3-D Magnetotelluric studies of Pre-Cambrian basement beneath southern Alberta

The Pre-Cambrian basement rocks beneath Alberta record the tectonic events that led to the assembly of Laurentia in the Proterozoic. Since these rocks are covered with younger sedimentary rocks, they must be investigated with geophysical methods. In the 1990s, these basement rocks were studied with a number of long-period magnetotelluric (MT) profiles collected by the Lithoprobe project. Dimensionality analysis of these data show that they appear to be two dimensional (2-D) in the period band 1-1000 s. However 2-D inversion models were unable to reproduce these MT data with a realistic resistivity model. The inversion models were very rough and characterized by many closely spaced conductors. Since the Lithoprobe data gave indications of 3-D resistivity structure, especially in the Archean Loverna block, additional MT data were collected by the University of Alberta from 2006-2010 using NIMS instruments. The goal was to develop an array that would constrain a fully 3-D model of crustal and upper mantle resistivity. The data at periods 1-10,000 s were inverted using a 3-D inversion algorithm. Comparisons between 2-D and 3-D inversions show that both models fit the measured MT data equally well. The 3-D model shows that the structure is dominated by an upper mantle conductor beneath the Loverna Block (the Loverna conductor). This conductor was previously imaged by the 2-D inversion of the Lithoprobe data. Our 3-D model shows that the Loverna conductor extends throughout the Archean Loverna block (part of the Hearne Domain) and is bounded to the south by a potential field anomaly known as the Vulcan Structure. Initial interpretations of the Vulcan Structure explained it as an intracontinental rift zone, while more recent studies show that it is more likely a north dipping subduction zone between two Archean blocks. This interpretation is supported by our 3-D resistivity model, which shows a good correlation between north dipping reflectors and the top of conductivity anomalies in the upper mantle. This new 3-D MT array shows an application where 2-D modelling is insufficient to properly explain the data, and sheds new light on the nature of Precambrian tectonic boundaries in southern Alberta.