Timing and mechanisms of the exhumation of deeply buried crust: The genesis of major mountain belts

As little as 50 years ago, the idea that the surface of the earth was in constant motion was considered ludicrous by many earth scientists. Now it would be difficult to find a geologist who does not believe in the theory of moving plates. The Earth's crust forms the top part of the plates and moves with them. There are two types of crust: the crust beneath the ocean basins (oceanic crust) and the crust that makes up the continents (continental crust). As the plates are carried by the motion of the underlying mantle over the face of the earth, new oceans are born where continents split apart and old oceans die where continents collide. Throughout this history, the buoyant continents have usually remained at the earth's surface, whereas the ocean floor has been carried down (subducted) deep into the Earth's interior. But rocks in the high mountain chains of the Alps and Himalayas, in the deserts of Oman, and the fjords of Western Norway record another story - an extraordinary sequence of events that suggest an altogether different fate for both continental and oceanic rocks. In these regions, the discovery of rare and exotic minerals that have crystallised within slabs of continental rocks indicate that these rocks formed at very high pressures - clear evidence for the subduction of continental material. And the preservation of slices of ancient ocean floor which were forced up onto continental rocks indicates that not all oceanic crustal material is destroyed. The extraordinary combination of continental crust subduction and oceanic crust preservation only occurs when mountain belts start to rise: a period of time which is important not only to geologists interested in mountain building processes, but also to scientists who study how global climate has changed over time. As mountains rise, they form a barrier for masses of moisture-laden air, and create a rain shadow behind them. The rate at which mountain belts rise, and the rate at which they affected local and global climate, therefore has important implications for modelling the present-day rate of climate change. This research aims to analyse, in precise detail, how long it took for slabs of deeply subducted continental crust to rise back up to the surface, and when these slabs were pushed up next to the rocks they are found associated with today. New techniques of dating microscopic fragments of certain minerals can provide timescales for these events. The chemical composition of the minerals also yields clues about the temperatures and pressures under which these rocks crystallised, and their temperature-pressure history as they rose back through the crust. This information will help us understand in greater detail how quickly mountain belts form and start to rise. We can then compare these results with other scientists' work on how quickly mountain belts erode in order to understand the lag-time between topographic rise and local or global climate change.