Earth’s plate tectonic regime is driven by subduction, but how do brand new subduction zones initiate in the first place? To get one plate to sink beneath another, deformation has to localize: rigid oceanic plates must weaken along a narrow plate interface. Localization involves feedbacks between temperature, fluid/mass transfer, metamorphism, and rheology, but these feedbacks are difficult to study directly because in-progress subduction initiation is rare on Earth today. My approach is to use geologic records preserved in large-slab ophiolites (e.g., Oman, Newfoundland, New Caledonia etc.). Ophiolites commonly form in proto-forearc settings and therefore provide the upper plate’s perspective. Where they are well-preserved, ophiolites also contain remnants of the down-going plate: hundreds-of-meters thick ductile shear zones made of metamorphosed oceanic crust (± sediments) accreted beneath the ophiolite mantle—metamorphic soles. Soles are dynamic archives of how heat, fluids, reactions and deformation interacted as the plate interface was born. In this talk, I use the Ordovician Mont Albert Ophiolite (Gaspé Peninsula, Québec, Canada) and it’s remarkably exposed metamorphic sole to test two commonly-cited mechanisms for lithosphere-scale strain localization: (1) serpentinization and (2) partial melting. I will argue that—at least for Mont Albert—neither mechanism explains the observations, and therefore did not substantially localize strain. The payoff, however, is that investigating each of these ideas showcases the power of integrating field structures, geochemistry, and electron microscopy. This approach illuminates yet less-appreciated feedbacks between temperature evolution, mass transfer, and strength that do promote weakening. I’ll close by speculating how lessons from “well-preserved” Phanerozoic soles may shed new light on how subduction may have initiated on a hotter, more primitive Earth.
Venue
Seminar rooms
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