ELECTRICAL-CONDUCTIVITY STRUCTURE OF THE PURCELL ANTICLINORIUM IN SOUTHEAST BRITISH-COLUMBIA AND NORTHWEST MONTANA

Magnetotelluric data from almost 200 sites were acquired by a commerci al contractor over the Precambrian Purcell Anticlinorium west of the R ocky Mountains of Canada and the United States. Fifteen east-west prof iles cross the anticlinorium between latitudes of 48 and 49.5 degrees N, and provide a grid suitable for a regional three-dimensional study of the electrical structure of predominantly the upper crust. The data show essentially a resistive uppermost crust, varying from 2 to 6 km in thickness, over a strongly conductive widespread electrical ''basem ent.'' The general electrical strike of this conductive basement is fo und to be N30 degrees W, which concurs with the surface geological tre nd of the region. The data from ail profiles were inverted one dimensi onally, and from two of the profiles two dimensionally, using differen t algorithms, to test the accuracy of the one-dimensional images. The main features found are (i) the sediments in the Upper and Middle Belt -Purcell strata are, in general, more conducting than those in the Low er Belt strata of the anticlinorium; (ii) the basement conductor appea rs to be strongest just to the south of the Canada - United States bor der, with resistivities of around 1 Ohm . m or less; (iii) the western part of the region is more conducting than the eastern part, suggesti ng that the sources of sedimentation on the two sides of the region we re different; (iv) the enhanced conductivity observed can be explained by the presence of mineralization (copper, etc.), rather than other g eophysical causes; (v) near the western edge of the Rocky Mountain tre nch the conductivity increases downwards from near the surface, and ne ar the eastern edge it increases downwards from a depth of about 2 km, suggesting the presence of the asymmetric mineralization in it; (vi) a few kilometres west of the Rocky Mountain trench in the resistive te rrain there exist two narrow, vertical, and significantly conductive e lectrical ''conduits''; and (vii) the pervasive conductive basement ex tends farther east and north than the present location of the copper s ulphide mines in northwestern Montana.