Cratered cobbles in Triassic Buntsandstein conglomerates in northeastern Spain: An indicator of shock deformation in the vicinity of large impacts: Comment and Reply (original) (raw)
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Impact-related melting of sedimentary target rocks of the Rubielos de la Cérida structure in Spain
Berichte der …, 2002
The Rubielos de la Cérida impact structure forms a companion crater to the Late Eocene-Oligocene Azuara impact structure. Both are located more or less at the margin of the Iberian chains and the Ebro basin south of Zaragoza. Within the Rubielos structure, silicate melt rocks, carbonate-phosphate melts with small-scaled immiscibility features, very fine mixtures of silicate melt and carbonate forming clasts in suevite, as well as glassy particles of amorphous carbon were found. These melt rocks clearly reflect the chemical composition of various parts of the thick sedimentary pile in the target area and show the shockinduced high-temperature influence on these rocks.
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
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.
Impact cratering has been a fundamental geological process in Earth history with major ramifications for the biosphere. The complexity of shocked and melted rocks within impact structures presents difficulties for accurate and precise radiogenic isotope age determination, hampering the assessment of the effects of an individual event in the geological record. We demonstrate the utility of a multi-chronometer approach in our study of samples from the 40 km diameter Araguainha impact structure of central Brazil. Samples of uplifted basement granite display abundant evidence of shock deformation, but U/Pb ages of shocked zircons and the 40Ar/39Ar ages of feldspar from the granite largely preserve the igneous crystallization and cooling history. Mixed results are obtained from in situ 40Ar/39Ar spot analyses of shocked igneous biotites in the granite, with deformation along kink-bands resulting in highly localized, partial resetting in these grains. Likewise, spot analyses of perlitic glass from pseudotachylitic breccia samples reflect a combination of argon inheritance from wall rock material, the age of the glass itself, and post-impact devitrification. The timing of crater formation is better assessed using samples of impact-generated melt rock where isotopic resetting is associated with textural evidence of melting and in situ crystallization. Granular aggregates of neocrystallized zircon form a cluster of ten U–Pb ages that yield a “Concordia” age of 247.8 ± 3.8 Ma. The possibility of Pb loss from this population suggests that this is a minimum age for the impact event. The best evidence for the age of the impact comes from the U–Th–Pb dating of neocrystallized monazite and 40Ar/39Ar step heating of three separate populations of post-impact, inclusion-rich quartz grains that are derived from the infill of miarolitic cavities. The 206Pb/238U age of 254.5 ± 3.2 Ma (2r error) and 208Pb/232Th age of 255.2 ± 4.8 Ma (2r error) of monazite, together with the inverse, 18 point isochron age of 254 ± 10Ma (MSWD = 0.52) for the inclusion-rich quartz grains yield a weighted mean age of 254.7 ± 2.5 Ma (0.99%, 2r error) for the impact event. The age of the Araguainha crater overlaps with the timing of the Permo–Triassic boundary, within error, but the calculated energy released by the Araguainha impact is insufficient to be a direct cause of the global mass extinction. However, the regional effects of the Araguainha impact event in the Paraná–Karoo Basin may have been substantial.
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.
SEPM Journal of Sedimentary Research, 1985
The base of the Triassic in the Iberian Range is represented by detrital sediments (Buntsandstein facies) deposited initially in a continental environment, finally evolving into a marine environment that is represented at the top of the sequence. The lithology of this facies is dominated by arkosic sandstones. The aim of this study is to reconstruct the nature and position of the source areas of these sandstones. Provenance research was carried out by quartz-grain typology. Eleven stratigraphic sections were sampled. The amount of interstitital matrix has been considered in selecting samples because mechanical compaction suffered by sandstones with little matrix may give rise to a significant increase in the undulosity of monocrystalline quartz. The 0110 de Sapo gneissic formation, located in the Hesperian Massif, has been mentioned as source rocks offeldspathic sandstones in previous works. In order to verify the provenance of feldspathic sandstones, in artificial sands derived by grinding gneisses, and sand samples collected at stream heads that drain the gneiss outcrops, we followed the methodology ofBasu et al. (1975). Analytical results indicate that two different areas within the Triassic basin were notably influenced by different source areas: a) a western zone, the nearest to the gneissic source rocks, where monocrystalline, nonunduIatory quartz grains predominate (Qm :s 5"); and b) an eastern zone farther from the gneissic source area, where polycrystalline quartz grains (Qp) and undulatory, monocrystalline quartz grains (Qm > 5") increase. Sediment evolution during transport processes is markedly reflected by the increase in Qm :s 5°/(Qp + Qm > 5") ratios in the westernmost zone, away from the source area. Low values in the above-mentioned ratios in the eastern zone are interpreted as results oflocal influence by low-ranking metamorphic source areas. Finally, this method also allows for the monitoring of the evolution of sediment maturity throughout the basin.
Earth and Planetary Science Letters, 2008
Different quartz microstructures from Rochechouart impact breccias, indicative of shock-induced compression and shear deformation are compiled, to obtain information on the mean stress and the deviatoric components of the shock wave-associated stress. Annealed rhombohedral planar deformation features (PDFs) are widespread in quartz from Rochechouart impact breccias, as analysed by optical microscopy, scanning electron and transmission electron microscopy. Shocked quartz can show mosaicism with domains of b 200 nm in diameter that are misoriented to each other. This is interpreted as the result of post-shock annealing of a high density of rhombohedral PDFs crosscutting each other. Pockets of newly crystallised quartz aggregates, interpreted as annealed diaplectic glass, are surrounded by optically almost isotropic quartz and occur in intensely shocked quartz. These areas are proposed to represent gradually increasing shock intensities from the host quartz grain to the newly crystallised quartz aggregate due to local pressure and temperature variations on the mm-scale. These microstructures indicate compression at high shock pressures (20-35 GPa) and they show no evidence of shear deformation at high differential stress. Annealing of these microstructures probably took place shortly after shock when post-shock temperatures were still high. Quartz microstructures in the Rochechouart impact breccias that record shock-induced shear deformation are mechanical Brazil twins and planar fractures that are supposed to represent microfaults. These shock effects indicate high differential stress on the order of a few GPa. Quartz that contains Brazil twins and planar fractures shows no or few rhombohedral PDFs, indicating relatively low shock pressures (b 15 GPa). In general, shock effects indicating high differential stress do not occur together with those indicative of high shock pressure. These findings reveal that high shock pressures (N 20 GPa) are apparently not accompanied by high differential stresses. Only at attenuated shock pressure (b 15 GPa), differential stresses are effective to cause shear deformation. As a result of deformation during shock compression, the mean stress decreases with time and distance from the point of impact, whereas the deviatoric components can be expected to increase due to the high anisotropy of rocks and minerals.