Shatter Cones Research Articles

KOEBERL, C. & HENKEL H. (eds) 2005. Impact Tectonics. Impact Studies Series. xix + 552 pp. Berlin, Heidelberg, New York: Springer-Verlag. Price Euros 129.95 (+ VAT at local rate), SFr 220, £100, US $169 (hard covers). ISBN 3 540 24181 7

Impact Tectonics is the latest in a series resulting from the European Science Foundation’s programme ‘The Response of the Earth System to Impact Processes’ and, like its predecessors, the result of papers presented at an international workshop. This book presents a wide-ranging view of some of the latest research into impacts and some useful reviews. The coverage of many of the various topics represents progress on a variety of individual research projects, rather than an attempt to produce a comprehensive guide to impact-related tectonics. The international author list includes most of the current key players in impact-related studies and the quality of the individual contributions is correspondingly high. These are grouped into four major areas: general aspects, structural and tectonic aspects, numerical and experimental methods and finally one paper on economic aspects. The first section, General Aspects, deals with three of the characteristic products of impacts: pseudotachylites, shatter cones and impact-induced dykes. An in-depth discussion (Reimold & Gibson) of the correct definition and terminology for rocks formed by frictional melting (‘pseudotachyllites’) provides a welcome contribution to the clarification of the characterization of these materials. It also illustrates the problems of definition and of realizing a generally applicable consensus when the evidence is so heavily dependent upon research on a single impact structure – Vredefort in South Africa. The …

Investigation of Shuttle Radar Topography Mission data of the possible impact structure at Serra da Cangalha, Brazil

Abstract— The Serra da Cangalha crater structure in northeast Brazil, ˜13 km in diameter, has long been widely considered to be a confirmed impact structure, based on reports of shatter cone findings. Only very limited field work has been carried out at this crater structure. Landsat Thematic Mapper (TM) and Shuttle Radar Topography Mission (SRTM) data sets for the region around this crater structure are compared here with regard to their suitability to determine first‐order structural detail of impact crater structures. The SRTM data provide very detailed information regarding drainage patterns and topography. A pronounced central ring of up to 300 m elevation above the surrounding area, two comparatively subdued intermediate rings of 6 and 10.5 km diameter, respectively, and the broad, complex crater rim of up to >100 m elevation can be distinguished in the Serra da Cangalha data. The maximum cratering‐related regional deformation (radial and concentric features) seems to be limited to a radial distance of 16–18 km from the center of the structure. A first comparison of macrostructural information from several impact structures with that from Serra da Cangalha does not yield firm trends, but the database is still very small at this stage. The varied nature of the target geology strongly influences the development of structural features in any impact event.

Discovery of the largest impact crater field on Earth in the Gilf Kebir region, Egypt

Using orbital imaging radar, we have detected a large number of circular structures in the southwestern Egyptian desert, covering more than 4500 km 2 close to the Gilf Kebir plateau in sandstones of Upper Cretaceous. Fieldwork confirmed that it is a new impact crater field: 13 craters from 20 m to 1 km in diameter were studied. The impact origin is confirmed by the observation of shock-related structures, such as shatter cones a nd planar fractures in quartz grains of breccia. Considering the extension of the crater field, it was possibly created by several meteorites that broke up when entering the atmosphere. To cite

Response to Comment on "Bedout: A Possible End-Permian Impact Crater Offshore of Northwestern Australia"

Response to Comment on "Bedout: A Possible End-Permian Impact Crater Offshore of Northwestern Australia"

Shatter cones: Branched, rapid fractures formed by shock impact

Shatter cones are rock discontinuities known only from sites of extraterrestrial impacts, and they are assumed to be formed by impact‐induced shock waves. Here we characterize the structure of shatter cones by field and microanalyses and explain their formation by dynamic fracture mechanics. Our analyses reveal that shatter cones always occur as multilevel, three‐dimensional networks, 0.01–100 m in size, with hierarchal branched fractures. A typical, individual shatter cone is a curved, oblate branch that bifurcates from its parent fracture (e.g., a larger shatter cone) and expands to form a spoon‐like surface. The unique shatter cone striations are arranged in V‐shaped pairs whose enclosed angle is constant for a given sample. We propose that shatter cones are the natural consequence of tensile rock fracturing at extreme velocities. First, the structure of shatter cone networks is strikingly similar to the structure of branched networks of experimental dynamic fractures that propagate at high velocities (velocities that approach the Rayleigh wave speed, VR). Second, “fracture front waves,” generated experimentally by the interaction of a rapidly moving tensile fractures and material inclusions, create tracks on the fracture surface that correspond to the V‐shaped striations of shatter cones. Third, applying the front wave concept to our field measurements (Vredefort impact, South Africa) shows that the shatter cones propagated at velocities of 0.98–0.90VR, with a systematic velocity decrease from the impact center. These extreme asymptotic velocities require the intense energy flux of impacts. Our model explains all of the structural features of shatter cones (curved surfaces, cone directivity, unique striations, hierarchic, multilevel structure) and their exclusive occurrence at impact sites.

Discovery of a double impact crater in Libya: the astrobleme of Arkenu

Abstract Using orbital imaging radar, we detected a double circular structure, located in the southeastern part of the Libyan Desert, which is partially hidden under sandy sediments. Fieldwork confirmed it to be an unknown double impact crater, each crater having a diameter of about 10 km, younger than 140 Ma. Sampling on the site enabled the observation of quantities of shatter cone structures and impact breccias containing planar fractures. To cite this article: P. Paillou et al., C. R. Geoscience 335 (2003).

The formation of shatter cones by shock wave interference during impacting

In this paper we present a new model for the formation of shatter cones. The model follows earlier suggestions that shatter cones are initiated by heterogeneities in the rock, but does not require the participation of an elastic precursor wave: the conical fractures are initiated after the passage of the main plastic compression pulse, not before. Numerical simulations using the hydrocode SALE 2D, enhanced by the Grady–Kipp–Melosh fragmentation model, support the model. The conditions required for the formation of shatter cones are explored numerically and are found to be consistent with the pressure range derived from both explosion experiments and the analysis of shock metamorphic features in impact structures. This model permits us to deduce quantitative information about the shape of the shock wave from the shape and size of the observed shatter cones. Indeed, the occurrence of shatter cones is correlated with the ratio between the width of the compressive pulse and the size of the heterogeneity that initiates the conical fracture. The apical angles of the shatter cones are controlled by the shape of the rarefaction wave.

Shoemaker impact structure, Western Australia

The Shoemaker impact structure, on the southern margin of the Palaeoproterozoic Earaheedy Basin, with an outer diameter of ∼⃒30 km, consists of two well‐defined concentric ring structures surrounding a granitoid basement uplift. The concentric structures, including a ring syncline and a ring anticline, formed in sedimentary rocks of the Earaheedy Group. In addition, aeromagnetic and geological field observations suggest that Shoemaker is a deeply eroded structure. The central 12 km‐diameter uplift consists of fractured Archaean basement granitoids of syenitic composition (Teague Granite). Shock‐metamorphic features include shatter cones in sedimentary rocks and planar deformation features in quartz crystals of the Teague Granite. Universal‐stage analysis of 51 sets of planar deformation features in 18 quartz grains indicate dominance of sets parallel to ω {1013}, but absence of sets parallel to π {1012}, implying peak shock pressures in the range of 10–20 GPa for the analysed sample. Geophysical characteristics of the structure include a —100 μs ‐2 gravity anomaly coincident with the central uplift and positive circular trends in both magnetic and gravity correlating with the inner ring syncline and outer ring anticline. The Teague Granite is dominated by albite‐quartz‐K‐feldspar with subordinate amounts of alkali pyroxene. The alkali‐rich syenitic composition suggests it could either represent a member of the Late Archaean plutonic suite or the product of alkali metasomatism related to impact‐generated hydrothermal activity. In places, the Teague Granite exhibits partial to pervasive silicification and contains hydrothermal minerals, including amphibole, garnet, sericite and prehnite. Recent isotopic age studies of the Teague Granite suggest an older age limit of ca 1300 Ma (Ar–Ar on K‐feldspar) and a younger age limit of ca 568 Ma (K–Ar on illite‐smectite).The significance of the K–Ar age of 568 Ma is not clear, and it might represent either hydrothermal activity triggered by impact‐related energy or a possible resetting by tectonothermal events in the region.

Sedimentary signatures and processes during marine bolide impacts: a review

Studies of submarine impact craters resulting from impacts of comets or asteroids demonstrate that the presence of water and the physical properties of target rocks have a major influence on sedimentary processes associated with meteorite impacts. This results in difference in sedimentary signature of bolide impacts in marine environments compared to subaerial impact craters. In subaerial impacts, the targets are commonly hard rocks, frequently of igneous and/or metamorphic origin, whereas in submarine impacts, the targets are mostly unconsolidated or poorly lithified sediments, or sedimentary rocks, with high volumes of pore water. Such differences result in variability in crater morphology and in sedimentary processes inside and outside the impact area. Impacts in shallow-water marine (neritic) environments produced craters with low or absent rims and wide and shallow brims, as characterize by both the Montagnais (on the Scotian shelf), the Mjølnir (in the Barents Sea), 45 and 40 km in diameter, respectively, and the Chesapeake Bay (90 km in diameter). Lack of elevated rims is thought to be the result of current reworking and resurge of the water back into the excavated cavity, as the water in the crater is vaporized. During this process, resurge gullies can be cut across the rim, while mass- and debris-flows, turbidites, and other gravity deposits are produced as results of tsunami and crater-wall and central high collapse, during and after the crater excavation stage. Such deposits are found both within and outside the crater structure. The only difference between gravity deposits triggered by an impact or other rare events, such as earthquakes, is the admixture of various melt particles and possible enrichments in iridium in the former. Impacts near the shelf edge may cause partial collapse of the continental margin as shown by the Montagnais and Chicxulub impacts. Some of the gravity and debris flows generated by margin collapse may be channelized, with final deposits up to several hundred meters thick, extending for hundreds of kilometers from the impact site. Other impact features such as shatter cones, tektites, spherules and Ni-spinels, shocked quartz, isotropication, and partial melts, are common to both submarine and subaerial impacts. Theoretical calculations of the destructive forces of mega-tsunami waves triggered by meteorite impact in the ocean greatly exceed those based on geologic evidence. A dearth of gigantic tsunami evidence for the Chicxulub impact outside of the Gulf of Mexico, where from theoretical modelling the maximum near bottom orbital velocity of water flow crossing the deep North Atlantic basin should have been >1 m/s, could be the result of the mitigating effect of the bathymetry of surrounding area, causing wave diffraction and interference. Calculated maximum horizontal orbital velocity near the seafloor at the shelf for the Montagnais impact is 22 m/s for a 200-m-high wave, 5.5 m/s for a 50-m-high wave at 500 km, decreasing to 0.5 m/s at 1000 km, strong enough to scour the deep ocean bottom and produce distinct erosion surfaces and disconformities in marine sedimentary record. However, lack of cores across impact horizon prevents confirmation of occurrence of such bottom water flows.

Yarrabubba - a large, deeply eroded impact structure in the Yilgarn Craton, Western Australia

Yarrabubba is a newly discovered impact structure situated within the complex granite–greenstone terrain of the Yilgarn Craton. Shock-metamorphic effects including shatter cones, planar deformation features in quartz grains, and pseudotachylites, were found in deeply eroded Archean granites near Yarrabubba station, southeast of Meekatharra, Western Australia. Aeromagnetic images reveal arcuate demagnetization features at diameters between 11 and 25 km, which roughly correspond to the outcropping of the Yarrabubba Granite, and are centered on a magnetic-high halo around the Barlangi Granophyre (119°50′E, 27°10′S). Rapid-quench textures in the Barlangi Granophyre and its inter-fingering relationships with pseudotachylites suggest that it is an impact melt that was injected into the Yarrabubba Granite and spread along fault discontinuities. Both the potassic Yarrabubba Granite and the felsic Barlangi Granophyre are atypical in the northern Yilgarn, as are the abundant fracturing and frictional melting within the local granitoids. These anomalous geological features associated with shock-metamorphic effects are indicative of a hypervelocity impact origin. The age of the impact is uncertain, but geological and geophysical relationships suggest that the Yarrabubba structure was formed during the early Proterozoic.

The Mesoproterozoic Beaverhead Impact Structure and Its Tectonic Setting, Montana‐Idaho: 40Ar/39Ar and U‐Pb Isotopic Constraints

New 40Ar/39Ar and uranium‐lead (U‐Pb) zircon data from the Beaverhead impact structure, first identified by extensive shatter coning of Proterozoic quartzite and gneiss from the Beaverhead Mountains near the Montana‐Idaho border, indicate that the structure formed at or after 900 Ma. The 40Ar/39Ar age spectra from fine‐grained muscovite and biotite from a breccia zone in high‐grade gneiss show significant argon loss but yield dates for highest‐temperature steps that cluster between 899 and 908 Ma. The dated minerals probably formed by recrystallization of impact glass, so on both geologic and isotopic grounds, the dates probably represent the minimum age of impact. U‐Pb data for zircons from the same breccia are strongly discordant and yield an upper intercept apparent age of $$2464\pm 56$$ Ma and a lower intercept apparent age of $$779\pm 69$$ Ma. Another brecciated gneiss about 7 km to the northeast that does not contain secondary mica does contain zircons that yield a concordant apparent age of $$2455\pm 9$$ Ma. Nearby gneiss that neither is brecciated nor contains shatter cones yields an apparent age of $$2451\pm 46$$ Ma. The 40Ar/39Ar results constrain the age of the shatter‐coned quartzite and indicate that it is >900 Ma and possibly correlative with the Gunsight Formation of the Mesoproterozoic Lemhi Group. The upper intercept U‐Pb age of ∼2450 Ma from all three dated samples also shows that the Paleoproterozoic basement rocks of the area are among the youngest in the mostly Archean Wyoming province of North America. The impact site lies near the margin of the province, along the northeast‐trending Great Falls tectonic zone, and the relatively young crustal age may reflect Early Proterozoic marginal accretion.

Dynamic fracture by large extraterrestrial impacts as the origin of shatter cones.

A large impact by a comet or meteorite releases an enormous amount of energy, which evaporates, melts and fractures the surrounding rocks. Distinctive features of such impacts are 'shatter cones', deformed rocks characterized by hierarchical striated features. Although such features have been used for decades as unequivocal fingerprints of large-body impacts, the process by which shatter cones form has remained enigmatic. Here we show that the distinctive shatter-cone striations naturally result from nonlinear waves (front waves) that propagate along a fracture front. This explains the observed systematic increase of striation angles with the distance from the impact. Shatter-cone networks, typically spanning many scales, can be understood as hierarchical bifurcations of the fracture front, which is generated by the immense energy flux carried by the initial, impact-generated, shock waves. Our quantitative predictions based on this theory are supported by field measurements at the Kentland and Vredefort impact sites. These measurements indicate that shatter cones near to the impact site were formed by fractures propagating at nearly the Rayleigh wave speed of the host rocks, whereas the furthest shatter cones observed (about 40 km from the impact site) were formed by fronts moving more slowly. These results provide insight into impact dynamics as well as dissipative mechanisms in solids subjected to sudden, extremely intense fluxes of energy.

Vredefort shatter cones revisited

Shatter cones have been described from a number of circular and polygonal structures worldwide, the origin of which has been alternatively ascribed to the impacts of large extraterrestrial projectiles or to catastrophic endogenic processes. Despite their association with enigmatic, catastrophic processes, the nature of shatter cones and the physics involved in their formation have not been comprehensively researched. Results of detailed field and laboratory studies of shatter cones from three areas in the collar of the Vredefort Dome in South Africa are presented. Vredefort shatter cones are directly related to a widely displayed fracture phenomenon, termed “multiply striated joint sets (MSJS)”. MSJs are planar to curviplanar fractures occuring at spacings of < 1 to several millimeters. The joint sets have a fractal character. When a new measurement protocol is used in the field, involving study of all joint surfaces and all steps and striae exposed on these surfaces, new information is gained on the genesis and significance of the MSJS and on their relationship to striated conical fractures. The internal constitution of a rock specimen with MSJS was examined in detail, by documenting the precise geometry of many fractures in a suite of parallel thin sections transecting the specimen. The steps and striae on shatter cone surfaces have the characteristics of displacement fractures (microfaults), along which evidence of melting is observed. Shatter cone and MSJS surfaces are often covered with glassy films; we evaluate whether these fracture phenomena are linked to the formation of pseudotachylitic (friction) melt. Our field and petrographic observations can be interpreted as consistent with the generation of shatter cones/MSJS relatively late in the formation of the Vredefort structure. This scenario contrasts sharply with the widely held view that shatter cones are formed during the early “compression” phase of a shock event that affected horizontal strata.

Argument supporting explosive igneous activity for the origin of “cryptoexplosion” structures in the midcontinent, United States: Comments and Reply

There has been a long-lived controversy over the origin of a well-defined linear set of eight cryptoexplosion structures in the southern midcontinent, United States. The linearity suggests that these structures must be related to either (1) impact of a string of bolides derived from breakup of a large parent bolide, or (2) volatile-rich, explosive ultramafic magmatism associated with structural weaknesses in the crust. That these structures are of various ages, are at or near intersections of major regional tectonic features, and have closely associated ultramafic magmatism, rules out the impact-string hypothesis. The only remaining hypothesis is that these cryptoexplosion structures were produced by explosive ultramafic volcanic activity. An important corollary to this hypothesis is that some shock-metamorphic features, including shatter cones and shocked quartz, can be produced by explosive ultramafic igneous activity.

Inaugeral lecture - Meteorite impacts on Earth and on the Earth Sciences

Plate tectonics has become established during the last thirty years as a coherent explanation for the major features of the Earth today. The formation of new crust at mid-ocean ridges, the movement of relatively rigid lithospheric plates over the softer underlying asthenosphere, and the collision of plates throwing up the Earth's major mountain ranges, are familiar concepts to many people. However, recent discoveries in Earth sciences challenge us to a newer concept of the Earth's evolution. These discoveries indicate that large meteorite impacts have had enormous effects on the Earth, perhaps most dramatically illustrated 65 million years ago, when an impact at Chicxulub in the Caribbean may have been responsible for mass extinctions (including the dinosaurs) on a global scale. The evidence for these catastrophic events is surprisingly enigmastic in the geological record, and has been the subject of intense scientific debate and disagreement. Some of the major types of direct evidence for meteorite impacts include the craters formed by impact, and the effects of impact on rocks, including melting the formation of shatter cones, and a whole variety of small-scale features that are revealed under the microscope. Perhaps paradoxically, these microstructures are the most unambiguous evidence for meteorite impacts, because they formed under radically greater stresses, temperatures and strain rates than those of plate tectonic processes, and are therefore quite distinct from plate tectonic microstructures. These features of meteorite impacts on Earth are illustrated by examples from several known impact sites, including the Vredefort structure in South Africa and a possible impact structure at Highbury south of Mhangura in Zimbabwe. Large meteorite impacts on Earth have significantly augmented Earth's mineral resources since the formation of the Earth, by creating diamonds in the ultra-high pressure conditions of impact, by providing structures that trap petroleum, and possibly by creating base metal deposits. The likely environmental consequences of impacts include a searing heat wave followed by global cooling, causing episodes of mass extinction that may occur in cycles. There is some controversial evidence for the theory that the first life on Earth itself may have been transported here on meteorites from Mars. The possibility of a major meteorite impact on Earth in the near future emphasizes the dramatic nature of these recent discoveries, which are having deep impacts in the Earth sciences, possibly even constituting a scientific revolution. Journal of Applied Science in Southern Africa Vol.5, No.1 pp. 63-80

Petrographic and geochemical study of rocks at Shakiso, Ethiopia: no evidence for shock metamorphism

We have performed field work and petrographic and geochemical studies on rocks from the area around Shakiso, Ethiopia, where the existence of an impact crater, based on the asserted presence of shatter cones and breccias, was previously suggested. We did not find any evidence that would support this claim.

Constraints on the size of the Vredefort impact crater from numerical modeling

Abstract— The Vredefort structure in South Africa was created by a meteorite impact about two billion years ago. Since that time, the crater has been deeply eroded; so to estimate its original size, researchers have had to rely heavily upon comparison to other terrestrial impact structures. Recent estimates of the original crater diameter range from 160 km to as much as 400 km. In this study, we combined the capabilities of both hydrocode and finite‐element modeling, using the former to predict where the pressure of an impact‐generated shock wave would have been high enough to form planar deformation features (PDFs) and shatter cones and the latter to follow the subsequent displacement of these shock isobars during the collapse of the crater. We established constraints on the sizes of the projectile and the transient crater (and, thus, on the size of the final crater) by comparing the observed locations of PDFs around Vredefort to the results of our simulations of impacts by projectiles of various sizes. These simulations indicate that a rocky projectile with a diameter of ∼10 km, impacting vertically at a velocity of 20 km/s generates shock pressures that are consistent with the distribution of PDFs around Vredefort. These projectile parameters correspond to a transient crater ∼80 km in diameter or a final crater ∼120–160 km in diameter. Allowing for uncertainties in our modeling procedures, we consider final craters 120 to 200 km in diameter to be consistent with the observed locations of PDFs at Vredefort. The shock pressure contour corresponding to the formation of shatter cones is almost horizontal near the surface, making the locations of these features less useful constraints on the crater size. However, they may provide a constraint on the amount of erosion that has occurred since the impact.

Shock‐induced melting and vaporization of shatter cone surfaces: Evidence from the Sudbury impact structure

Abstract— Analytical scanning electron microscopy has been used to investigate the surface textures and compositions of newly exposed shatter cones from the 1.85 Ga Sudbury impact structure, Canada. Unusual surface microstructures are observed at the micron scale, including silicate melt smears, melt fibres and melt splats. Silicate and Ni‐rich spherules up to 5 μm in diameter adorn earlier‐formed surface features, and we interpret these to be condensates formed due to shock‐induced vaporization of the shatter cone surfaces. The development of striations on the shatter cones is attributed to shock‐related fracture and slip. Formation of melts and spherules indicates that the highest ranks of shock metamorphism (Stages IV and V) were realized, but only on a very localized scale. Shatter cone surfaces are, therefore, likely sites for the development of high‐pressure polymorphs and, if the chemistry is appropriate, fullerenes. As such, they may be equivalent to “Type A” pseudotachylytes and shock veins in meteorites.

Exogenic and endogenic breccias: a discussion of major problematics

Pseudotachylitic breccias, impact melt rock, (ultra)cataclasites, and even ultramylonites may macroscopically be of very similar appearance. Even detailed optical microscopic work may not suffice to identify the actual type of melt rock studied. The difficulties to distinguish different tectonically produced breccias (fault rocks) and impact breccias (for which no uniformly accepted nomenclature exists) are examined, and the various formational processes (friction melting or shock melting), at the macro- and micro-scales, are discussed. Special emphasis is placed on the occurrence of different and multiple breccia types within impact structures, where not only impact-induced friction melt and impact melt may occur, but where tectonically produced fault rocks that are not related to the impact event may further complicate the breccia situation. Melting on shatter cone surfaces and its likely formation, the proposed origin as distal impact breccia of certain diamictites, and problems with carbonate breccias and caused by metamorphic overprint on breccias are discussed. It is cautioned not too easily to reach generalized conclusions, for example with regard to the usage of the A- and B-type pseudotachylite terminology. Areas in need of further breccia studies – if possible in close collaboration between impact and tectonic researchers – are emphasized.

Shock metamorphism and shock barometry at a complex impact structure: Slate Islands, Canada

The Slate Islands archipelago is believed to represent the central uplifted portion of a complex impact structure. Planar microstructures in quartz and feldspars and shock vitrification of rocks are the most common shock metamorphic features encountered. No diaplectic quartz was identified in the exposed rocks, but minor maskelynite is present. Shatter cones occur on all islands of the archipelago suggesting minimum pressures of 4 ± 2 GPa. The relative frequency of low index planar microstructures of specific, optically determined crystallographic orientations in quartz are correlated with results from shock barometric experiments to estimate peak shock pressures experienced by the exposed target rocks. In general, there is a decrease in shock pressure recorded in the target rocks from about 20–25 GPa in east-central Patterson Island to about 5–10 GPa at the western shore of this island and on Mortimer Island. The shock attenuation gradient is ∼4.5 GPa/km across this section of the island group. However, the shock attenuation has a roughly concentric plan only over the western part of the archipelago. There is no distinct shock center and there are other deviations from circularity. This is probably the result of: (1) the shock wave not having expanded from a point or spherical source because of the ∼1. 0 to 1.5 km size of the impactor; (2) differential movement of large target rock blocks during the central uplift and crater modification phases of the impact process. The orientation of planar deformation features in quartz appears to be independent of the shock wave direction suggesting that crystal structure exerts the primary control on microstructure development. Based on the results of XRD analyses, residual, post-impact temperatures were high enough to cause annealing of submicroscopic damage in shocked quartz.