IMAGING THE 3-D LITHOSPHERIC STRUCTURE OF CRATONS WITH MAGNETOTELLURIC DATA : EXAMPLES FROM WESTERN LAURENTIA (Invited Presentation)

Understanding the lithospheric structure and evolution of cratons requires a multidisciplinary approach. Magnetotelluric (MT) exploration complements seismic studies and can determine the electrical conductivity structure of the lithosphere and asthenosphere. Studies of a number of cratons worldwide have reported an increase in conductivity at depths that correspond to the lithosphere – asthenosphere boundary (LAB). They have also reported major conductors within the lithosphere. However the cause of these zones of high conductivity is unclear. The ability of MT to provide useful constraints on lithospheric structure has improved greatly in the last decade. The first area of improvement has come from the ability to invert MT data to give a 3-D model, in contrast to previous 2-D approaches. The second area of improvement has come from new laboratory experiments that have provided a better understanding of the mantle composition required to explain zones of high conductivity. In this abstract, new 3-D models for the Slave Craton and regions of the Hearne and Rae provinces beneath Alberta are presented.

The study of the Slave Craton used legacy data which was a combination of broadband and long-period MT data. A 3-D inversion of data at 92 stations produced a model that imaged the LAB at a depth of 200 - 220 km. A number of mantle conductors were imaging, including a major conductor beneath the Lac de Gras area that was reported in previous studies. The origin of this feature will be discussed in the context of recent laboratory studies. This mantle conductor is connected to the surface by a number of conductive pathways, some of which reach the surface close to areas of mapped mineralization.

In Alberta, legacy long-period MT data were collected on a set of profiles during the Lithoprobe project. Additional long-period data were collected by the University of Alberta to form a 3-D grid. Inversion of data at 405 stations produced the first 3-D conductivity model of the lithosphere for the entire province of Alberta. The LAB depth was in the range 150 - 300 km and a number of upper mantle conductors were imaged. Significantly no conductor was found beneath the Snowbird tectonic zone. The mantle of the southern Hearne province has a conductivity two orders of magnitude higher than the northern Hearne province in Alberta.