Metamorphic architecture of orogenic belts

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Most of the Earth's continental crust consists of metamorphic rocks formed deep within ancient and modern mountain belts by the heat and pressure generated during plate collisions. Studies of metamorphic rocks therefore help us to understand how continents and mountain belts have evolved through geologic time. Metamorphism cannot be observed directly, but computer simulations of heating and deformation of the Earth's crust and mantle during plate collisions can be used to investigate metamorphic processes indirectly. In the proposed research, predictions of computer models will be tested against data from metamorphic belts of different age. Results show that when mountain belts get very large and hot, weak lower crust can flow outward from the centre of the mountain belt, forming a sub-surface 'channel' that can be exposed by erosion on the flanks of the system. Patterns of metamorphism and deformation predicted by the computer models closely resemble observations from the Himalayas, and similar concepts have now been extended to some ancient mountain belts. In the Muskoka-Georgian Bay area of Ontario, metamorphic rocks formed at great depths indicate that a vast mountain range (Grenville orogen) occupied the southeastern margin of the Canadian Shield about 1 billion years ago. The style of ductile flow in these rocks can be accounted for by models in which crustal strength varies across the system. Over the past 25 years, metamorphic data (eg. microdiamonds in continental rocks) have demonstrated that continental crust can be subducted during plate collisions; however, it is still unclear how the subducted rocks are returned to the Earth's surface. Our computer models show that a combination of buoyancy and weakening of subducted crust can lead to its rapid expulsion from the subduction zone, with results that match observations from some natural examples. This work will be extended to investigate formerly subducted crustal rocks in Norway, where this type of metamorphism was first described. This project will contribute to a better understanding of how mountain belts are created and destroyed, and thus how the ancient mountains of the Canadian Shield once grew and then vanished.