Magnetic investigations of the West Hawk, Deep Bay and Clearwater impact structures, Canada

Abstract

Abstract-Due to the effects of erosion, tectonism and burial, impact structures are often obscured or destroyed. Geophysical methods are increasingly being used in detecting the signatures of impact structures. While gravity lows associated with impact structures are well understood, associated magnetic anomaly lows are not. In this study, drill cores from three Canadian impact structures were analyzed for rock magnetic properties and mineralogy, in order to explain the magnetic anomaly lows associated with these structures. Samples from the drill cores were cut and measured for anisotropy of magnetic susceptibility (AMS) and natural remanent magnetization (NRM) parameters. Drill cores from the twin impact craters of the Clearwater structure exhibited different NRM characteristics, and samples from their respective drill cores were subject to demagnetization by alternating field and thermal techniques. The differenct: noted in their NRM characteristics was attributed to the acquisition of a viscous remanent magnetization (VRM) at depth in Clearwater East.At all three structures, both magnetic susceptibilities and remanent magnetizatioc s are well below regional values in impact generated breccias, melt rocks, shocked crystalline rocks, and in postimpact sedimentary infill. The processes of brecciation, alteration, shock, and infill by nonmagnetic szdiments contribute to the development of the magnetic lows. However, a significant contribution to the observed magnetic anomalies was found, by first-order forward modelling, to arise from basement rocks beneath the impact structures. This zone of reduced magnetization may be caused by the partial demagnetization of magnetite by the impact-induced transient stress wave traveling away from the point of impact.

Comment on “Geophysical evidence for a large impact structure on the Falkland (Malvinas) Plateau”

“…While we do not dispute that some large impact structures (>40 km) display a high‐amplitude anomaly of ~1000 nT limited to the central dome, it should be noted that most craters have a dominantly low magnetic response (Cisowski & Fuller, ; Henkel & Guzman, ; Kukkonen, Kivekäs, & Paananen, ; Pilkington & Grieve, ; Plado, Pesonen, Elo, Puura, & Suuroja, ; Plado, Pesonen, Koeberl, & Elo, ; Robertson & Roy, ) (For a more complete review of the magnetic responses of impact craters see Scott et al. ). It is therefore considered unlikely that such a widespread positive magnetic high as observed to the west of the Falkland Islands would be related to a cratering process.…”

“…() describe a “rose”‐like circular positive magnetic anomaly; contrary to their assertions, magnetic anomalies rarely have a consistent or simple correlation with impact structures. This is largely due to the fragmentation and mixing of rock during cratering (French, ; Pilkington & Grieve, ; Scott, Pilkington, & Tanczyk, ). While we do not dispute that some large impact structures (>40 km) display a high‐amplitude anomaly of ~1000 nT limited to the central dome, it should be noted that most craters have a dominantly low magnetic response (Cisowski & Fuller, ; Henkel & Guzman, ; Kukkonen, Kivekäs, & Paananen, ; Pilkington & Grieve, ; Plado, Pesonen, Elo, Puura, & Suuroja, ; Plado, Pesonen, Koeberl, & Elo, ; Robertson & Roy, ) (For a more complete review of the magnetic responses of impact craters see Scott et al.…”

Viscous and impact demagnetization of Martian crust

“…Other near surface processes may also modify the magnetization of the crust. Brecciation reduces the coherent magnetization of the crust by randomizing the directions of fragmented blocks [ Scott et al , 1997]. Shock metamorphism and melting can result in formation of new minerals that have lower or higher susceptibility and remanent magnetism [ Pilkington and Hildebrand , 2000; Werner et al , 2002].…”

Calcul de l’érosion à long terme en région de socle autour de grands astroblèmes du Québec et de France

“…la partie nord du lac (Plante et al, 1990). Ce bassin est spatialement associé à des anomalies gravimétriques et magné-tiques de même diamètre (Dence et al, 1965 ;Plante et al, 1990 ;Scott et al, 1997). Le cratère d'impact originel de l'astroblème oriental du lac à l'Eau Claire devait donc présenter un diamètre de 16 ± 2 km.…”