Azuara impact structure: The Daroca thrust geologic enigma – solved? A Ries impact structure analog (original) (raw)
Related papers
Evidence of an impact origin for the Azuara structure (Spain)
Earth and Planetary Science Letters, 1985
Some 50 km south of Zaragoza (northeast Spain) there is a tectonically peculiar area, which is suspected to be a large impact site. It has a morphologically conspicuous ring structure with a diameter of roughly 30 km and consists mainly of Mesozoic sediments emerging from the Ebro Tertiary Basin, and partly of Palaeozoic rocks from the Iberian System. The strata of the ring, in general, dip to the center and are highly folded and faulted. Intense and unusual deformation is indicated by abundant mixed and monomict breccias. A single poorly exposed outcrop of a mixed breccia with sedimentary fragments exhibits shock-metamorphic effects. Within quartz grains, systems of crystallographically oriented microscopic planar features and planar fractures can be observed, as well as kink bands in micas within the breccia matrix. The age of the impact is estimated to be between Lower Cretaceous and Miocene.
Geological Magazine, 2013
The tectonothermal evolution of a unit in the foreland fold-and-thrust belt of the Iberian massif is established using the conodont colour alteration index (CAI). The unit consists of two parts with different tectonothermal histories -the Esla nappe region and the Valsurbio region -separated by a synorogenic Carboniferous basin (Guardo-Valderrueda basin). The Esla nappe region evolved in diacaizonal conditions (corresponding to the diagenetic conditions of the pelites) whose palaeotemperatures were controlled by rock burial. Maximum values were reached before the emplacement of the thrust nappes, so tectonic superimposition is not registered by the CAI. Overburial due to the emplacement of the thrust units was prevented by simultaneous intense erosion. The geothermal gradient obtained for burial was c. 35 • C km −1 and the temperature reached by the older Cambrian rocks was c. 210-230 • C. The Valsurbio region was affected by an extensional tectonothermal post-orogenic event that gave rise to metamorphism with ancaizonal or epicaizonal conditions (corresponding to anchizone or epizone of the pelites). The most common maximum palaeotemperatures reached in this event fall within the range 305-415 • C, although higher palaeotemperatures could be reached locally as a consequence of contact metamorphism. This event gave rise to subhorizontal cleavage that cuts the main Variscan folds. Coal rank data indicate an increase in maximum palaeotemperatures eastwards from the Esla nappe region to the Valsurbio region through the Guardo-Valderrueda basin.
Sedimentary record of impact events in Spain
A review of the evidence of meteorite-impact events in the sedimentary record of Spain reveals that the only proven impact-related bed is the clay layer at the Cretaceous-Tertiary boundary (at Zumaya and Sopelana in the Bay of Biscay region, and at Caravaca, Agost, and Alamedilla in the Betic Cordilleras). Other deposits previously proposed as impact related can now be rejected, or are dubious and still debated. These include the Pelarda Formation, alleged to represent proximal ejecta from the Azuara structure; the Paleocene-Eocene boundary near Zumaya (western Pyrenees) and Alamedilla (Betic Cordillera); and the Arroyofrío Oolite Bed, which has been alleged as distal ejecta of an unknown Callovian-Oxfordian impact event. The scarcity of evidence for meteorite-impact events in the sedimentary record is possibly due to a lack of detailed studies. We propose several sedimentary units that could potentially be related to impact events, and where future research should focus.
Tectonophysics, 2008
This paper presents a multidisciplinary approach (stratigraphic, sedimentologic, structural, and mineralogic) to document the evidences for Cadomian and earliest-Cambrian tectonic processes recorded in the Iberian Chains, which were strongly overprinted during Variscan and Alpine deformation episodes. The contact of the Neoproterozic Paracuellos Group and the lower Cambrian Bámbola Formation is commonly identifiable by the presence of a distinct erosive unconformity related to the onset of alluvial plain sequences, or associated with synsedimentary faults. A meso-structural analysis of the deformation recorded in the Paracuellos slates reveals the existence of an earlier slaty cleavage (S 1N ) overprinted by Variscan contractional structures, some of them related to positive inversion of pre-Variscan normal faults. The comparative geometrical study with structures observed in the Bámbola Formation (essentially, a single, NW-SE to N-S striking cleavage S 1C ) is not conclusive about the record of the Cadomian orogeny. However, micro-scale deformation associated with the earliest cleavage is much more intense in the Paracuellos slates. In addition, the metamorphic grade and the pressure conditions of the lower Cambrian anchizonal shales contrast with the Neoproterozoic epizonal slates. Both observations are consistent with a Cadomian contractional deformation associated with a low-grade, intermediatepressure metamorphism, absent in the overlying rocks. Postdating both processes, stratigraphically marked by a tectono-thermal discontinuity located at the Paracuellos/Bámbola contact, the onset of the lowermost Cambrian Bámbola Formation is accompanied by strong palaeogeographic transformations in the sedimentary basin, including: (i) a sharp relative sea-level fall leading to deposition of alluvial plain sediments; and (ii) an increase in sedimentation rates and tectonically induced subsidence at fault-bounded depressions. All these changes, together with the presence of normal or transtensional faults at the base of Bámbola Formation, point to an earliest-Cambrian extensional regime similar to that described for other neighbouring post-Cadomian basins.
The mid-Tertiary Azuara and Rubielos de la Cérida paired imapct structures (Spain).
Treb. Mus. Geol. Barcelona, 11, 5-65., 2002
We report on the Azuara impact structure and its Rubielos de la Cérida companion crater, which establish the largest terrestrial doublet impact structure presently known. Both structures have diameters of roughly 35 - 40 km and they have been formed in a purely sedimentary target. From stratigraphic considerations and palaeontologic dating, an Upper Eocene or Oligocene age is very probable. Geological mapping has established abundant geologic impact evidence in the form of monomictic and polymictic breccias and breccia dikes, megabreccias, dislocated megablocks, remarkable structural features, extensive impact ejecta and impact signatures even in distant autochthonous deposits. The most striking impact evidence for both structures is given by strong shock metamorphism, including melt and diaplectic glass, planar deformation features (PDFs), different kinds of impact melt rocks (from former silicate melt, carbonate melt, carbonate-phosphate melt) and suevite breccias. Glassy amorphous carbon particles in a solid C-O compound may be related with fullerenes and may originate from a quenched melt of extremely shocked coal or from extremely shocked limestones. It is assumed that the impact had considerable influence on the Mid-Tertiary regional geology of the Iberian System, and we suggest that respective geologic models which have so far not considered this peculiar and far-reaching event, need considerable revision.
Gondwana Research, 2010
Models concerning the tectonic evolution of accretionary complexes typically relate outcrop-scale to platescale multiphase deformation as a smooth variation of strain on all scales. However, at oblique convergent margins, regional scale brittle faults in the shallow crust are commonly parallel to the main orogenic grain. These faults impose a strong structural anisotropy and can subsequently control deformation at subordinate scales. As a result, finite strain in each domain may not record local kinematics consistent with the overall orogenic-scale motion implying that structural data must be analyzed selectively from a large area in order to relate outcrop-scale kinematics to global plate-scale dynamics. Field mapping and preliminary structural analysis of the Late Devonian Pulo do Lobo (PDL) Formation, and suspect "exotic" South Portuguese Zone (SPZ) in southern Iberia indicate tectonic juxtaposition of diverse deposits such as foreland basin flysch, sedimentary and tectonic mélange, and passive margin sediments showing an overall geometry consistent with an accretionary wedge setting. Variations in finite strain, lithology and regional structure were used as proxies for defining tectonic domains for structural analysis. Numerous local kinematic indicators within the PDL suggest a complex regional deformation with several enigmatic features that can be explained by sequential compartmentalization of strain during the development of the imbricate stack followed by latestage bulk strain imposed across the entire complex. Structural data produced by local strain partitioning reveals kinematic indicators, which contradict the overall regional structural style (e.g. spatial juxtaposition of sinistral and dextral fabrics). When viewed at larger scales (i.e. regional scale), however, these data indicate that significant sinistral strike-slip movement occurred in conjunction with both an extension and shortening. Outcrop-scale deformation in polydeformed domains is controlled by local conditions resulting from brittle deformation coeval with orogenic-scale bulk strain. The entire Pulo do Lobo Zone is dominated by a pervasive late-stage vertical to sub-vertical E-W cleavage axial planar to chevron folds which overprint earlier deformation in the older passive margin units. This overprinting suggests that in the late stages of the evolution of the accretionary complex, bulk strain was imposed over the entire complex as a result of internal locking of the accretionary complex and reduced strain rates during the waning stages of collision between Gondwana and Laurussia. Stereographic analysis of fabric elements from each distinct tectonic domain, together with regional geological constraints, support this hypothesis and are indicative of progressive deformation imposed on the PDL during the Variscan Orogeny. Crown Please cite this article as: Braid, J.A., et al., Structural analysis of an accretionary prism in a continental collisional setting, the Late Paleozoic Please cite this article as: Braid, J.A., et al., Structural analysis of an accretionary prism in a continental collisional setting, the Late Paleozoic
http://www.impact-structures.com, 2019
The Pelarda Formation (Fm.), located in the Iberian System in northeast Spain, is a sedimentary deposit with an extension of roughly 12 km x 2.5 km and an estimated thickness of no more than 400 m. The formation was first recognized as a peculiar unit in the early seventies and underwent interpretations like a fluvial or an alluvial fan deposit having a postulated age between Paleogene and Quaternary. Since the early nineties the Pelarda Formation has been considered an impact ejecta deposit originating from the large ca. 40 km-diameter Azuara impact structure and meanwhile being among the largest and most prominent terrestrial impact ejecta occurrences, which however is questioned by regional geologists still defending the fluvial and alluvial fan models. Roughly speaking, the Pelarda Fm. is a grossly unsorted, matrix-supported diamictite with grain sizes between silt fraction and meter-sized clasts and a big intercalated megablock. Strong clast deformations and abundant shock metamorphic effects like planar deformation features (PDF) are observed throughout the Pelarda F. deposit compatible with its impact ejecta origin. Aligned bigger clasts and smaller intercalated bands of sandstones, siltstones and clayey material indicate some local stratification obviously adjusted to flow processes within the impact ejecta curtain. This suggests that gravitational flows predominated in a transport by water in both liquid and gas states. Transport and deposition as a kind of pyroclastic surge are discussed. A sketch sequence describes the emplacement process of the Pelarda Fm. as 2 part of the Azuara crater formation and the integration in the general frame of pre-impact geology and some post-impact layering.
ERNSTSON CLAUDIN IMPACT STRUCTURES – METEORITE CRATERS Research on impact geology, geophysics, petrology, and impact cratering WEB, 2020
The Iberian System in NE Spain is characterized by a distinctive graben/basin system (Calatayud, Jiloca, Alfambra/Teruel), among others, which has received much attention and discussion in earlier and very recent geological literature. A completely different approach to the formation of this graben/basin system is provided by the impact crater chain of the Rubielos de la Cérida impact basin as part of the important Middle Tertiary Azuara impact event, which has been published for about 20 years. Although the Rubielos de la Cérida impact basin is characterized by all the geological, mineralogical and petrographical impact findings recognized in international impact research, it has completely been hushed up in the Spanish geological literature to this day. The article presented here uses the example of the Jiloca graben to show the absolute incompatibility of the previous geological concepts with the impact structures that can be observed in the Jiloca graben without much effort. Digital terrain modeling and aerial photography together with structural and stratigraphic alien geology define a new lateral Singra-Jiloca complex impact structure with central uplift and an inner ring, which is positioned exactly in the middle of the Jiloca graben. Unusual topographic structures at the rim and in the area of the inner ring are interpreted as strike-slip transpression and transtension. Geological literature that still sticks to the old ideas and develops new models and concepts for the graben/basin structures, but ignores the huge meteorite impact and does not even enter into a discussion, must at best cause incomprehension.