Breccia dikes in the Azuara impact structure
In impact structures, breccia dikes have played an important role in the understanding of the impact cratering process [see, e.g., Lambert, P. (1981): Breccia dikes: geological constraints on the formation of complex craters. - In: Multi-ring Basins (R. B. Merrill and P. H. Schultz, eds.) Proc. Lunar Planet Sci., 12A, 59-78, New York (Pergamon Press)]. They have been reported from many impact structures, and they have in detail been investigated e.g. for the Rochechouart (France) and Azuara (Spain) structures.
In the Azuara structure the abundance of impact breccia dikes and their great variability is striking [see FIEBAG, J. (1988): Contributions to the geology of the Azuara structure: Geological mapping in the area between Herrera de los Navarros and Aladrén and SE of Almonacid de la Cuba, and special investigations of the breccias and breccia dikes in the light of the impact origin of the Azuara structure. - Doctoral thesis, Univ. Würzburg, 271 pp., Würzburg (in German)].
It is generally suggested that for the most part the breccia dikes are formed in the excavation stage by injection of brecciated material into the walls and the floor of the expanding excavation cavity. Later formations of breccia dikes in the modification stage incorporating earlier formed ones may lead to generations of breccia dikes.
Fig. 1. Breccia dikes in Liassic limestones south of Belchite. Frequently, systems with roughly orthogonal breccia dike orientation can be observed in the Azuara (and also in the Rubielos de la Cérida) structure. We call them H-type dikes, and we suggest the systems have originated from coaction of compression and subsequent tension in the cratering process (Fig. 2).
Fig. 2. Simple model of H-type dike formation
Fig. 3. H-type breccia dike system in Jurassic limestone. Jaulín, Azuara northern rim zone.
Fig. 4. Breccia dike generations: The younger dike partly runs within the older one. Whitish Muschelkalk dike within Keuper dike in Carniolas host rock (also see Ernstson & Fiebag 1992). Monforte de Moyuela; coin diameter 23 mm. Dikes-within-dikes are known also from the Vredefort impact structure.
Fig. 5. Large breccia dike composed of Mesozoic (?Keuper) and ?Lower Tertiary material cutting through Paleozoic siltstones. Near Santuario Virgen de Herrera. In earlier mappings the dike was interpreted as a tectonic graben structure with 200 m throw.
Fig. 6. Extended breccia dike system in Dogger limestones. The sharp-cut emplacement of the dikes excludes any karstification processes. - Barranco de Bocafóz, near Almonacid de la Cuba.
Fig. 7. Polymictic breccia dike cutting through Malmian limestones. The dike can be traced in the field over nearly 300 m. Three dike generations can be differentiated. Many components are themselves breccias. Azuara impact structure; north of Muniesa.
Fig. 8. Breccia dikes cutting through brecciated (mortar texture) Muschelkalk limestone. Azuara impact structure; Monforte de Moyuela. Coin diameter 23 mm.
Figs. 9, 10. Breccia dikes in Malmian limestone. Note the sharp-edged splinters within the dike (arrows) incompatible with any karst phenomena. Fuendetodos; Azuara impact structure.
Fig. 10. Another aspect. Photo-cap diameter 50 mm.
Fig. 11. Breccia dikes in Liassic limestone. Match-box length 40 mm. Blesa, Azuara impact structure.
Fig. 12. System of breccia dikes and breccia pockets (dark) in light Malmian limestones. The origin of the dark material is unknown. Small quarry between Muniesa and Ventas de Muniesa. Hammer length 42 cm.
Fig. 13. Crossing breccia dikes (dike generations) in Malmian limestone. The first emplaced dike (N – S trending) is cut by the younger dike showing ribbon texture probably originating from chemical interactions between dike material and the host rock. Azuara impact structure; near Ventas de Muniesa, Corral de Cámaras. Match-box length 40 mm.
Typical breccia pockets in rocks from the Azuara impact structure
Frequently, breccia dikes in the Azuara structure merge into characteristic breccia pockets.
Fig. 12. Breccia dikes and breccia pockets in Liassic limestone, near Belchite.
Fig. 14. Breccia dikes and breccia pockets in Liassic limestone, near Belchite.
Fig. 15. Breccia pocket in Malmian limestone, near Ventas de Muniesa.
Fig. 16. Breccia dikes and breccia pocket in Muschelkalk limestone, near Olalla.
Fig. 17. Breccia pipe injected from below into Paleozoic schists. Note the ring-wall of uplifted country rock. Azuara impact structure; near Santa Cruz de Nogueras. Coin diameter 23 mm.
Fig. 18. Azuara impact structure, near Nogueras: Photomicrograph of microscopic breccia dike with bifurcation; plane parallel light and crossed nicols. Sandstone fragment from a shocked polymict dike breccia. Note that the thin dike cuts through several quartz grains (arrows). - The field is 2.5 mm high.
Figs. 19, 20. Impact dike breccias from the Azuara and Charlevoix impact structures. Here: Azuara polymictic dike breccia; near Santa Cruz de Nogueras. Note the grading of the components and the distinct flow texture. Fig. 18: Comparable texture in a dike breccia from the Charlevoix impact structure (sample courtesy of J. Rondot). Similar breccias have been described to occur also in the Sierra Madera impact structure (Wilshire et al. 1972).
Fig. 20.