Azuara impact structure

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The Azuara impact structure is a 35 to 40 kilometers large impact crater in northeastern Spain 50 kilometers south of Zaragoza . Stratigraphic considerations as well as paleontological dating methods assign the structure an age of 30 to 40 million years Before Present (BP) and thus place it in the Upper Eocene or Oligocene .

location

Morphological expression of the Azuara impact structure in the digital elevation map of Spain, scale 1: 250,000

The city of Azuara in the Spanish province of Zaragoza , after which the structure is named, is located in the center of the crater. The crater itself has a roughly circular shape with a distinct outer ring. The interior of the crater is covered by upper chalk sediments , but the outcropping conditions are excellent in the outer rim zone.

history

As early as 1980, Wolfgang Hamann made the first indication of a possible impact structure at Azuara. In the early 1980s, Johannes Fiebag discovered the first site findings. In 1985, Kord Ernstson and colleagues reported evidence of shock metamorphosis and the structure was then recognized as an authentic meteorite impact by Grieve and Shoemaker in 1994, by Hodge (also in 1994), and by Norton in 2002 .

Impact structures

Shocked polymictic breccia of the Azuara impact structure

The impact nature of the Azuara structure is documented by extensive polymictic and monomictic breccias , breccia ducts, extensive megabreccias, but also by ejecta, dislocated giant blocks, geophysical anomalies and the signs of shock metamorphosis. The latter are available as fused glasses, diaplectic glasses and planar elements (PDFs). The planar elements ( English : planar deformation features or abbreviated PDF) are found in breccias and breccia ducts , very often they occur in quartzite dumps in the ejecta ( Pelarda formation ).

Planar elements (PDFs) in quartz of the Azuara impact structure

Shocked crystals in the Azuara impact structure show planar elements. The crystallographically controlled microdeformation planes in the affected grains can be well represented in a histogram . In particular, the {10 1 3} and {10 1 2} levels are diagnostic and are generally considered to be a sure sign of deformation that has occurred in shock.

Controversial interpretation

As with other impact structures such as the Nördlinger Ries , the Vredefort Crater or the Sudbury Basin , the extraterrestrial cause of the Azuara impact structure was (and is) extremely controversial. Spanish geologists still categorically reject an impact event. They explain the shock effects as being of tectonic origin, the ejecta of the Pelarda Formation are in their eyes the remains of Quaternary alluvial rubble fans, and the impact breccias and breccia dikes are interpreted by them as karst and soil formations.

This opposition to an impact in Azuara was nourished by Langenhorst and Deutsch (1996) in their scientific study, in which they believe they do not recognize any signs of shock metamorphosis, but rather ascribe the structures in question to an Alpine thrust orbit. Azuara was then removed from the Canadian Impact Data Base . In other databases, however, Azuara is still listed as a recognized impact structure, for example in J. Moilanen (2009) and also in the Expert Database on Earth Impact Structures (EDEIS).

Accompanying impact structures

Impact glass from a polymictic breccia dike in Azuara

Since 1994 there has been speculation that Azuara is only part of a multiple impact, to which an elongated impact basin is added, which was probably excavated by a chain of impactors. This so-called Rubielos-de-la-Cérida basin contains all indications of an impact origin, be it its morphological configuration, its poly- and monomictic breccias, its megabreccias, its ejecta blankets, its signs of shock metamorphosis, its suevites and its impact melts.

Individual evidence

  1. ↑ Ernstson K., Hammann W., Fiebag J. and Graup G .: Evidence of an impact origin for the Azuara structure (Spain) . In: Earth and Planetary Science Letters . tape 74 , 1985, pp. 361-370 .
  2. ^ Grieve, RAF and Shoemaker, EM: The record of past impacts on Earth . In: T. Gehrels, Hazards Due to Comets and Asteroids (Ed.): Space Science Series . Univ. Arizona Press, Tucson, Arizona, USA 1994, p. 417-462 .
  3. ^ Hodge, P .: Meteorite craters and impact structures of the Earth . Cambridge University Press, Cambridge, UK 1994, pp. 124 .
  4. ↑ Ernstson, K. and Claudin, F .: Pelarda Formation (Eastern Iberian Chains, NE Spain): Ejecta of the Azuara impact structure. In: N. Jb.Geol.Paleont. Mh. 1990, p. 581-599 .
  5. ↑ Ernstson, K. and Fiebag, J .: The Azuara impact structure (Spain): new insights from geophysical and geological investigations . In: International Journal of Earth Sciences (Geologische Rundschau) . tape 81 (2) , 1992, pp. 403-427 .
  6. ^ Ernstson, K., Claudin, F., Schüssler, U. and Hradil, K .: The mid-Tertiary Azuara and Rubielos de la Cérida paired impact structures (Spain). In: Treb. Mus. Geol. Barcelona . tape 11 , 2002, p. 5-65 .
  7. ↑ Stöffler, D. and Langenhorst, F .: Shock metamorphism of quartz in nature and experiment: I. Basic observation and theory. In: Meteoritics . tape 29 , 1994, pp. 155-181 .
  8. ↑ Cortés AL, Diaz-Martínez E., Sanz-Rubio E., Martínez-Frías J. and Fernández C .: Cosmic impact versus terrestrial origin of the Azuara structure (Spain): A review. In: Meteoritics & Planetary Science . tape 37 , 2002, p. 875-894 .
  9. ↑ Langenhorst F. and German A .: The Azuara and Rubielos structures, Spain: Twin impact craters or Alpine thrust systems? 1996.
  10. ↑ Ernstson, K., Claudin, F., Schüssler, U., Anguita, F. and Ernstson, T .: Impact melt rocks, shock metamorphism, and structural features in the Rubielos de la Cérida structure, Spain: evidence for a companion to the Azuara impact structure . In: Impact markers in the stratigraphic record, 6th ESF-IMPACT workshop . abstract book: 23-24. Granada 2001.
  11. ↑ Ernstson, K., Rampino, MR and Hiltl, M .: Cratered cobbles in Triassic Buntsandstein conglomerates in northeastern Spain: An indicator of shock deformation in the vicinity of large impacts . In: Geology . tape 29 , 2001, p. 11-14 .