AZUARA IMPACT STRUCTURE (SPAIN) CURVED JOINT SETS: INDICATION OF IMPACT-INDUCED FRACTURING

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.
A B
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.
 C  D
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.

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.
A  B
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
 D  E
Comparable strong and continuous deformations (Images F, G) can be observed in a limestone quarry located in another block at UTM coordinates 0683000, 4583000.
 F  G
H and I Ries impact structure; Iggenhausen quarry
 H  I
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.

Rubielos de la Cérida impact structure (Spain): impact melt glass from the central uplift

 A  B
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).
 C  D
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.

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:

Fieldguide – Stages of Crater

Fieldguide – Stop 7

rubielos,

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

Shortly after the impact … A
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.
 B

The 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.

 C

 D

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.

Ries impact structure (Germany); Azuara and Rubielos de la Cérida impact structures (Spain): peculiar structural setting

 B (close-up)

Peculiar structural setting in autochthonous Jurassic limestones at the eastern rim of the Ries impact structure (Wemding; formerly Schneider quarry). Photos: July, 2001.
Interpretation: The strange abrupt change from horizontal layering to steeply dipping and strongly deformed limestone beds has resulted from horizontal thrust under very high overburden pressure in the excavation and ejection process.Similar strange deformations can be observed at the rims of the Azuara and Rubielos de la Cérida impact structures (Spain):
C
Aguilón; Jurassic limestones (Azuara structure). Note the bedding in the base speaking against a tectonic fault.
D
near Santa Eulalia; Muschelkalk limestones (Rubielos de la Cérida structure). Note the block of bedded limestone floating in the highly brecciated material.
In all three cases, a tectonic interpretation of the layering offers considerable difficulties.

Rubielos de la Cérida impact structure (Spain) – compressive signature – megabreccia

B

Megabrecciation of Jurassic limestones in the southern central uplift near Bueña. Note the chaotic criss-cross layering (A) and some “ghost” layering having survived the intense brecciation (B).
Interpretation: A distinct megabrecciation is a typical structural feature in the central uplift of complex impact structures and well known from many craters.The giant compression occurs in the modification stage of impact cratering, when the transient cavity collapses and large rock volumes undergo a centripetal accelleration towards the center of the structure.In the Rubielos de la Cérida impact structure, the enormous compressive signature with strong deformations up to continuous megabrecciation is evident nearly everywhere and can best be observed in cuts from road constructions.

Azuara impact structure, Spain: shock metamorphism

A comprehensive article on the Azuara shock effects with emphasis on the F. Langenhorst and A. Deutsch quarrel may be clicked HERE.

Highly shocked polymictic dike breccia (near Santa Cruz de Nogueras, 30660971E, 4553223N). Typical shock effects in the breccia are
A

A: Melt glass with vesicles, schlieren and mineral fragments; photomicrograph, plane polarized light and xx nicols. The field is 9 mm wide.

B
B: 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.CC: 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
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

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).

 

 

 

 

 

 

Rubielos de la Cérida impact structure, Spain:


A

B

C

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.

Are bent planar deformation features (PDFs) no PDFs?

PDFs are planar shock deformations in minerals (especially quartz) in the form of closely spaced isotropic lamellae following crystallographic planes (Fig. 1). They may be decorated by fluid and/or mineral inclusions as the result of annealing.

Fig. 1. Planar deformation features in quartz; Ries impact structure. Photomicrograph, crossed polarizers; the field is 460 µm wide.

According to current knowledge, PDFs cannot form in endogenetic geologic processes. Therefore, the occurrence of PDFs plays an important role in the establishment of authentic impact structures. And here, a problem turns out. In the past, curved PDFs have repeatedly been claimed to be of non-impact origin, and we mention for example the paper written by 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). In this paper we read:However, ‘planar’ and ‘locally curved’ represent a contradiction. There are several papers in the literature (e.g. [2,3]) that demonstrate that non-planar lamellae are not shock-diagnostic evidence.”

What are the facts? In general, PDFs in quartz are indeed straight and parallel (Fig. 1) due to crystallographic planes in an undeformed crystal lattice. However, as is well known, quartz crystals may undergo plastic deformations resulting in a deformation of the lattice. In thin section, these deformations are easily seen in the form of undulatory extinction on rotation of the stage of the polarization microscope. Obviously, the crystallographic “planes” in the crystal are no longer plane.

What happens when a shock wave impinges on an already plastically deformed quartz grain to produce PDFs in it? We think that according to definition, the PDFs will develop with respect to the deformed lattice, and, consequently, they must be bent.

What happens when a shock wave impinges on an undeformed quartz crystal to produce straight and parallel PDFs, and the crystal experiences a post-shock plastic deformation? According to the critics of bent PDFs, it’s true the lattice becomes deformed displaying undulatory extinction, but the PDFs must remain straight. As, however, they are no longer following the crystallographic orientation, these PDFs are no PDFs. Everything all right? We to the contrary see things much simpler and predict bent PDFs reflecting the deformation of the crystal lattice.

As an impact may affect a tectonically overprinted target, bent PDFs in pre-shock plastically deformed quartz must not be surprising. On the other hand, bent PDFs from post-shock plastic deformation are expected to develop in the impact cratering process itself (in the excavation and modification stages) not excluding a later tectonic overprint and further bending of PDFs.

Fig. 2. Two sets of slightly bent PDFs in quartz. Photomicrograph, crossed polarizers; the field is about 1.5 mm  wide. – Shocked quartzite cobble from crater 004 of the Chiemgau impact strewnfield (see. http://www.chiemgau-impact.com). 

We conclude and express (what we have been doing since many years) that curved PDFs (Fig. 2) must belong to the shock inventory of impact structures and that they may even be diagnostic if the bending is correlated with undulatory extinction proving crystallographic orientation. In Fig. 3 we show photomicrographs obtained from the rotation of the stage of a polarization microscope (xx polarizers), and we can clearly observe that the bent PDFs are intimately associated with the undulatory extinction.

Fig.3. Bent PDFs closely corresponding with undulatory extinction of the quartz grain. Photomicrographs taken by stepwise rotation of the stage of the polarization microscope. Same quartzite cobble as in Fig. 2.

In March and April 2000, Dr. Ann Therriault (Geological Survey of Canada, and co-worker of Dr. R.A.F. Grieve) made a very thorough PDF analysis of sedimentary samples from the Azuara impact structure in Spain. She analyzed quartz from a strongly shocked dike-breccia clast and from the Pelarda Fm. ejecta and found among 48 measured PDF sets 9.3 % basal, 40.7 % ω, 12.9 % π, 12.9 % ξ, 7.4 % r,z and subordinate s, x  and m PDFs. And Dr. Therriault explicitly stated in her report “… many curved PDFs!! (see Fig. 4, and for the PDFs histogram:http://www.impact-structures.com/impact-spain/the-azuara-impact-structure/shock-effects-shock-metamorphism-in-rocks-from-the-azuara-impact-structure/).

 

 

Fig. 4. Sets of bent PDFs in quartz; quartzite cobbles from the Pelarda Fm. ejecta of the Azuara impact structure (Spain).

 

We want to add that meanwhile there are other impact researchers having become aware of peculiar post-shock processes in impact target rocks, and we especially point to an LPSC XXXV (2004) abstract article (http://www.lpi.usra.edu/meetings/lpsc2004/pdf/1730.pdf) on the Charlevoix impact structure.  In Fig. 1 of that article nicely bent PDFs in quartz parallel to {0001} are shown (copied to our Fig. 5), and we cite from the text: “Bent planar deformation features (PDFs) can reflect the continuous bending of the crystal lattice (Fig. 1).”

Fig. 5. Bent planar deformation features (PDFs) in quartz from the Charlevoix impact structure. (from Trepmann and Spray 2004).

In fact and strictly speaking, “bent planar deformation features” is a contradiction in terms, but as long as we understand that PDFs refer to the crystal lattice and not to the mathematical definition of a plane, a renaming seems to be dispensable.