MAGNETIC ANOMALY

MAGNETIC ANOMALY

by: Charles O’Dale

  1. introduction;
  2. confirmed impact;
  3. suspected impact;
  4. non-impact.

1. INTRODUCTION

MAGNETIC ANOMALIES

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.

 

MAGNETIC STUDIES

Unraveling the simultaneous shock magnetization and demagnetization of rocks

J. Gattacceca, M.Boustie, E.Lima, B.P.Weiss , T.de Resseguier, J.P.Cuq-Lelandais

ABSTRACT

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

Geophysical method to measure variations of the Earth’s magnetic field related with rocks of different magnetic properties. Magnetic anomalies in and around impact structures may result from displacement of magnetized rocks in the impact cratering process, decomposition of existent rock magnetization (by shock, for example), and formation of new magnetic phases in rocks (e.g., by chemical alterations, by acquiring a thermal remnant magnetization). Magnetic geo-signatures are instrumental in identifying impact structures ie: Carswell.
Several mechanisms related to impacts may radically change the magnetic properties of target rocks. Peak pressures in autochthonous rocks may reach ~30 GPa at impact, which is sufficient to produce shock demagnetization and remagnetization effects.

 

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.

As a rock sample ages, the radiometric isotope decays into more and more daughter products. Measuring the ratio of the original isotope to the daughter products can yield the age of the sample. Credit: John Schmidt .

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
November 2003.
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.

From: Proceedings of the Symposium on Planetary Cratering Mechanics, Flagstaff, Ariz., September 13-17, 1976.

2. CONFIRMED IMPACT CRATERS

  • Carswell;
  • Deep Bay;
  • Glover Bluff;
  • Holleford;
  • Wanapitei;
  • Whitecourt.

CARSWELL

Innes, M. J. S., Recent advances in meteorite crater research at the Dominion Observatory, Ottawa, Canada. 1964

DEEP BAY

Deep Bay Crater magnetic variations (Sander 1964).

GLOVER BLUFF

Bouguer gravity map showing the location of the Glover Bluff disturbed area (gravity data from Koenen, 1956). Contours in milligals. B, Ground magnetic map showing the location of the Glover Bluff disturbed area (magnetic data from Koenen, 1956). Contours in gammas.

HOLLEFORD

The magnetic studies performed by the Geological Survey of Canada document that there is a minimum magnetic disturbance within the impact structure, here outlined by the circle on the aeromagnetic map (Grieve 2006).

WANAPETEI

Lake Wanapetei magnetic survey (2002).
A shallow magnetic survey was run in August 2002. It indicated a circular low approximately 2.5 km wide and placed over the greatest depths of the lake (>100m). It is therefore not known whether the magnetic low can be absolutely attributed to an impact structure or whether it is due to the large water depths. This magnetic low however is slightly more south than the location of the gravity low. The estimated maximum and minimum sizes for the impact crater are indicated.

WHITECOURT

Combined results of two magnetic surveys performed using the GEM Systems GSM 19-TW. This represents the diurnally corrected data. Dense vegetation along the southern half of the grid made surveying slightly more difficult. A large magnet was found at the large positive anomaly on the NW crater rim (one of three found to date, likely left behind by meteorite hunters). With the exception of the magnet, large meteorites, about several hundred grams each, were found at all the major anomalies. Several other meteorites of similar scale were recovered from additional localized anomalies evident only in the raw data.

3. SUSPECTED IMPACT CRATERS

  • Can-Am Structure;
  • Charity Shoal;
  • Charron Lake;
  • High Rock Lake;
  • Merewether;
  • Skeleton Lake.

CAN-AM (PROBABLE) IMPACT STRUCTURE

Regional shaded relief map of residual magnetic anomaly field of the Can-Am structure located within Lake Huron.

Can-Am vertical derivative of residual magnetic anomaly field. G.F. = Grenville front.

CHARITY SHOAL

A. Total magnetic intensity (TMI) B. Residual magnetic field map produced by subtraction of 200 m upward continued grid.

CHARRON LAKE

A strongly negative magnetic anomaly coincides with Charron Lake, Manitoba. D. H. Hall (University of Manitoba) calculated that the removal of a block of the slightly magnetic country rock granite would produce the negative anomaly over the lake.
Lake Charron, top centre, contains a very strong magnetic anomaly (Natural Resources Canada).

HIGH ROCK LAKE

Location of coreholes in the High Rock Lake structure magnetic map and of cross-section A-A’ (after McCabe, 1982).

MEREWETHER

Magnetic contour map of vicinity of Merewether (J. Vise and L.I. Cowan).

SKELETON LAKE

The type of magnetic field change, a negative magnetic anomaly of ~80nT centered over the crater, is consistent with that found at other impact sites. The model suggests it is 3.4 km wide and the undisturbed bedrock is at ~750 m (Unpublished geomagnetic and electrical survey report Energy Mines and Resources, Earth Physics Branch (GSC) by J.F. Clark). Courtesy of Dr. James Whitehead, Planetary and Space Science Centre, University of New Brunswick. http://www.unb.ca/passc/ImpactDatabase/This Aeromagnetic Chart was researched and forwarded to me by fellow RASC member Eric Briggs.

4. NONIMPACT STRUCTURES

  • Croker Island Complex;
  • Manitou Island Complex.

 

CROKER ISLAND COMPLEX

Generalized geological map, showing sampling sites (large dots) of the Croker Island Complex, Lake Huron, Ontario. Older rocks are stippled and younger Paleozoic rocks are ruled. Total magnetic contours, with values in gammas, outline the circular plan of the complex. Map prepared from Card (1965). Inset map shows general location of the Croker Island Complex in Lake Huron. (PALMER, 1969)

MANITOU ISLAND COMPLEX

Aeromagnetic map of the Manitou Island complex (from ODM-GSC 1965c) Abstract, (Rowe 1954) Two concentric fenitic zones comprise the outer part of the complex : 1 ) an outer zone of quartz fenite as much as 400 feet wide ; and 2 ) an inner zone of aegirine-potassic feldspar fenite as much as 1,500 fee t wide .