by: Charles O’Dale
- confirmed impact;
- suspected impact;
In geophysics, a magnetic anomaly is a local variation in the Earth’s magnetic field resulting from variations in the chemistry or magnetism of the rocks. The natural process of hypervelocity impact where a rock carrying a remanent magnetization is shocked in the presence of an ambient field can be studied as the simple superimposition of shock demagnetization and shock magnetization. For this there are now a variety of techniques that allow experimental study of both phenomena separately or simultaneously as in this study. Mapping of variation over an area is valuable in detecting structures obscured by overlying material.
J. Gattacceca, M.Boustie, E.Lima, B.P.Weiss , T.de Resseguier, J.P.Cuq-Lelandais
In the natural case of a hypervelocity impact on a planetary or asteroidal surface, two competing phenomena occur: partial or complete shock demagnetization of pre-existing remanence and acquisition of shock remanent magnetization (SRM). In this paper, to better understand the effects of shock on the magnetic history of rocks, we simulate this natural case through laser shock experiments in controlled magnetic field. As previously shown, SRM is strictly proportional to the ambient field at the time of impact and parallel to the ambient field. Moreover, there is no directional or intensity heterogeneity of the SRM down to the scale of ∼0.2 mm3. We also show that the intensity of SRM is independent of the initial remanence state of the rock. Shock demagnetization and magnetization appear to be distinct phenomena that do not necessarily affect identical populations of grains. As such, shock demagnetization is not a limiting case of shock magnetization in zero field.
As a consequence, when it can be recognized in a rock, SRM must be considered as a reliable record of the direction and intensity of the ambient magnetic field at the time of impact. The natural process of hypervelocity impact where a rock carrying a remanent magnetization is shocked in the presence of an ambient field can be studied as the simple superimposition of shock demagnetization and shock magnetization. For this there are now a variety of techniques that allow experimental study of both phenomena separately or simultaneously as in this study.
These results have potential implications for the paleomagnetic study of meteorites, and lunar rocks, and for the understanding of the magnetic signature (as studied through paleomagnetism and/or magnetic anomalies) of terrestrial, lunar and Martian impact craters
MAGNETIZATION OF ROCKS
The magnetic field of the Earth can be “captured” by certain types of rocks, and this magnetic signature can be used to study the Earth’s magnetic field throughout history. The magnetic poles of the Earth are not fixed, and pole reversals have occurred many times in the past.
The rocks from the West Lake show that they were formed during a “superchron,” which is an unusually extended period of time where no reversals occurred. This superchron, known as the Permo-Carboniferous Reversed Superchron, lasted from 316 to 265 million years ago, which agrees with the age found by the argon dating.
The rocks from the East Lake tell a different story. They have a number of different magnetic polarizations, which indicate viscous remnant magnetization. This is magnetization that is acquired slowly over a long period of time. The more complex magnetic history points to the rocks being much older than the West Lake, as they have had more time to be altered.
Transient and disruption cavity dimensions of complex terrestrialimpact structures derived from magnetic data
Mark Pilkington Geological Survey of Canada, Ottawa, Ontario, Canada
Alan R. Hildebrand Department of Geology and Geophysics, University of Calgary, Calgary, Alberta, Canada
Accurate transient and disruption cavity dimensions are critical for estimating the energy release associated with impact. Transient and disruption cavity size can, in principle, be inferred from morphometric relationships based on crater diameter. However, locating the crater rim can be difficult for eroded terrestrial craters, and existing morphometric relationships are mostly based on observations of extraterrestrial craters where morphologic features at best provide imprecise constraints on the collapsed disruption cavity margin. Fortunately, magnetic survey data collected over terrestrial impact structures demonstrate that collapsed disruption cavity size can be estimated directly from changes in the magnetic anomaly character. A lower bound on this parameter can be defined by the outer limit of short-wavelength, intense magnetic anomalies produced by impact melt and/or suevite deposits. An upper bound is given by the inner limit of magnetic anomaly trends associated with the pre-impact target rock configuration. Using published values of crater diameters (D) and values of collapsed disruption cavity diameters (D CDC ) derived from magnetic data for 19 complex terrestrial impact structures, we derive the relationship D CDC = 0.49D. These data and the possibility of geometrical similarity in crater collapse suggest that this relationship is independent of complex crater size over more than a decade of size variation.
2. CONFIRMED IMPACT CRATERS
- Deep Bay;
- Glover Bluff;
- Ile Rouleau;
3. SUSPECTED IMPACT CRATERS
- Can-Am Structure;
- Charity Shoal;
- Charron Lake;
- High Rock Lake;
- Skeleton Lake.
Can-Am vertical derivative of residual magnetic anomaly field. G.F. = Grenville front.
4. NONIMPACT STRUCTURES
- Croker Island Complex;
- Manitou Island Complex.