Publisher： John Wiley & Sons Inc
E-ISSN： 2156-2202|91|B7|7403-7416
ISSN： 0148-0227
Source： Journal Of Geophysical Research, Vol.91, Iss.B7, 1986-06, pp. : 7403-7416
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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.
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