Bringing lower crustal rocks closer to the surface: testing exhumation models in the Grenville Province

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It is fascinating to think that the core-zone of mountain belts generally exposes large areas of rocks formerly located very deep in the crust (i.e.20-50 km). Understanding how these rocks were brought towards the surface, a phenomenon termed "exhumation", is one of the most challenging problem faced by geologists. It has traditionally been ascribed to extension that commonly follows relaxation of compressive stresses at the end of a mountain building phase, but a growing body of evidences indicate that rocks are transported towards the surface during the compressive stages as well. Advanced numerical models even point to complex coupling between the flow of hot and ductile rocks located deep in the core of a growing mountain belt and climate-driven surface erosion. The archetypal of this later model, commonly referred to as "channel flow", is the Himalayas, for which numerous studies have indicated a relatively good match between modeled and observed features. **My research program focuses on the much more accessible Laurentians in Québec where the vestige of an ancient Himalayan-scale mountain belt, the Grenville Province, is exposed. Investigating this one billion years old mountain belt is an excellent complement for the Himalayan studies, because it has obviously completed its mountain building and vanishing phases and it exposes a much larger areas of rocks formerly located in the deep crust. The backbone of the program is a diagnostic test of exhumation models adapted to the Grenville Province from the generic test published during my doctoral studies. The scientific approach of my team, led by graduate students shouldered by undergraduate students, Canadian and international collaborators, provincial surveys, is to: 1) conduct detailed geological mapping and structural analysis of key areas; 2) date the main structures; 3) determine pressure-temperature-time paths followed by rocks across studied transects. The current research program is the first conducted in the Grenville Province to integrate geophysics, field mapping, and advanced structural analyses, with cutting-edge laboratory analyses such as phase equilibria modeling, numerical modeling of diffusion in minerals, and high-resolution U-Pb geochronology. The improved understanding of the tectonic evolution of the Grenville Province will allow for testing the validity of numerical models and, in turn, provides information on parameters needed to improve them. Along the way, publications of the advancement methods required to complete the various projects will benefit to all researchers working in high-grade rocks by providing them an efficient methodology to study these complex rocks and by giving insights into deformation and metamorphic processes taking place in the deep crust. **Zooming out a bit, it is important to realize that although it has traditionally not been targeted for mineral exploration, the Grenville Province do contains interesting mineral deposits, notably in metals identified as strategic because of the urgency to meet the exponentially increasing World needs. This is most likely because they sit in complexly deformed and metamorphosed rocks, and thus requires a strategic and efficient exploration strategy, which is impossible without the kind of solid scientific foundation that this research program aims at building. Finally, the most significant impact of the proposed research program will be the formation of a future generation of high-quality field geologists, who are becoming a rarity in this world of technological advancements, but without whom such advancements would be impossible, for they provide its essential raw constituents.