Three-dimensional resistivity structure of Southern Alberta, Canada: implications for Precambrian tectonics

The Precambrian basement rocks in southern Alberta are hidden beneath the Western Canada Sedimentary Basin, making studies of these rocks dependent on geophysical measurements. Magnetotelluric (MT) data were used to study the structure of these basement rocks through measurements of electrical resistivity. Long-period MT data collected in Southern Alberta during the Lithoprobe project were combined with new data to produce a grid of data that permitted a 3-D approach to data analysis. Dimensionality analysis suggested that data at periods less than 1000 s were relatively 2-D. However, 2-D inversion models of MT data in Alberta resulted in low resistivity features in the crust which moved dependant on the data included in the inversion. These features were previously attributed to crustal anisotropy. 3-D inversion yielded a resistivity model that fit the measured MT data and was well correlated with both the Precambrian domain boundaries and interpretations of other geophysical data. This MT data set defines a major upper-mantle conductor coincident with the Archean Loverna Block of the Hearne Domain. This anomaly is called the Loverna Conductor, and its southern boundary is defined by a pronounced increase in upper-mantle resistivity along the Vulcan Structure, which is an approximately 300-km-long linear potential field anomaly completely buried beneath the western Canada Sedimentary Basin. Since the lithosphere in this region was assembled ca. 1.9-1.8 Ga, the low resistivity anomaly in the upper mantle is not associated with recent tectonic activity. The Loverna Conductor was likely formed by the enrichment of the lithospheric mantle through subduction along the Vulcan Structure during the Proterozoic assembly of Laurentia. In particular, this model is consistent with recent interpretations which attribute the origin of the Vulcan Structure to collision along a north dipping subduction zone.