In this Article, the middle initial of author Kosei E. Yamaguchi (of the IODP-ICDP Expedition 364... more In this Article, the middle initial of author Kosei E. Yamaguchi (of the IODP-ICDP Expedition 364 Science Party) was missing and his affiliation is to Toho University (not Tohu University). These errors have been corrected online.
A major tool in the initial recognition and study of terrestrial impact craters,-20% of which are... more A major tool in the initial recognition and study of terrestrial impact craters,-20% of which are buried beneath postimpact sediments, is geophysics. The general geophysical character of terrestrial impact craters is compiled and outlined with emphasis on its relation to the impact process and as an aid to the recognition of additional impact craters. The most common and conspicuous geophysical signature is a circular gravity low. For simple bowl-shaped craters, gravity models indicate that the anomaly is largely due to the presence of an interior allochthonous breccia lens. In complex craters, modeling indicates that the main contribution to the gravity anomaly is from fractured parautochthonous target rocks in the floor of the crater. The gravity signature of both simple and complex crater forms can be modeled well, using known morphometric parameters of impact structures. The size of the gravity anomaly generally increases with increasing crater diameter reaching a maximum of-20-30 mGal at diameters D of-20-30 km. Further increases in D have a negligible effect on the magnitude of the gravity anomaly due to lithostatic effects on deep fractures. The general gravity signature of a simple low can be modified by target rock and erosional effects, and there is a tendency for larger complex structures (D > 30 km) to exhibit a relative gravity high restricted to the crater center and extending out to <0.5D. The magnetic signature of craters is more varied. The dominant effect is a magnetic low due to a reduction in susceptibility. Large structures (D > 40 km) tend to exhibit central high-amplitude anomalies, with dimensions of <0.5D, due to remanently magnetized bodies in the target rocks. The sources of these bodies are wide ranging and include the effects of shock, heat, and chemical alteration. The few studies over craters involving electrical methods indicate resistivity lows coinciding with the extent of the potential field anomalies and related to fracturing. Seismic techniques, particularly reflection surveys, have provided details of the subsurface structure of craters. Incoherent reflections and reduced seismic velocities due to brecciation and fracturing are expected, the degree of coherency of reflections increasing away from and below the center of the structure. From the various geophysical techniques a set of general criteria can be established that correspond to the geophysical signature of impact craters. These criteria can be used to evaluate the hypothesis that any particular set of geophysical anomalies is due to impact. Confirmation of an impact origin, however, is based on geologic evidence. geologic studies, for example, Eagle Butte, Canada. Terrestrial data are currently the main source of information on the nature of the third dimension of impact craters ranging up to over 100 km in diameter. This
Proceedings of the International Conferences on Basement Tectonics, 1992
... 7 STRUCTURAL FABRIC OF THE NORTH AMERICAN CONTINENT, AS DEFINED BY GRAVITY TRENDS MO Thomas, ... more ... 7 STRUCTURAL FABRIC OF THE NORTH AMERICAN CONTINENT, AS DEFINED BY GRAVITY TRENDS MO Thomas, RAF Grieve'and VL Sharptorr ... Stewart, 1978) and Tertiary sedimentary rocks less than 17 Ma old, and Quaternary sediments (Christiansen and McKee, 1978 ...
The currently known terrestrial record of impact cratering stands at over 160 impact structures a... more The currently known terrestrial record of impact cratering stands at over 160 impact structures and several new examples are identified each year. The record, however, is a biased sample of an originally much larger population, favoring younger, larger structures in geologically stable areas of the Earth's continental crust. The largest and oldest known structures are limited to diameters of approx. 250-300 km and ages of less than 2 Ga. Care must be taken, therefore, in making generalised statements regarding the record with respect to such time-integrated effects as variations in cratering rate, periodicities, etc. The terrestrial record, however, does provide cumulative observations of aspects of the cratering process and is the only available source of ground truth with respect to the structural and lithological results of large-scale natural impact events.
In this Article, the middle initial of author Kosei E. Yamaguchi (of the IODP-ICDP Expedition 364... more In this Article, the middle initial of author Kosei E. Yamaguchi (of the IODP-ICDP Expedition 364 Science Party) was missing and his affiliation is to Toho University (not Tohu University). These errors have been corrected online.
A major tool in the initial recognition and study of terrestrial impact craters,-20% of which are... more A major tool in the initial recognition and study of terrestrial impact craters,-20% of which are buried beneath postimpact sediments, is geophysics. The general geophysical character of terrestrial impact craters is compiled and outlined with emphasis on its relation to the impact process and as an aid to the recognition of additional impact craters. The most common and conspicuous geophysical signature is a circular gravity low. For simple bowl-shaped craters, gravity models indicate that the anomaly is largely due to the presence of an interior allochthonous breccia lens. In complex craters, modeling indicates that the main contribution to the gravity anomaly is from fractured parautochthonous target rocks in the floor of the crater. The gravity signature of both simple and complex crater forms can be modeled well, using known morphometric parameters of impact structures. The size of the gravity anomaly generally increases with increasing crater diameter reaching a maximum of-20-30 mGal at diameters D of-20-30 km. Further increases in D have a negligible effect on the magnitude of the gravity anomaly due to lithostatic effects on deep fractures. The general gravity signature of a simple low can be modified by target rock and erosional effects, and there is a tendency for larger complex structures (D > 30 km) to exhibit a relative gravity high restricted to the crater center and extending out to <0.5D. The magnetic signature of craters is more varied. The dominant effect is a magnetic low due to a reduction in susceptibility. Large structures (D > 40 km) tend to exhibit central high-amplitude anomalies, with dimensions of <0.5D, due to remanently magnetized bodies in the target rocks. The sources of these bodies are wide ranging and include the effects of shock, heat, and chemical alteration. The few studies over craters involving electrical methods indicate resistivity lows coinciding with the extent of the potential field anomalies and related to fracturing. Seismic techniques, particularly reflection surveys, have provided details of the subsurface structure of craters. Incoherent reflections and reduced seismic velocities due to brecciation and fracturing are expected, the degree of coherency of reflections increasing away from and below the center of the structure. From the various geophysical techniques a set of general criteria can be established that correspond to the geophysical signature of impact craters. These criteria can be used to evaluate the hypothesis that any particular set of geophysical anomalies is due to impact. Confirmation of an impact origin, however, is based on geologic evidence. geologic studies, for example, Eagle Butte, Canada. Terrestrial data are currently the main source of information on the nature of the third dimension of impact craters ranging up to over 100 km in diameter. This
Proceedings of the International Conferences on Basement Tectonics, 1992
... 7 STRUCTURAL FABRIC OF THE NORTH AMERICAN CONTINENT, AS DEFINED BY GRAVITY TRENDS MO Thomas, ... more ... 7 STRUCTURAL FABRIC OF THE NORTH AMERICAN CONTINENT, AS DEFINED BY GRAVITY TRENDS MO Thomas, RAF Grieve'and VL Sharptorr ... Stewart, 1978) and Tertiary sedimentary rocks less than 17 Ma old, and Quaternary sediments (Christiansen and McKee, 1978 ...
The currently known terrestrial record of impact cratering stands at over 160 impact structures a... more The currently known terrestrial record of impact cratering stands at over 160 impact structures and several new examples are identified each year. The record, however, is a biased sample of an originally much larger population, favoring younger, larger structures in geologically stable areas of the Earth's continental crust. The largest and oldest known structures are limited to diameters of approx. 250-300 km and ages of less than 2 Ga. Care must be taken, therefore, in making generalised statements regarding the record with respect to such time-integrated effects as variations in cratering rate, periodicities, etc. The terrestrial record, however, does provide cumulative observations of aspects of the cratering process and is the only available source of ground truth with respect to the structural and lithological results of large-scale natural impact events.
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