The Delineation of Deep Crustal Magnetic Bodies from Total Field Aeromagnetic Anomalies

Abstract

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.