Using electromagnetic geophysics to understand global tectonics and resource distribution

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Plate tectonics has shaped the surface of the Earth over the course of geological history. Many important processes occur at the boundaries where tectonic plates move relative to each other. Subduction zones are formed when two plates converge and an oceanic plate is forced downwards into the mantle. Subduction zones are important because they transport water into the mantle, generate molten rock that forms volcanoes, cause large earthquakes and also form important types of mineral deposit. A subduction zone is present in Western Canada beneath Vancouver Island and is responsible for both volcano and earthquake hazards. Subduction zones vary over time and sometimes the down going plate flattens out, stopping the volcanoes from erupting and changing the pattern of deformation. This behaviour occurs today beneath the Andes mountains where the Nazca Plate subducts beneath the South American Plate. In Peru a flat slab is present and grows southward. In Colombia, a previously flat slab has recently steepened, with volcanoes becoming active once again. This research program will investigate subduction zones by using geophysical imaging to map the location of fluids such as water and partial melt. Mapping fluid distribution within the Earth requires the use of geophysical data measured at the surface to determine the properties of rocks at depth. Seismic imaging uses acoustic waves but is not effective at imaging low concentrations of fluids. However, fluids dramatically change the electrical resistivity of a rock and this property can be determined with magnetotellurics (MT) - a geophysical exploration method that uses low frequency radio waves to image the subsurface to depths in excess of 200 km. In this proposal, MT will be used to study the processes that occur in subduction zones in Canada and South America. Measurements of the natural radio waves will be converted into 3-D images of the electrical resistivity of the Earth. From these 3-D models the distribution of water and molten rock will be determined. In Canada this will improve understanding of where water is present in the subduction zone beneath Vancouver Island and where the molten rock originates that is erupted in the volcanoes of the Garibaldi Belt. In Peru and Columbia the 3-D models will show how Earth structure changes when slabs flatten and steepen. In related research, the MT method will be used to investigate the tectonic events that occurred in the past as the North American continent was formed. The resulting models will be used to investigate how these past events formed mineral deposits. This process is being used to improve the methods used in mineral exploration by searching not just for mineral deposits, but regions of past fluid flow in the mineral systems approach to exploration. A major emphasis of the research program will be to train undergraduate and graduate students who will be qualified to work in academic research, hazard reduction and mineral exploration.