Metamorphism and ductile flow in the deep crust - controls and tectonic consequences

The behaviour of the lower continental crust controls the growth and decay of mountain belts. In particular, crustal thickening during mountain-building can lead to melting in the deep crust, causing a profound reduction in crustal strength. The melt-weakened crust can then flow outward or upward in response to the pressure gradients associated with mountain-building, and may be transported hundreds of kilometres laterally as mid-crustal channels or reworked into large ductile folds. Melting of continental crust that has been subducted to mantle depths (=100 km) may even assist its return to the surface. Although the deep crust is inaccessible to direct observation, geological evidence for these processes comes from metamorphic gneiss terranes in deeply eroded continental crust and recently exposed regions of younger mountain belts. These rocks contain mineral assemblages and structures characteristic of flow on scales of tens to hundreds of kilometres at high temperature and pressure, with textures indicating that melt was present during deformation. In the upper continental crust, transport of magma into cool near-surface regions can profoundly change the heat budget, strength, and behaviour of the surrounding rocks.***This proposal focuses on interactions between melting and mountain-building processes. To get around the problem of inaccessibility I use a combination of field observations from deeply eroded mountain belts and computer models that simulate the physics of mountain building. In western Norway, formerly subducted continental crust displays widespread evidence of melting, but there is continuing debate about whether melting started when the rocks were still at mantle depths or during their return to the surface. In central Ontario, the well-exposed roots of a billion-year-old mountain belt offer an ideal natural laboratory for examining the relationship between melting and orogenesis. In this case, melting began relatively early, leading to weakening and stacking of the crust and transport of dismembered high-pressure metamorphic rocks toward the surface. In the world's largest active mountain belt, the Himalayan-Tibetan system, melting of the middle crust is postulated to have driven lateral flow of weak crust as a mid-crustal channel for hundreds of kilometres toward the mountain front. This "channel flow" hypothesis is still controversial more than a decade after it was first proposed, but new concepts and methods for testing competing ideas have emerged that should help to resolve the debate. Rocks in southern Nova Scotia and northeastern Newfoundland preserve excellent evidence of how magma intrusions affect the upper crust. Using a novel combination of petrological and computer methods, the proposed research will resolve some persistent questions concerning cause vs. effect with respect to metamorphism, melting, and mountain-building processes.**