A magnetotelluric survey over the East Bull Lake gabbro‐anorthosite complex

Abstract

Tensor and scalar magnetotelluric and audiomagnetotelluric (MT/AMT) data have been acquired on a gabbro-anorthosite pluton in northern Ontario, about 100 km west of Sudbury. The scalar data from 102 sites were used to map resistivity contrasts associated with near-surface geology and overburden. Data from stations located away from possible geological inhomogeneities provided a structural interpretation which was subsequently confirmed by tensor MT soundings and by drilling. The scalar data also provided a means of interpreting the bedrock structure associated with a major fault that bisects the pluton. The apparent resistivity curves from the eight tensor stations ranged from nearly isotropic to strongly anisotropic with large shifts in apparent resistivity between the different locations. However, the measured impedances, both modulus and phase, strongly reflected the predominant northwest-southeast trend of faulting and diabase dykes in the region. Modelling indicates the presence of an anisotropic, subhorizontal conducting layer beginning at a depth of about 800 m. The direction of fluid transport within this layer is expected to be parallel to the structural trends and to the E polarization axis of the impedance tensor. The presence of this conductor has been confirmed by borehole resistivity measurements. A conductive zone in the lower crust has also been detected, commencing at a depth of about 18 km. Finally, the tensor measurements at the longest periods are consistent with an increase in conductivity in the upper mantle suggesting the existence of an electrically conducting asthenosphere in this part of the Canadian Shield. INTRODUCTIONA gabbro-anorthosite complex near East Bull Lake, approximately 100 km west of Sudbury, Ontario (Figure 1), is one of a small number of Precambrian crystalline intrusions or plutons selected by Atomic Energy of Canada, Limited, to test the capability of geotechnical methods for defining structure within the bedrock. Geological, hydrogeological, and geophysical methods need to be developed and evaluated for the purpose of identifying suitable rock bodies for the long-term storage of nuclear waste. Therefore these techniques must provide information relating to the uniformity and integrity of the rock mass. Is it homogeneous in its chemical composition and physical properties? To what degree has it been subjected to fracturing and deformation? What is the nature and distribution of any alteration 9. Of particular importance is the detection of subsurface fracture zones or permeable regions that might be capable of transporting fluids resulting in radionuclide migration. Electrical and electromagnetic methods are well suited to resolving this problem because the resistivity of a rock mass is strongly dependent on both its structural integrity and its fluid content. This paper describes the results obtained from tensor magnetotelluric (MT) and scalar audiomagnetotelluric (AMT) measurements made at the East Bull Lake test site during 1982 and 1983.Preliminary results from geop...

New constraints on the geometry of the Lac Bouchette gabbro – anorthosite from magnetotellurics and magnetics

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A controlled-source, time-domain electromagnetic survey over an upthrust section of Archean crust in the Kapuskasing Structural Zone

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