『Abstract
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』
Contents
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
3.2.2.1. Reliance on Ir analyses alone
3.2.2.2. Analysis of associated target rocks
3.2.2.3. 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
4.3.1.1. Twinning
4.3.1.2. Growth lines
4.3.2. Internal strain features
4.3.2.1. Extinction bands
4.3.2.2. Deformation bands (kink bands)
4.3.3. Fracturing
4.3.3.1. Irregular (random) fractures
4.3.3.2. 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
6.3.1.1. Questionable shatter cones
6.3.1.2. 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
Acknowledgments
Appendix A. Critical characteristics and measurements to determine
the presence of diagnostic shock effects
References