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An equivalent layer magnetization model obtained from inversion of long wavelength satellite magnetic anomaly data indicates a very magnetic source region centered in south central Kentucky. The magnetization maximum nearly coincides with a gravity high elongated north-south and extending into Tennessee. Previous refraction profiles suggest that the source of the gravity anomaly is a large mass of rock occupying much of the crustal thickness. The outline of the source delineated by gravity contours is also discernible in aeromagnetic anomaly patterns. Taken together, the geophysical data suggest a large, localized mass of intracrustal rock which is both dense and very magnetic. A simple magnetization/density model is given which accounts for the gravity and long wavelength aeromagnetic anomalies due to the body. We interpret it as a mafic plutonic complex, and several lines of evidence are consistent with a rift association. The body is, however, clearly related to the inferred position of the Grenville Front. It is bounded on the north by the fault zones of the 38th Parallel Lineament. The inferred mean magnetization (4 A/m) of the body is large, but not inconsistent with values reported by others for deep crustal bodies associated with long wavelength magnetic anomalies. Such magnetization levels can be ach'eved with :magnetic mineralogies produced by normal oxidation and.metamorph°c processes and enhanced by viscous build-up, especially in mafic rocks of alkaline character.
Long-wavelength aeromagnetic anomalies and deep crustal magnetization in Manitoba and Northwestern Ontario, Canada
. A new type of aeromagnetic anomaly map (a long-wavelength anom aly map with anomaly widths in the range 60 km. <A.< 4000 km) is presented for the area. It is believed that this group of anomalies represents a physically distinct field. This field shows considerable correlation with the broad features of deep crustal structure as derived from seismic sounding; linear relationships were found between the field and depths to the bottom of the crust and to the boundary between the upper and the lower crustal layers, as well as to the thickness of the lower crustal layer. A theoretical relationship connecting structure on magnetized layers to magnetic anomalies is given showing that the linear relationships are to be expected. It is shown that the lower crustal layer is the most likely source of the anomalies, with an intensity of magnetization of 5.3 X 10-3 emu/cc. It is also indicated that the upper crustal layer could also be the source, but that a shallow plate of magnetization could not explain the anomalies. Thus deep crustal magneti zation must (on all interpretations made in the present paper), be responsible for the long-wavelength anomalies. Also, these anomalies are strongly related to major features in surface geology.
The Delineation of Deep Crustal Magnetic Bodies from Total Field Aeromagnetic Anomalies
Studies were made of accurate, total field aeromagnetic data of excellent uniformity over an area in N. W. Ontario extending from 48°45′N to 53°N latitude and from 86°W to 94°W longitude. The rocks in this area are of early Precambrian age (Archaean era). The regional effects from the data were removed. The residual values were then analytically filtered to obtain the total field stripped of its near-surface, high amplitude components. Eighty-five individual anomalies were selected and analyzed to determine the horizontal dimensions and the depths to the tops and bottoms of the causative bodies and the magnitude and direction of total polarization vectors associated with the bodies. The bodies were assumed to be vertical rectangular prisms of arbitrary polarization. A brief summary of the method used is presented.The horizontal dimensions of the bodies vary from 0.8km (.5mile) to 5.1kms (3.2miles). The depths to the top of the bodies in the filtered aeromagnetic map are found to be in the range 4.8kms (3miles) to 8kms (5miles) from the surface of the ground. The depths to the bottom of the bodies are between 17.7kms (11miles) and 24kms (15miles) for most of the anomalies, with a mean depth of 20kms (12.4 miles), which is interpreted as the mean depth to the Curie point geotherm. The horizontal vectors of the polarizations are subparallel to the magnetic lineation of the filtered aeromagnetic map. An interesting and surprising result of this analysis is the detection of the presence of alternatingly normal and reverse polarization roughly in the East-West direction. The mean inclination and declination for normal polarizations are 67.6° and 258.7° respectively whereas those for reverse polarizations are -67.9° and 81.3° respectively. The consistency in the directions of magnetization in the two clusters for normal and reverse polarization indicates strongly the absence of any dominant component of magnetization parallel to the present field. On the basis that the magnetizations are permanent and the reversals are due to field or selfreversal, the calculated pole position for the rocks of Archaean era is found to be at a latitude of 31.5°N and a longitude of 136.9°W. The intensity of magnetization of the bodies lies within a wide range from .0006 to .349cgs emu with the average of .055cgs emu.