Early evolution of the middle crust in continental collisions

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Large-hot orogens form following continent-continent collision as a result of significant thickening of the lithosphere and melt weakening of the middle crust (15-40 km depth). Melt weakening facilitates lateral spreading at depth, controls the topographic relief and climate at surface, affects magma and fluid migration in the crust, and influence orogenic architecture. The transition from thin, small and cold orogenic wedges dominated by mechanical deformation processes to thick, large and hot orogenic plateaus dominated by thermomechanical deformation processes is poorly documented and forms a major knowledge gap in tectonic research. The development of the melt-weakened middle crust during the early stages of orogenesis can be targeted in specific structural settings and is the primary objective of this proposal. Mid-crustal rocks that are exhumed to the surface during or after continent-continent collisions preserve crucial constraints on the state of the middle crust during orogenesis. Numerical models based on rocks exposed in the present-day hinterland (main mountain belt) of orogens provide much of our understanding of mid-crustal thermal and mechanical properties. However, these rocks rarely preserve any information on the initial stages of continental collision because they are generally overprinted by subsequent metamorphic events. In this proposal, I hypothesize that far-travelled, high metamorphic grade external nappes transported closer to the foreland (front) are exhumed early during the collision evolution, and that such rocks are more likely to record the early stages of continental collision. Two iconic large-hot orogens are targeted by this proposal: the well-characterized active Himalaya of Nepal and India, where early exhumation of mid-crustal units in the foreland is known, and the old Grenville (i.e. the current Laurentides in Québec), where, in comparison, scarce modern petrochronologic and metamorphic studies have been conducted. The Grenville (> 1 billion years old) offers the opportunity to compare Himalayan geodynamic models to a continental collision that evolved to completion. HQP-led projects, including fieldwork and newly available, multifaceted cutting-edge laboratory investigations, will reconstruct the pressure-temperature-time-deformation path and characterize the nature of early-stage metamorphic rocks involved in these two orogens. This research program will provide robust constraints on mid-crustal properties (temperature, pressure, structure) following initial collision. This is fundamental to understand how convergence is accommodated at continent-continent tectonic plate boundaries, how material is transported within the crust, and how the middle crust contributes to shape the earth surface. These refinements to plate tectonic theory provide the necessary foundation for a critical examination of numerical models of continental collision and feedback effects between geological and surface processes.