| The type locality of the peculiar curved joint pattern (Images A, B) has been found by H. Müller in the course of his diploma thesis mapping at the south-western ring of the Azuara impact structure (UTM coordinates, 684500/4555400, near Moneva) some 15 km from its center. The exposure shows fossiliferous Dogger limestones, which have undergone strong brittle fracturing. The joint sets under discussion are well exposed by their strong curvature. Two evidently conjugate sets with parallel strike form a system, which is nearly symmetrical to the vertical, and cut the rock into bars of approximately rhomboid shape. This often results in a rhomboid-within-rhomboid structure. Small displacements with slickensides parallel to dip have been observed to occur on the order of centimeters. | |
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| Proceeding with field examinations, more joint systems with quite similar shape were found throughout the ring terrain of the Azuara structure. However, in contrast with the sets in Image A , the curved joints in Image C (south of Belchite) display counter convexity, and Image D (near Almonacid de la Cuba) shows the phenomenon on a smaller scale with a more irregular pattern. | |
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E: All locations are displayed in Image E where the strike directions of the curved sets are plotted. Although statistically only weakly exemplified, there is a trend of radial strike related to the center of the impact structure |
 Discussion. – In contrast to well established rhomboid structures resulting from the intersection of linear joints, strongly curved conjugate joint sets producing rhomboid-within-rhomboid structures are virtually unknown up to now. In a proceedings paper, pp. 257-263 (Image F), Müller and Ernstson excluded a relation to listric faulting, a formation by sedimentational and diagenetic processes, and presented a model of a dynamic formation which considers the modulation of running fractures in the impact cratering process. According to this model, the stress field of the shock-driven excavation flow combines with the stress field of the rising rarefaction pulse to a time-varying stress field causing the propagation of fractures along curved paths. Such a process is well known in experimental fracture mechanics: The modulation of a running fracture by ultra-sonic waves produces an undulating fracture surface.In our paper, we compute and show that in the early (excavation) stage of the impact cratering process, the conditions of the formation of curved conjugate joint sets can be met locally and during a short period of time.The model is not only consistent with the Azuara observations (radial strike with respect to the center, convex and concave curvature, different radii of curvature, rhomboid-within-rhomboid structures) but also predicts curved joint sets to belong to the regular structural inventory of impact craters. |
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Azuara impact structure (Spain), Ries impact structure (Germany): impact as a geologic process
| A few kilometers outside the northern ring of the Azuara impact structure near Belchite, a handful of isolated large blocks of Jurassic limestones emerge from the post-impact Upper Tertiary Ebro basin sediments. Quarrying in these blocks has enabled instructive insight into the drastic impact deformations experienced by very large rock volumes. | |
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| Image A shows part of a large quarry located at UTM coordinates 0687000, 4583000. The visible length in the image is roughly 300 m. The limestones are drastically destroyed through and through to form a more or less continuous breccia displaying grit (gries) brecciation and mortar texture (see Images B – E). |  C |
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| Comparable strong and continuous deformations (Images F, G) can be observed in a limestone quarry located in another block at UTM coordinates 0683000, 4583000. | |
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| H and I Ries impact structure; Iggenhausen quarry | |
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| Comment: The Azuara region and the Jurassic limestones underwent Alpidic tectonics with some folding and block faulting, but we emphasize that Alpidic tectonics can not possibly have caused these disastrous deformations over hundreds of meters. Impact cratering is the only reasonable process to have produced this impressive geologic scenario, and the same deformations are well known to occur in large allochthonous limestone megablocks ejected from the 25km-diameter Ries impact structure (Germany) (Images H, J; Iggenhausen quarry).We suggest that those geologists from the Zaragoza university and the Center of Astrobiology (Madrid) vehemently refusing an Azuara impact visit these highlighting outcrops. Since they like to contrast the Azuara structure with the Ries crater (see their MAPS paper referred to in the Controversy section), they will get a lot of illustrative material.There is one more point we want to refer to. As already said, impact is the only reasonable geologic process that explains these desastrous and voluminous deformations. In other words, there’s actually no need for the well documented strong shock effects in Azuara polymictic breccias to establish Azuara as an impact structure (see below in the Archives and http://www.impact-structures.com/impact-spain/the-azuara-impact-structure/shock-effects-shock-metamorphism-in-rocks-from-the-azuara-impact-structure/ ). The outcrops under discussion here are as well a convincing proof.Usually, the impact nature of a structure under discussion is established by the occurrence of shock metamorphism. Reasonably, it is argued that there are no endogenetic processes known to produce, e.g., diaplectic glass or planar deformation features (PDFs) in quartz. Likewise, we argue that there are no endogenetic geologic processes known to have catastrophically destroyed the Jurassic limestones near Belchite. Therefore, geologists should be aware of their competence to establish in some cases an impact structure from pure field evidence. The time has come to give up the very limited point of view of some impact researchers that TEM analyses of PDFs or geochemical signature of the projectile are the ultimate requirement for establishing an impact structure. |
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Rubielos de la Cérida impact structure (Spain): impact melt glass from the central uplift
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| The glass shown in A, B (B: the field is 14 mm wide) is coating a sandstone exposed in the central uplift of the Rubielos de la Cérida impact structure. The glass has a greenish to whitisch color and is transparent to milky. In thin section (C, D (xx nicols) – the field is 6 mm wide), the sandstone shows heavily damaged, and intense cataclastic flow texture is observed to merge with the glass. Quartz grains are strongly fractured and show multiple sets of planar fractures (PFs) and planar deformation features (PDFs). | |
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| Interpretation: Despite the occurrence of shock features in the sandstone, the glass probably did not form by shock melting. We suggest frictional melting by extreme dynamic metamorphism in the impact event (excavation or – more probably – modification stage when the uplift formed) and the glass to be pseudotachylite. Temperatures in excess of 2,000 °C were probably required for the homogenization of this glass (David Griscom, pers. com.). The location of this spectacular exposure of the glass-bearing sandstone remains secret for the moment in order to prevent it from destruction by rock hunters. |
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Rubielos de la Cérida impact structure, Spain: at the crater floor
This peculiar fold is exposed in a region of an extended megabreccia near the village of Barrachina in the Rubielos de la Cérida impact structure. The fold is portrayed by a competent, however heavily brecciated Lower Tertiary limestone layer. The core of the fold is a pulp of nearly pulverized carbonate rock without any regular internal structure. Only a few limestone fragments are preserved.
Interpretation: The exposure is assumed to be located at or near the crater floor of the Rubielos de la Cérida impact structure (for more details see:
where giant rock masses moved in the excavation and modification stage of impact cratering to form the now exposed megabreccia. The fold is interpreted to be the result of a high-pressure injection of extremely brecciated material from below. A tectonic origin of this peculiar structure is hardly to understand. Local geologists (from the Zaragoza university and the Center of Astrobiology, Madrid) suggest collapse by dissolution of gypsum to have produced the megabrecciation – need we comment?
Azuara impact structure (Spain) – Ries impact structure (Germany)
Shortly after the impact … A |
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| The exposure in image A (details in B, C) results from the construction of an irrigation channel and is located near Blesa village about 14 km from the center of the Azuara impact structure. | |
 BThe channel cuts through highly fractured and brecciated Liassic limestone megablocks in sharp and steep contact with well-bedded Tertiary sands. Near the contacts, a few disintegrated limestone blocks are floating in the sands. The sand is composed of predominantly calcite and quartz grains and some altered glass fragments. In thin section (D, plane polarized light; the field is 1 cm wide), the quartz grains show to be mostly sharp-edged indicating fragmentation and short transport. |
Many quartz grains display shock features like multiple sets of planar fractures (PFs) and multiple sets of planar deformation features (PDFs). |
| Interpretation: The peculiar contact between the sands and the overhanging and highly fractured rocks gives evidence of an obviously sudden and very short-term depositional process. The highly brecciated and partly overhanging flanks of the limestone megablocks would not have survived any substantial period of time, and faults can basically be excluded. Therefore, we suggest that the outcrop reflects the earliest phase of the post-impact sedimentation at the crater floor shortly after the impact. In some respects, the sandy unit may be compared with the so-called «graded unit» which has been found as a 17 m core section in the research borehole Nördlingen 1973 in the Ries impact structure (Germany). The «graded unit» occurs within the crater between the suevite impact breccia and aquatic sediments, and it is assumed to be the result of a single-phase sedimentation. Alternative processes consider airfall of ejected impact material or a turbidity current-type transport mechanism in water or steam. Both are possible explanations also for the deposition of the sandy unit in the Blesa irrigation channel, which is currently investigated in more detail. |
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Ries impact structure (Germany); Azuara and Rubielos de la Cérida impact structures (Spain)
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 CAguilón; Jurassic limestones (Azuara structure). Note the bedding in the base speaking against a tectonic fault.
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 Dnear Santa Eulalia; Muschelkalk limestones (Rubielos de la Cérida structure). Note the block of bedded limestone floating in the highly brecciated material. |
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| In all three cases, a tectonic interpretation of the layering offers considerable difficulties. | |||
Rubielos de la Cérida impact structure (Spain)
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Azuara impact structure, Spain: shock metamorphism
Highly shocked polymictic dike breccia (near Santa Cruz de Nogueras, 30660971E, 4553223N). Typical shock effects in the breccia are |
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 BB: Diaplectic glass; photomicrograph of a sandstone fragment completely transformed to diaplectic quartz; plane polarized light and xx nicols. Note that there are a few holes in the thin section not to be confused with diaplectic quartz grains. The field is 600 µm wide.
C: Planar deformation features (PDFs) in quartz grains; sandstone fragment from the shocked breccia. Photomicrograph, plane polarized light; the field is 800 µm wide. Note the large number of grains showing PDFs, their high density, the small spacing and the multiple sets. Up to five sets of different PDF orientation per grain have been observed in the dike breccia.
D: Planar fractures (PFs; cleavage) in quartz. Photomicrograph, xx nicols; the field is 450 µm wide. Note that at least six sets of different orientation can be observed. Cleavage in quartz is very uncommon in tectonically deformed quartz. In rare cases, rhombohedral fracturing is observed to occur in rocks which underwent strong regional metamorphism. In rocks from impact structures, PFs in quartz belong to the regular shock inventory. E: Kink bands in biotite from the shocked polymictic breccia. Photomicrograph, crossed nicols; the field is 840 µm wide. – Although kink bands can form under static conditions of strong regional metamorphism, the high frequency of the kink bands shown here, their narrow width, and their high kink-angle asymmetry point to shock deformation. The shock-metamorphic effects shown here correspond to a broad range of shock pressures. The melt glass, however, shows that parts of the breccia must have experienced shock peak pressures exceeding 500 kbars (50 GPa).
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ERubielos de la Cérida impact structure, Spain:
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Rubielos de la Cérida impact structure, Spain:
Part of a large (some 300 m size) quarry exposing limestones (Muschelkalk Fm.) drastically destroyed through and through (A).
Within the completely brecciated rocks (displaying gries brecciation and mortar texture), white blocks (up to cubic-meter size) of carbonate material (B) are intercalated.
The low-density, highly porous material shows a distinct vesicular texture (C – the field is 7 mm wide).
Interpretation: A compressive strength of perhaps 150 – 200 MPa (= 1.5 – 2 kbar) for these massive and dense Muschelkalk limestones assumed, they must have experienced pressures clearly exceeding these values not only locally but throughout the huge volume. Whereas a tectonic origin can be excluded without any doubt, deformations like that are expected to occur in the cratering process (excavation and/or modification stage) of large impact structures. The intercalated white vesicular material is considered to be the relics from decarbonization and/or carbonate melt produced by shock or strong frictional heating.
¿No son los rasgos de deformación planar (PDFs) curvos, rasgos de deformación planar?
Las PDFs son estructuras de deformación por choque planares presentes en los minerales (especialmente en el cuarzo) bajo la forma de lamelas isotrópicas de espaciado pequeño que siguen los planos cristalográficos (Fig. 1). Pueden estar decoradas por inclusiones de fluidos o de minerales como efecto del recalentamiento.
Fig. 1. Estructuras de deformación planar en el cuarzo; estructura de impacto de Ries. Microfotografía realizada con nícoles cruzados; la anchura de campo es de 460 µm.
De acuerdo con los conocimientos actuales, las PDFs no se pueden formar por procesos geológicos endogenéticos (esto es, ligados a la actividad terrestre “normal”). De este modo, la presencia de PDFs juega un papel importante en el establecimiento o verificación de estructuras de impacto auténticas. Llegados a este punto, surge un problema. En el pasado, las PDFs curvas habían sido repetidamente declaradas como estructuras no ligadas a un origen por impacto. Podemos mencionar a este respecto el artículo redactado por Reimold, W.U. & Koeberl, C. (2000): Critical Comment on: A.J. Mory et al. ‘Woodleigh, Carnavon Basin, Western Australia: A New 120 km Diameter Impact Structure EPSL v. 184, pp. 353-357. En dicho artículo podemos leer: “No obstante, los terminus “planar” y “localmente curvados” representan una contradicción. Existen diversos artículos en la literatura (e.g. [2, 3] que demuestran que las lamelas que no son planares no son evidencia diagnóstica de choque”.
¿Cuáles son los hechos? En general, las PDFs presentes en el cuarzo acostumbran a ser rectas y paralelas (Fig. 1) debido a la existencia de planos cristalográficos en la red no deformada del cristal. No obstante, y como es bien conocido, los cristales de cuarzo pueden sufrir deformaciones plásticas que dan como resultado una deformación de la red cristalina. En sección delgada estas deformaciones son fácilmente visibles bajo la forma de extinción ondulante al rotar la platina del microscopio polarizante. En este caso, y de un modo obvio, los “planos” cristalográficos del cristal no son ya planos.
¿Qué sucede cuando una onda de choque afecta granos de cuarzo que previamente ya estaban deformados y genera PDFs? Pensamos que de acuerdo con la definición de las PDFs, éstas se desarrollaran con respecto a la red cristalina deformada y, por tanto, serán curvas.
¿Qué sucede cuando una onda de choque afecta a un cristal de cuarzo no deformado y produce PDFs rectas y paralelas, experimentando con posterioridad una deformación plástica post-choque? De acuerdo con los críticos de las PDFs curvas, es cierto que la red esta deformada mostrando extinción ondulante, pero las PDFs deben permanecer rectas. Pero, no obstante, como ya no siguen la orientación cristalográfica, estas PDFs no son PDFs. ¿Todo bien? Nosotros, al contrario, vemos los hechos de una manera más simple y predecimos PDFs curvadas que reflejan la deformación de la red cristalina.
Dado que un impacto puede afectar a un objetivo previamente tectonizado, no deberían ser sorprendentes PDFs curvas en cuarzos pre-choque deformados plásticamente. Por otra parte, PDFs curvas desarrolladas por deformación plástica post-choque son de esperar durante el mismo proceso de impacto (durante los estadios de excavación y modificación) no excluyendo una acción tectónica posterior con el consecuente curvado de las PDFs.
Fig.2. Dos conjuntos de PDFs ligeramente curvadas en un cuarzo. Microfotografía realizada a nícoles cruzados; la anchura de campo es de alrededor de 1.5 mm. – Canto de cuarcita chocado procedente del cráter 004 del campo de cráteres de impacto de Chiemgau (ver. http://www.chiemgau-impact.com).
Concluimos y expresamos (lo cual hemos estado realizando desde have varios años) que las PDFs curvadas (Fig.2) deben pertenecer al inventario de choque de las estructuras de impacto y que ellas deben ser consideradas diagnósticas si el curvamiento está correlacionado con la extinción ondulante proveniente de la orientación cristalográfica. En la Fig. 3 mostramos microfotografías obtenidas mediante rotación de la platina del microscopio de polarización (a nícoles cruzados), donde puede observarse de manera clara como las PDFs curvadas esran íntimamente asociadas con la extinción ondulante.
Fig.3. PDFs curvadas estrechamente relacionadas con la extinción ondulante de un grano de cuarzo. Microfotografías tomadas mediante la rotación a intervalos de la platina del microscopio de polarización. Se trata del mismo canto de cuarcita de la Fig. 2.
Entre Marzo y Abril del año 2000, la Dra. Ann Therriault (del Geological Survey of Canada, colobarodora del Dr. R.A.F. Grieve) realizó un extenso análisis sobre las PDFs procedentes de muestras sedimentarias de la estructura de impacto española de Azuara. Analizó cuarzos de un clasto intensamente chocado procedente de un dique de brechas y de clastos procedentes de la Fm. Pelarda. Entre los 48 conjuntos de PDFs identificados halló las siguientes direcciones: 9.3 % basales, 40.7 % ω, 12.9 % π, 12.9 % ξ, 7.4 % r,z y subordinados s, x and m. La Dra. Ann Thierrault mencionó de manera explícita en su informe “….algunas PDFs curvadas!!” (ver Fig. 4, y para el histograma de las PDFs: http://www.impact-structures.com/impact-spain/the-azuara-impact-structure/shock-effects-shock-metamorphism-in-rocks-from-the-azuara-impact-structure/).
Fig. 4. Conjuntos de PDFs curvadas presents en un cuarzo; cantos de cuarcita procedentes de la Fm. Pelarda, eyecta de la estructura de impacto de Azuara (España).
Queremos añadir que a pesar de todo existen otros investigadores de impactos que si estan al corriente de los peculiares procesos post-impacto que acaecen en las rocas del objetivo. Queremos destacar aquí el abstract sobre la estructura de impacto de Charlevoix publicado en el LPSC XXXV (2004) (http://www.lpi.usra.edu/meetings/lpsc2004/pdf/1730.pdf ). En la Fig. 1 del artículo pueden observarse unas preciosas PDFs curvadas paralelas a {0001} ( que hemos copiado en nuestra Fig. 5). En el mismo texto, los autores del abstract, citan: “ Las estructuras de deformación planar (PDFs) curvadas pueden ser el reflejo del continuo curvamiento de la red cristalina (Fig.1)”
Fig. 5. Estructuras de deformación planar (PDFs) curvadas halaldas en un cuarzo de la estructura de impacto de Charlevoix (de Trepmann y Spray, 2004).
De hecho y estrictamente hablando, “las estructuras de deformación planar curvas” son una contradicción de términos. Esta circunstancia puede ser corregida si pensamos que en realidad las PDFs hacen referencia a la red cristalina y no a una definición matemática de planos. Ante esto, parece necesario una redefinición de los términos.