French,B.M. and Koeberl,C.(2010): The convincing identification of terrestrial meteorite impact structures: What works, what doesn't, and why. Earth-Science Reviews, 98, 123-170.


 In the geological sciences it has only recently been recognized how important the process of important cratering is on a planetary scale, where it is commonly the most important surface-modifying process. On the Moon and other planetary bodies that lack an appreciable atmosphere, meteorite impact craters are well preserved, and they can commonly be recognized from morphological characteristics, but on Earth complications arise as a consequence of the weathering, obliteration, deformation, or burial of impact craters and the projectiles that formed them.These problems made it necessary to develop diagnostic criteria for the identification and confirmation of impact structures on earth. Diagnostic evidence for impact events is often present in the target rocks that were affected by the impact. The conditions of impact produce an unusual group of melted, shocked, and brecciated rocks, some of which fill the resulting crater, and others which are transported, in some cases to considerable distances from the source crater. Only the presence of diagnostic shock-metamorphic effects and, in some cases, the discovery of meteorite, or traces thereof, is generally accepted as unambiguous evidence for an impact origin. Shock deformation can be expressed in macroscopic form (shatter cones) or in microscopic forms (e.g., distinctive planar deformation features [PDFs] in quartz). In nature, shock-metamorphic effects are uniquely characteristic of shock levels associated with hypervelocity impact. The same two criteria (shock-metamorphic effects or traces of the impacting meteorite) apply to distal impact ejecta layers, and their presence confirms that materials found in such layers originated in an impact event at a possibly still unknown location. As of 2009 about 175 impact structures have been identified on Earth based on these criteria. A wide variety of shock-metamorphic effects has been identified, with the best diagnostic indicators for shock metamorphism being features that can be studied easily by using the polarizing microscope. These include specific planar microdeformation features (planar fractures [PFs], PDFs), isotropization (e.g., formation of diaplectic glasses), and phase changes (high pressure phases; melting). The present review provides a detailed discussion of shock effects and geochemical tracers that can be used for the unambiguous identification of impact structures, as well as an overview of doubtful criteria or ambiguous lines of evidence that have erroneously been applied in the past.

Keywords: impact craters; shock metamorphism; shocked quartz; spherules; craters; crater identification』

1. Introduction
2. Conditions of shock metamorphism
3. Shock-deformation features in impact structures
 3.1. Shock metamorphism and the identification of impact structures
 3.2. Traces of the impacting projectile
  3.2.1. Preserved meteorite fragments
  3.2.2. Chemical and isotopic signatures from the projectile Reliance on Ir analyses alone Analysis of associated target rocks Interpretation of null results
 3.3. Unique target-rock deformation features formed by shock-wave conditions
  3.3.1. Shatter cones
  3.3.2. High-pressure (diaplectic) mineral glasses
  3.3.3. High-pressure mineral phases
  3.3.4. High-temperature glasses and melts
4. Planar microdeformation features in quartz: impact-produced and endogenic features
 4.1. Background
 4.2. Shock-produced planar microdeformation features
  4.2.1. Planar fractures (PFs)
  4.2.2. Planar deformation features (PDFs)
  4.2.3. Basal microdeformation features
 4.3. Endogenic planar microdeformation features
  4.3.1. Growth features Twinning Growth lines
  4.3.2. Internal strain features Extinction bands Deformation bands (kink bands)
  4.3.3. Fracturing Irregular (random) fractures Healed fractures
  4.3.4. Metamorphic deformation lamellae (MDLs)
5. Non-diagnostic impact deformation effects
 5.1. Background
 5.2. General geological and geophysical features
  5.2.1. Circular morphology and circular structural deformation
  5.2.2. Circular geophysical anomalies
 5.3. Deformational effects in the target rock
  5.3.1. Brecciation
  5.3.2. Kink banding in micas
  5.3.3. Mosaicism in quartz and other minerals
  5.3.4. Pseudotachylite and pseudotachylitic breccia
  5.3.5. Igneous rocks and glasses
 5.4. Spherules and microspherules in distal ejecta layers
 5.5. Other problematic criteria
  5.5.1. Fullerenes
  5.5.2. Iron-rich nanophase particles
  5.5.3. Impact-produced damage in microfossils
6. Problematic reports of impact events, structures, and shock-deformation effects
 6.1. Background
 6.2. Morphological, structural, and geophysical studies
  6.2.1. Introduction
  6.2.2. Circular features and patterns
  6.2.3. Structural and geological studies
  6.2.4. Geophysical studies
 6.3. Field and petrographic studies
  6.3.1. Megascopic features Questionable shatter cones Quartz fracturing
  6.3.2. Misidentification of coesite
  6.3.3. Incorrect and questionable identifications of PDFs in quartz
 6.4. Microspherules
7. Questionable “impact” effects at major extinction boundaries
 7.1. The Permian-Triassic boundary
  7.1.1. Background
  7.1.2. Quartz PDFs and siderophile elements
  7.1.3. Micrometeorites and Cr isotope anomalies
  7.1.4. Spherules
  7.1.5. Fullerenes and noble gases
  7.1.6. The Bedout Permian (?) impact (?) structure
 7.2. The Younger Dryas (Pleistocene) Event
8. Discussion
 8.1. Constraints on the Detection of Shock Effects
 8.2. Shock Effects and Target Rock Characteristics
  8.2.1. Maffic crystalline rocks and basalt lavas
  8.2.2. Carbonate Rocks
  8.2.3. Fine-grained sediments
  8.2.4. Unconsolidated sediments
 8.3. Identification and Documentation of Quartz PDFs
 8.4. Documentation of Extraterrestrial PGE (including Ir) and Siderophile Anomalies
 8.5. Structural and Geophysical Criteria for Impact
9. Conclusions
Appendix A. Critical characteristics and measurements to determine the presence of diagnostic shock effects