alteration and degassing. This process was followed by local structural adjustments, likely due to thermal contraction of the whole edifices along circular fractures. Erosion did its cycle and finally revealed the present architecture.
  13. 13. NON-IMPACT ORIGIN OF THE ARKENU CRATERS (LIBYA) M. Di Martino (1), C. Cigolini (2), L. Orti (3) 1) INAF-Osservatorio Astronomico di Torino, Italy 2) Dipartimento di Scienze Mineralogiche e Petrologiche, Università di Torino, Italy 3) Dipartimento di Scienze della Terra, Università di Firenze, Italy. Results presented at the: “Large Meteorite Impacts and Planetary Evolution IV” LPI conference Vredeford, South Africa, 17-21 August 2008 – Extended abstract
  14. 14. Huge impact crater found in Egypt The crater dwarfs the next largest known Saharan crater A giant crater made by a meteorite impact millions of years ago has been discovered in Egypt's western desert. (F. El-Baz and E. Ghoneim, Boston Univ.)
  15. 15. Lesson The analysis of satellite imagery is fundamental for looking for impact craters, but has to be considered only a preliminary step BUT Before claiming that a circular structure has been originated by a cosmic body impact, accurate field investigations and laboratory analyses are necessary
  16. 16. The Kamil crater Moon a “lunar” crater on the Earth Earth (Eastern Sahara) Luna Mars
  17. 17. The crater in false color images
  18. 18. Radar image (X band) by Cosmo-SkyMed satellite
  19. 19. The Kamil crater
  20. 20. Chunks of cosmic iron
  21. 21. The tracks of our ancestors Crater age ≤ 5,000 years
  22. 22. The expedition “Kamil 2010” February
  23. 23. The “baby” (83 kg)
  24. 24. Geophysical Data a) Digital elevation model with superimposed magnetic anomaly map detected after systematic searches and collection of meteorites >1 g. Maxima are localized along the northern, southeastern and southwestern ejecta rays where abundant microscopic melt particles of the projectile are interspersed within the ejecta blanket. b) A representative section of the crater based on digital elevation model and ground penetrating radar survey of the crater floor (lower panel). c) Diameter ≈ 45 m Depth ≈ 12 m
  25. 25. The iron meteorite (1) Basic observations - density = 7.90 g/cm3 (typical density of iron meteorites) - Very fresh appearance, no fusion crust, no “heat alteration zone”, “twisted” edges --> the examined specimens, except one, are fragments formed during the meteorite impact event (“schrapnels”) “twisted” edges “twisted” edges
  26. 26. The iron meteorite (2) Macroscopic internal structure: Polished endcut etched with Nital o FeCl3 - No Widmannstätten pattern; - Abundant inclusions of schreibersite Schreibersite [(Fe,Ni)3P, troilite (FeS) and daubreelite (FeCr2S4) with a large rim of kamacite; Schreibersite - Evidences of shock effects (brecciation, shear zones); Brecciation
  27. 27. An unusual iron meteorite Chemical analysis Iron meteorites with Ni contents between 15 and 25 Ni = 198 mg/g wt%, Ir < 1g g-1, Ge 100 -150 g g-1 and Ga 30-60 g Co = 7.5 mg/g g-1 are very rare. The one most similar is Morradal (Norway), “ungrouped” iron. Ga = 49.5 g g-1 Ge = 121 g g-1 Ir = 0.39 g g-1 (the following trace elements have been also determined: V, Cr, Cu, Zn, As, Mo, Ru, Rh, Pd, Sn, Sb, W, Re, Pt, Au)
  28. 28. An unusual iron projectile 1) A very rare type of Ni-rich iron meteorite 2) A very fresh and moderately shocked iron meteorite, structurally transitional between the ataxites and the plessitic octahedrites 3) Only one individual meteorite (83 kg) has been found. The other meteorite fragments (shrapnels) have been formed during the impact event 4) The estimated total mass of the impactor is of the order of 5-10 x 103 kg, corresponding to a pre-atmospheric mass of ~20-40 x 103 kg.