Traces of Catastrophe: A Handbook of Shock-Metamorphic

20,1

MB

Effects in Terrestrial Meteorite Impact Structures

130

stron

4746

ID

Smithsonian Institution

2000

rok

Contents

Chapter 1: LANDSCAPES WITH CRATERS: METEORITE IMPACTS, EARTH, AND THE SOLAR

SYSTEM. 1

1.1. The New Geology: Meteorite Impacts on the Earth 1

1.2. The Planetary Perspective 4

1.3. A Peculiar Process: Why Impacts are Different 6

1.3.1. Rarity 7

1.3.2. Immense Energy . 7

1.3.3. Instant Effects . 7

1.3.4. Concentrated Energy Release . 8

1.3.5. Extreme Physical Conditions . 9

1.3.6. Unique Deformation Effects 10

Chapter 2: TARGET EARTH: PRESENT, PAST, AND FUTURE .11

2.1. Comets and Asteroids: The Killer Neighbors? . 11

2.1.1. Asteroids . 11

2.1.2. Comets 11

2.1.3. Close Encounters . 12

2.2. In Our Time: Small Catastrophes 12

2.3. The Problems of Prediction: How Big, How Often? 12

2.3.1. Ingredients of Catastrophe 12

2.3.2. Uncertain Estimates . 13

2.3.3. An Uncertain Future? . 16

Chapter 3: FORMATION OF IMPACT CRATERS 17

3.1. Shock Waves and Crater Formation . 17

3.1.1. Contact/Compression Stage . 18

3.1.2. Excavation Stage: The Transient Crater . 20

3.1.3. Modification Stage 23

3.2. Simple and Complex Impact Structures 23

3.2.1. Simple Craters . 23

3.2.2. Complex Craters 24

3.2.3. Multiring Basins 27

3.3. Subsequent Development of Impact Structures 28

Chapter 4: SHOCK-METAMORPHIC EFFECTS IN ROCKS AND MINERALS .31

4.1. Formation Conditions and General Characteristics . 31

4.2. Stages of Shock Metamorphism . 36

4.3. Megascopic Shock-Deformation Features: Shatter Cones 36

4.4 High-Pressure Mineral Polymorphs 40

4.5. Planar Microstructures in Quartz . 42

4.5.1. Planar Fractures . 42

4.5.2. Planar Deformation Features (PDFs) 42

4.5.3. PDF Orientations 49

4.5.4. PDFs in Sedimentary Rocks . 52

4.6. Planar Microstructures in Feldspar and Other Minerals 53

4.7. Shock Isotropization and Diaplectic Glasses . 55

4.8. Selective Mineral Melting . 57

Chapter 5: SHOCK-METAMORPHOSED ROCKS (IMPACTITES) IN IMPACT STRUCTURES .61

5.1. Rock Types in the Final Impact Structure . 61

5.2. Classification of Impactites 62

5.3. Subcrater Rocks 62

5.3.1. Formation Conditions . 62

5.3.2. In-Place Shock-Metamorphosed Rocks 63

5.3.3. Lithic Breccias (Parautochthonous) . 64

5.3.4. Cross-Cutting (Allogenic) Breccias . 64

5.3.5. Pseudotachylite 65

5.4. Crater Interior: Crater-Fill Deposits (Breccias and Melt Rocks) . 69

5.4.1. Formation Conditions . 69

5.4.2. Lithic Breccias (Allogenic) 71

5.4.3. Melt-Fragment Breccias (Allogenic) (Suevites) . 71

5.4.4. Melt-Matrix Breccias (Impact-Melt Breccias) . 74

5.5. Crater Rim Zone and Proximal Ejecta Deposits . 74

5.6. Distal Ejecta 78

Chapter 6: IMPACT MELTS .79

6.1. Formation Conditions . 79

6.2. Impact Melt Volumes and Crater Size 81

6.3. Impact Melt Varieties in the Near-Crater Environment 82

6.3.1. Small Glassy Bodies . 82

6.3.2. Impact Melt Breccias 82

6.3.3. Large Crystalline Bodies (Dikes and Sills) . 86

6.4. Impact Melt in Distal Ejecta . 87

6.4.1. Spherules . 88

6.4.2. Tektites and Microtektites . 89

6.4.3. Miscellaneous Impact Glasses . 90

6.5. Recognition of Impact Melt Rocks . 90

Chapter 7: HOW TO FIND IMPACT STRUCTURES 97

7.1. Reasons for the Search 97

7.2. Detection of Candidate Impact Sites 97

7.2.1. Geological Features 98

7.2.2. Geophysical Features 98

7.3. Verification of Impact Structures 99

Chapter 8: WHAT NEXT? CURRENT PROBLEMS AND FUTURE INVESTIGATIONS 101

8.1. Identification of New Impact Structures . 101

8.2. Impact Events and Extinctions . 101

8.3. Distal Impact Ejecta 102

8.4. Carbon Chemistry in the Impact Environment 102

8.5. Postimpact Processes and Effects 103

8.6. Petrogenesis of Igneous Rocks: Impact Melts 103

8.7. Impacts and the Early Earth 104

Appendix 107

References .. 111