Shocked Quartz Research Articles

Noble gases in shocked igneous rocks from the 380 Ma-old Siljan impact structure (Sweden): A search for paleo-atmospheric signatures

Finding direct records of the ancient Earth's atmosphere is key to understand the evolution of the entire planet. Nevertheless, such records are scarce, and only a few geological samples with preserved paleo-atmospheric signatures are available today. In this study, we analyzed noble gases contained in shocked granitic and volcanic rocks collected across the 380 Ma-old Siljan impact structure (in Sweden) to investigate the potential of shocked quartz crystals with planar deformation features (PDFs), often decorated by fluid inclusions, to preserve atmospheric signatures.Our results reveal that most of the investigated samples show relative elemental abundances of noble gases falling between those of air and meteoric water values. Atmospheric gases are present in the samples but excesses in radiogenic, nucleogenic, and fissiogenic noble gas isotopes highlight interactions of fluids with crustal rocks during the impact-generated hydrothermal circulation of fluids after the impact. Such fluid circulation event(s) could also have remobilized older fluids originally present in fluid inclusions preserved in the mineral lattices. Furthermore, results obtained on the two pure quartz fractions highlight a stronger atmospheric signature correlated with the presence of PDFs. The atmospheric component detected in this study could represent either a paleo-atmospheric signature trapped shortly after the Siljan impact event, or it may result from modern atmospheric contamination. Future studies of noble gases contained within shocked quartz crystals from other impact structures will unveil if such samples present advantages compared to other paleo-atmospheric proxies.

Aouelloul impact crater, Mauritania: New structural, lithological, and petrographic data

The Aouelloul circular structure (Mauritania), 0.4 km in diameter, is a simple impact crater, which formed approximately 3 Myr ago. The Aouelloul crater was confirmed to be an impact crater based on the presence of an extraterrestrial component in glass fragments found around the crater, but so far, no convincing evidence of shock metamorphism in minerals, such as shocked quartz grains, is reported. The crater has been visited by a small number of geologists and limited information is available on deformations and impact-related effects at the macroscopic and microscopic scales. Detailed petrographic investigations of eight sandstone samples, all collected at the rim of Aouelloul during a 2019's field campaign, show that tectonic deformation lamellae occur in a number of quartz grains, but did not reveal any evidence of shock metamorphism. New observations on the distribution of the impact glass at the Aouelloul crater, exclusively located in the southern and eastern outer parts of the crater, suggest an oblique impact, from possible directions ranging from south-west to north (clockwise). Geological and lithostructural data are also presented and show that the rim of the crater is characterized by more or less disturbed metric to pluri-metric blocks of rocks showing fracturing and dipping toward either the interior or the exterior of the crater. This study confirms previous works conducted at the Aouelloul crater and illustrates the difficulty in demonstrating the impact origin of relatively small craters.

Modeling airbursts by comets, asteroids, and nuclear detonations: shock metamorphism, meltglass, and microspherules

Asteroid and comet impacts can produce a wide range of effects, varying from large crater-forming events to high-altitude, non-destructive airbursts. Numerous studies have used computer hydrocode to model airbursts, primarily focusing on high-altitude events with limited surface effects. Few have modeled so-called “touch-down” events when an airburst occurs at an altitude of less than ∼1000 m, and no known studies have simultaneously modeled changes in airburst pressures, temperatures, shockwave speeds, visible materials, and bulk material failure for such events. This study used the hydrocode software Autodyn-2D to investigate these interrelated variables. Four airburst scenarios are modeled: the Trinity nuclear airburst in New Mexico (1945), an 80-m asteroid, a 100-m comet, and a 140-m comet. Our investigation reveals that touch-down airbursts can demolish buildings and cause extensive ground-surface damage. The modeling also indicates that contrary to prevailing views, low-altitude touch-down airbursts can produce shock metamorphism when the airburst shockwave or fragments strike Earth’s surface at sufficiently high velocities, pressures, and temperatures. These conditions can also produce microspherules, meltglass, and shallow impact craters. Regardless of modeling uncertainties, it is known that bolides can burst just above the Earth’s surface, causing significant damage that is detectable in the geologic record. These results have important implications for using shocked quartz and melted materials to identify past touch-down airbursts in the absence of a typical impact crater. Although relatively rare, touch-down events are more common than large crater-forming events and are potentially more dangerous.

Rebuttal of Holliday et al.’s Comprehensive Gish Gallop of the Younger Dryas Impact Hypothesis

In an article comprising some 96,000 words, Holliday et al. (2023) (HEA) claim to have “comprehensively refuted” the Younger Dryas impact hypothesis (YDIH), even though it has been corroborated by scores of articles in dozens of peer-reviewed journals based on the discovery of some combination of synchronous nanodiamonds, exotic microspherules and platinum enrichment at more than 50 Younger Dryas boundary sites on five continents. No hypothesis or theory is immune from criticism, but to “comprehensively refute” one so well established should require dispositive falsifying evidence. However, HEA provide no new evidence of their own and many of their arguments are based on faulty reasoning. Their remaining differences of opinion do not lend themselves to the falsification of an active hypothesis supported by an abundance of reproducible evidence, which now includes shocked quartz which is generally accepted to be produced only by cosmic impacts. Their article can therefore be called a Gish gallop; a long series of weak or flawed arguments designed to overwhelm an opponent. Since HEA’s claims are too many to respond to individually, we instead have selected portions of their article for critical analysis. By providing strong line-by-line counterarguments to their text we crucially show that they demonstrate a poor understanding of the logic required to test the YDIH and a poor understanding of uncertainty in experimental data analysis, and these problems propagate through many parts of their review. We also show that they repeatedly distort the facts and make misleading claims or derisory remarks. In summary, their approach is a corruption of the scientific method. In fact, the YDIH remains in a very strong position and probably represents a second example to go along with the Alvarez Theory of an extraterrestrial event that affected life on Earth. A hypothesis with such potential should not be so casually dismissed and instead should continue to be the subject of research.

Shocked quartz in sandstone from the buried Ilkurlka impact structure, Officer Basin, Western Australia

AbstractThe Ilkurlka structure is an ~12 km diameter buried circular aeromagnetic anomaly within the Officer Basin in Western Australia. Prior studies postulated a range of origins, including meteorite impact. We report the presence of pervasive deformation in the first drill cores from the structure. Brecciated sandstone and siltstone contain arrays of quartz grains with concussion fractures and rare shocked quartz grains with planar deformation features (PDF). Universal stage measurements of two quartz grains reveal one grain with PDF parallel to (0001) orientation and three PDF sets parallel to . A second grain contains three PDF sets parallel to and one set parallel to . The shocked grains are interpreted to have formed in situ, rather than representing transported detrital shocked grains. These results suggest local shock compression of at least 10 GPa; however, preservation of primary porosity and overall paucity of shocked grains may indicate lower mean shock pressures. (U‐Th)/He dating of 58 apatite grains from four samples across both cores shows a dominant age population at ~265 Ma and a minor age population at ~135 Ma. These dates overlap with regional events and thus do not provide an unambiguous impact age. An upper Carboniferous to lower Permian maximum impact age is provisionally proposed based on inferred missing target rock stratigraphy.

Evidence for a rosiaite-structured high-pressure silica phase and its relation to lamellar amorphization in quartz

When affected by impact, quartz (SiO2) undergoes an abrupt transformation to glass lamellae, the planar deformation features (PDFs). This shock effect is the most reliable indicator of impacts and is decisive in identifying catastrophic collisions in the Earth´s record such as the Chicxulub impact. Despite the significance of PDFs, there is still no consensus how they form. Here, we present time-resolved in-situ synchroton X-ray diffraction data of single-crystal quartz rapidly compressed in a dynamic diamond anvil cell. These experiments provide evidence for the transformation of quartz at pressures above 15 GPa to lamellae of a metastable rosiaite (PbSb2O6)-type high-pressure phase with octahedrally coordinated silicon. This phase collapses during decompression to amorphous lamellae, which closely resemble PDFs in naturally shocked quartz. The identification of rosiaite-structured silica provides thus an explanation for lamellar amorphization of quartz. Furthermore, it suggests that the mixed phase region of the Hugoniot curve may be related to the progressive formation of rosiaite-structured silica.

Microstructures in shocked quartz: linking nuclear airbursts and meteorite impacts

Many studies of hypervelocity impact craters have described the characteristics of quartz grains shock-metamorphosed at high pressures of >10 GPa. In contrast, few studies have investigated shock metamorphism at lower shock pressures. In this study, we test the hypothesis that low-pressure shock metamorphism occurs in near-surface nuclear airbursts and that this process shares essential characteristics with crater-forming impact events. To investigate low-grade shock microstructures, we compared quartz grains from Meteor Crater, a 1.2-km-wide impact crater, to those from near-surface nuclear airbursts at the Alamogordo Bombing Range, New Mexico in 1945 and Kazakhstan in 1949/1953. This investigation utilized a comprehensive analytical suite of high-resolution techniques, including transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). Meteor Crater and the nuclear test sites all exhibit quartz grains with closely spaced, sub-micron-wide fractures that appear to have formed at low shock pressures. Significantly, these micro-fractures are closely associated with Dauphiné twins and are filled with amorphous silica (glass), widely considered a classic indicator of shock metamorphism. Thus, this study confirms that glass-filled shock fractures in quartz form during near-surface nuclear airbursts, as well as crater-forming impact events, and by extension, it suggests that they may form in any near-surface cosmic airbursts in which the shockwave is coupled to Earth’s surface, as has been proposed. The robust characterization of such events is crucial because of their potential catastrophic effects on the Earth’s environmental and biotic systems.

Abu Hureyra, Syria, Part 1: Shock-fractured quartz grains support 12,800-year-old cosmic airburst at the Younger Dryas onset

A previous investigation revealed that shock-fracturing, a form of low-pressure shock metamorphism in quartz grains, can be produced during near-surface atomic airbursts and in cosmic impact structures, most likely at pressures lower than 8 GPa. This discovery implies that similar shock-fracturing may also form in quartz grains exposed to near-surface airbursts by comets and asteroids. Here, we investigate this hypothesis by examining quartz grains in a sedimentary profile from Abu Hureyra, a prehistoric archaeological site in northern Syria. This site was previously proposed to have experienced a nearby, low-altitude cosmic airburst at the onset of the Younger Dryas (~12,800 years ago). The Younger Dryas boundary layer (YDB) at Abu Hureyra has previously been shown to contain a rich assemblage of materials consistent in indicating a cosmic impact. These include anomalously high concentrations of melted micro-spherules displaying increased remanent magnetism; meltglass with low water content indicative of high-temperature melting; nanodiamonds, potentially including lonsdaleite; carbon spherules produced by biomass burning; black carbon or soot; total organic carbon; and abnormally high-temperature melted refractory minerals and elements, including platinum, iridium, chromite, and zircon. To further test this impact hypothesis, we searched for evidence of shocked quartz, a robust, widely accepted indicator of cosmic impacts. We used a comprehensive analytical suite of high-resolution techniques, including transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD), to examine and characterize quartz grains from the YDB layer at Abu Hureyra. Our analyses revealed the presence of quartz grains with sub-planar, sub-parallel, and sub-micron-wide intragranular fractures, most likely produced by mechanical and thermal shock or the combination of both. Furthermore, these fractures are typically filled with amorphous silica (glass), a classic indicator of shock metamorphism. Elemental analyses of the weight percentages of oxygen in the amorphous silica indicate that this could not have formed from the deposition of hydrated silica (e.g., opal and hyalite), which is enhanced in oxygen. Instead, the silica we observed is typically depleted in oxygen, consistent with melting under highly reducing conditions. The shock fractures in quartz grains also display Dauphiné twinning, which sometimes develops during the stress of high temperatures or pressures. This evidence is consistent with the hypothesis that the glass-filled fractures in quartz grains were produced by thermal and mechanical shock during a near-surface cosmic airburst at Abu Hureyra. These glass-filled fractures closely resemble those formed in near-surface atomic airbursts and crater-forming impact events.

Discriminating between impact or nonimpact origin of small meteorite crater candidates: No evidence for an impact origin for the Tor crater, Sweden

AbstractCompared to intensive research on km‐sized meteorite impact craters, fewer studies focus on smaller craters. The small craters are often hard or impossible to recognize using “classical” criteria like the presence of shatter cones, shocked quartz, and geochemical indicators. Therefore, a long list of candidate structures awaiting approval/disapproval of their origin has been formed over the last decades. One of them is the Tor structure in central Sweden. To test a hypothesis of an impact origin of this structure, we have performed topographical analysis, geophysical studies, 10Be exposure dating of boulders, and 14C dating of Tor‐associated charcoal. None of the methods gave us a reason to claim the Tor structure is of impact origin. Thus, we support a recently suggested idea of Tor being formed by a grounded iceberg within a glacial lake.

The Basque coastal sections of the K/T boundary – A key to understanding “mass extinction” in the fossil record

 lnvestigations at the K/T boundary section al Zumaya, northern Spain, underline the importance of this section in (1) establishing a Maastrichtian ammonite zonation and (2) in understanding "mass extinctions" in the fossil record. "Mass extinction" at the K/T boundary can be restricted to two groups of oceanic surface plankton. Ammonites and inoceramids exhibit a gradual decline through the late Cretaceous, as probably also do the Temperate belemnites and the bulk of involved vertebrates. Most of these groups disappear a long time before the boundary. Compared with the available environmental factors, the course of ammonite diversity through time perfectly parallels the course of global sea level changes. Times of high extinction rates (e.g. the era and system boundaries) coincide with times of maximum regressions, while the subsequent evolutionary radiations match with the new sea level rise. There is no need for any cosmic catastrophy to explain the decline of most invertebrate and vertebrate groups. Coeval with the short-time extinction of oceanic surface plankton at the boundary itself, the following environmental and depositional changes were observed: minor (positive as well as negative) changes in temperature, boundary clay sedimentation, enrichment of iridium and noble metals, occurrence of sanidium spheres and shocked quartz, and changes in carbon and strontium isotopes. Ali these events were used to support the Alvarez hypothesis of a cosmic impact at the boundary. But they are much easier explained by increasing explosive volvanicity which, on the other hand, might correlate also with the time-equivalent magnetic reversals.

A compound scenario for the end-Cretaceous mass extinctions

Two major controversies have arisen in research on the end-Cretaceous mass extinctions, concerning the extent to which they were sudden or gradual, and terrestrially or extraterrestrially induced. A review of recent work supports more or less gradual extinction for a number of terrestrial and marine groups such as dinosaurs and-ammonites, but the spectacular crash of the calcareous plankton and correlative ecological disaster in land plants in part of the northern hemisphere suggest a short-term catastrophic event. Chemical and physical signatures of the Cretaceous-Tertiary boundary, including iridium anomalies, shocked quartz, microspherules and carbon, oxygen and strontium isotopes, are discussed and the evidence for and against a bolide impact-induced or volcanic catastrophe is reviewed, with the conclusion that it is not yet decisive either way. Evidence for longer-term changes is also cited, with strontium-isotope data supporting that from stratigraphy in suggesting a significant fall in sea level shortly before the end of the Cretaceous. A large body of evidence also supports a latest Cretaceous fall in seawater and air temperature, but this has recently been disputed for western North America on the basis of leaf studies. It is concluded that both longer term causes, intrinsic to this planet, and a final catastrophe either involving bolide impact or volcanism on a spectacular scale, or perhaps a combination of the two, are required to account for the pattern of end-Cretaceous extinctions.

Identification of the ejecta deposit formed by the Australasian Tektite Event at Huai Om, northeastern Thailand

AbstractThe Australasian Tektite Event, approximately 0.8 Ma, is the youngest record of a large impact event on Earth. Although it is estimated that it occurred somewhere in Indochina based on the distribution of tektites, the crater has never been located. Here, we report the discovery and occurrence of shocked quartz with planar deformation features (PDFs) in the Quaternary depositional sequence at Huai Om in northeastern Thailand. Measurements of the orientation of lamellae using a universal stage microscope as well as observation using scanning electron microscopy and transmission electron microscopy were conducted to confirm the presence of PDFs. Together with the occurrence of in situ layered tektite fragments, we identify the depositional sequence as the ejecta deposit formed by the Australasian Tektite Event. We further describe the detailed lithostratigraphy of the ejecta deposit, which will allow the tracing of its distribution and lateral changes in its thickness, grain size, and grain composition. Further investigation of the lateral distribution of the ejecta deposit would provide information about the location, magnitude, and target rocks of the Australasian Tektite Event.

Timing and causes of forest fire at the K–Pg boundary

We report K–Pg-age deposits in Baja California, Mexico, consisting of terrestrial and shallow-marine materials re-sedimented onto the continental slope, including corals, gastropods, bivalves, shocked quartz grains, an andesitic tuff with a SHRIMP U–Pb age (66.12 ± 0.65 Ma) indistinguishable from that of the K–Pg boundary, and charred tree trunks. The overlying mudstones show an iridium anomaly and fungal and fern spores spikes. We interpret these heterogeneous deposits as a direct result of the Chicxulub impact and a mega-tsunami in response to seismically-induced landsliding. The tsunami backwash carried the megaflora offshore in high-density flows, remobilizing shallow-marine fauna and sediment en route. Charring of the trees at temperatures up to > 1000 °C took place in the interval between impact and arrival of the tsunami, which on the basis of seismic velocities and historic analogues amounted to only tens of minutes at most. This constrains the timing and causes of fires and the minimum distance from the impact site over which fires may be ignited.

Shock‐metamorphic microstructures in quartz grains from Albian sandstones from the Tin Bider impact structure, Algeria

AbstractTin Bider is a 6‐km‐diameter complex impact structure, the largest one recognized in Algeria. The crater was excavated in Cretaceous sedimentary rocks composed of, from the base to the top, Albian sandstones, Cenomanian clays, Cenomanian‐Turonian limestones, undifferentiated Coniacian to Maastrichtian clays and limestones. The age of the impact event is poorly constrained to <66 Ma by stratigraphy, the youngest geological unit affected by the event being the ~66 Myr old Maastrichtian limestones. Albian sandstones outcrop in the central sector of the structure and represent the only occurrence at outcrop of this geological unit in the structure. Here we report on a detailed petrographic analysis of eight Albian sandstone samples that were searched for shock‐metamorphic features. We confirm the presence of rare shocked quartz grains with planar deformation features (PDFs) and report on their crystallographic orientations as determined using the universal stage microscope. PDFs oriented parallel to the π{} and ω{} orientations are the most abundant ones. For the first time in impactites from Tin Bider, PDFs with basal (0001) orientation, corresponding to amorphized mechanical Brazil twins, are reported. Our results indicate that locally the peak shock pressure was of at least 20 GPa, but much lower in average for the investigated samples.

A Tunguska sized airburst destroyed Tall el-Hammam a Middle Bronze Age city in the Jordan Valley near the Dead Sea

We present evidence that in ~ 1650 BCE (~ 3600 years ago), a cosmic airburst destroyed Tall el-Hammam, a Middle-Bronze-Age city in the southern Jordan Valley northeast of the Dead Sea. The proposed airburst was larger than the 1908 explosion over Tunguska, Russia, where a ~ 50-m-wide bolide detonated with ~ 1000× more energy than the Hiroshima atomic bomb. A city-wide ~ 1.5-m-thick carbon-and-ash-rich destruction layer contains peak concentrations of shocked quartz (~ 5–10 GPa); melted pottery and mudbricks; diamond-like carbon; soot; Fe- and Si-rich spherules; CaCO3 spherules from melted plaster; and melted platinum, iridium, nickel, gold, silver, zircon, chromite, and quartz. Heating experiments indicate temperatures exceeded 2000 °C. Amid city-side devastation, the airburst demolished 12+ m of the 4-to-5-story palace complex and the massive 4-m-thick mudbrick rampart, while causing extreme disarticulation and skeletal fragmentation in nearby humans. An airburst-related influx of salt (~ 4 wt.%) produced hypersalinity, inhibited agriculture, and caused a ~ 300–600-year-long abandonment of ~ 120 regional settlements within a > 25-km radius. Tall el-Hammam may be the second oldest city/town destroyed by a cosmic airburst/impact, after Abu Hureyra, Syria, and possibly the earliest site with an oral tradition that was written down (Genesis). Tunguska-scale airbursts can devastate entire cities/regions and thus, pose a severe modern-day hazard.

Shocked quartz in distal ejecta from the Ries impact event (Germany) found at\u2009~\u2009180\xa0km distance, near Bernhardzell, eastern Switzerland

Impact ejecta formation and emplacement is of great importance when it comes to understanding the process of impact cratering and consequences of impact events in general. Here we present a multidisciplinary investigation of a distal impact ejecta layer, the Blockhorizont, that occurs near Bernhardzell in eastern Switzerland. We provide unambiguous evidence that this layer is impact-related by confirming the presence of shocked quartz grains exhibiting multiple sets of planar deformation features. Average shock pressures recorded by the quartz grains are ~ 19 GPa for the investigated sample. U–Pb dating of zircon grains from bentonites in close stratigraphic context allows us to constrain the depositional age of the Blockhorizont to ~ 14.8 Ma. This age, in combination with geochemical and paleontological analysis of ejecta particles, is consistent with deposition of this material as distal impact ejecta from the Ries impact structure, located ~ 180 km away, in Germany. Our observations are important for constraining models of impact ejecta emplacement as ballistically and non-ballistically transported fragments, derived from vastly different depths in the pre-impact target, occur together within the ejecta layer. These observations make the Ries ejecta one of the most completely preserved ejecta deposit on Earth for an impact structure of that size.

The Mesoproterozoic Stac Fada Member, NW Scotland: an impact origin confirmed but refined

The origin of the Stac Fada Member has been debated for decades with several early hypotheses being proposed, but all invoking some connection to volcanic activity. In 2008, the discovery of shocked quartz led to the hypothesis that the Stac Fada Member represents part the continuous ejecta blanket of a meteorite impact crater, the location of which was, and remains, unknown. In this paper, we confirm the presence of shock-metamorphosed and -melted material in the Stac Fada Member; however, we also show that its properties are unlike any other confirmed and well documented proximal impact ejecta deposits on Earth. Instead, the properties of the Stac Fada Member are most similar to the Onaping Formation of the Sudbury impact structure (Canada) and impact melt-bearing breccias from the Chicxulub impact structure (Mexico). We thus propose that, like the Sudbury and Chicxulub deposits, Melt Fuel Coolant Interactions – akin to what occur during phreatomagmatic volcanic eruptions – played a fundamental role in the origin of the Stac Fada Member. We conclude that these rocks are not primary impact ejecta but instead were deposited beyond the extent of the continuous ejecta blanket as high-energy ground-hugging sediment gravity flows.

Characterization of shocked quartz grains from Chicxulub peak ring granites and shock pressure estimates

AbstractPlanar deformation features (PDFs) in quartz are a commonly used and well‐documented indicator of shock metamorphism in terrestrial rocks. The measurement of PDF orientations provides constraints on the shock pressure experienced by a rock sample. A total of 963 PDF sets were measured in 352 quartz grains in 11 granite samples from the basement of the Chicxulub impact structure’s peak ring (IODP‐ICDP Expedition 364 drill core), with the aim to quantify the shock pressure distribution and a possible decay of the recorded shock pressure with depth, in the attempt to better constrain shock wave propagation and attenuation within a peak ring. The investigated quartz grains are highly shocked (99.8% are shocked), with an average of 2.8 PDF sets per grain; this is significantly higher than in all previously investigated drill cores recovered from Chicxulub and also for most K‐Pg boundary samples (for which shocked quartz data are available). PDF orientations are roughly homogenous from a sample to another sample and mainly parallel to {103} and {104} orientations (these two orientations representing on average 68.6% of the total), then to {102} orientation, known to form at higher shock pressure. Our shock pressure estimates are within a narrow range, between ~16 and 18 GPa, with a slight shock attenuation with increasing depth in the drill core. The relatively high shock pressure estimates, coupled with the rare occurrence of basal PDFs, i.e., parallel to the (0001) orientation, suggest that the granite basement in the peak ring could be one of the sources of the shocked quartz grains found in the most distal K‐Pg boundary sites.

Tracing shock-wave propagation in the Chicxulub crater: Implications for the formation of peak rings

AbstractThe Chicxulub crater (Yucatán Peninsula, Mexico) is considered exceptional in many scientific aspects; morphologically it is the only known impact structure on Earth with a well-preserved peak ring. Recent drilling (International Ocean Discovery Program–International Continental Scientific Drilling Program Expedition 364) into this topographic feature provides insights into the structural properties and complex formation of a peak ring. By means of U-stage microscopy on shocked quartz grains from the granitic section of the recovered drill core, orientations of feather features (FFs) were determined and local principal axis of stress (σ1) orientations of the shock wave were derived. The FF orientations are strongly confined to a radially outward trend (WNW) relative to the crater center, which emphasizes a link between FF formation and the direction of shock-wave propagation. Thus, FFs represent an excellent tool as a stress-orientation indicator for the shock wave. Our microstructural data set shows that the granitic basement of the peak ring between ∼750 and ∼1200 m below seafloor behaved as a semi-coherent block above an imbricate thrust zone, and underwent both rotation and local folding during cratering. This validates the block sizes of acoustic fluidization employed in most Chicxulub-scale impact simulations. The folding of the upper part of the granitic basement may have developed by either (1) compression of the crater wall at the transient cavity and/or (2) dragging by the centripetal flow of the overlying crater material.

Preferred orientation distribution of shock-induced planar microstructures in quartz and feldspar.

Shocked quartz and feldspar grains commonly exhibit planar microstructures, such as planar fractures, planar deformation features, and possibly microtwins, which are considered to have formed by shock metamorphism. Their orientation and frequency are typically reported to be randomly distributed across a sample. The goal of this study is to investigate whether such microstructures are completely random within a given sample, or whether their orientation might also retain information on the direction of the local shock wave propagation. For this work, we selected samples of shatter cones, which were cut normal to the striated surface and the striation direction, from three impact structures (Keurusselkä, Finland, and Charlevoix and Manicouagan, Canada). These samples show different stages of pre‐impact tectonic deformation. Additionally, we investigated several shocked granite samples, selected at different depths along the drill core recovered during the joint IODP‐ICDP Chicxulub Expedition 364 (Mexico). In this case, thin sections were cut along two orthogonal directions, one parallel and one normal to the drill core axis. All the results refer to optical microscopy and universal‐stage analyses performed on petrographic thin sections. Our results show that such shock‐related microstructures do have a preferred orientation, but also that relating their orientation with the possible shock wave propagation is quite challenging and potentially impossible. This is largely due to the lack of dedicated experiments to provide a key to interpret the observed preferred orientation and to the lack of information on postimpact orientation modifications, especially in the case of the drill core samples.