Advancements in U-Pb dating of deep crustal structures and magmatic/meteoritic events in relation to biotic evolution in earth history

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Precise uranium-lead analyses of minute (microgram) quantities of the mineral 'zircon' that crystallize during formation, reheating, or ductile flow events provide detailed scenarios of the evolution of the Earth's crust during periods of mountain building. When enormous continental plates collide, this produces over-thickened stacks of crustal rocks such that the lower part of the thrust stack is heated from below and softened and slides further inland. This must have happened under the Rocky Mountains as this thick pile of sedimentary material, once at the ocean bottom, now sits above an older continent. To understand processes that took place along the interface, 10 - 20 km below the Earth's surface where we cannot see, we must find a very old mountain root where the mountains have since been removed by erosion. The Grenville Province, well exposed along Georgian Bay, Ontario, is such a root. Here, geologists who study the rock record observe both the old, cold solid lower plate and the soft, hot overlying rocks that moved overtop during a great plate collision more than a billion years ago. In short, we discover the nature of the processes that put the Rockies in their place.   The evolution of life on Earth has been subject to sudden drastic losses of species over brief time intervals that are recorded in the rock record. These mass extinction events can be correlated with meteorite impacts and major volcanic eruptions. Using high resolution U-Pb dating methods we can accurately pinpoint the time of these meteoritic/volcanic events and establish or refute synchronicity with the extinction events by dating volcanic ash beds that occur near the extinction horizons.   My research also involves new strategies in the field to select rocks in complex gneiss terranes that give age information directly tied to once active tectonic processes. I am also developing methods in the laboratory to better resolve complex, multi-stage, zircon populations that occur in metamorphic rocks, and to make U-Pb ages more precise.