Fossil Subduction Systems and the Evolution of Plate Tectonics

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The Earth is currently convecting through Global Plate Tectonics (GPT), a regime that is so far unique amongst all known planetary bodies. In GPT, the Earth's rigid outer shell, the lithosphere, is broken into plates that sink in the planet's interior, a process called subduction. We know that GPT played an important role in the emergence and sustainability of life, and exerts a control on the foci of metals as ore deposits. Yet we don't know how and when GPT began, whether it evolved since its onset, and if so how? To answer such fundamental questions, subduction zones, the main manifestation of GPT, are prime targets. However, all of the Earth's currently active subduction zones are under sea level, making the direct study of their rock record difficult. Moreover, even the oldest active subduction zones is relatively young, having initiated only in the last 10% of the period we think GPT was active. As an alternative, my research program aims at deciphering the geological record of ancient, extinct subduction systems preserved onland as ophiolites in intercontinental suture zones. These fossil subduction systems are exposed in mountain ranges, and are thus fully accessible for direct geological investigation of their rock record. These archives also cover a large portion of the Earth's history, potentially reaching as far back as the onset of GPT. The specific objectives of this research program are thus to constrain the processes leading to the initiation of subduction zones and to investigate whether these processes evolved since the onset of GPT. In order to do so, my research group will study three fossil subduction systems of contrasting age: the 2.0 billion years old ophiolite of the Ungava Peninsula, Nunavik, the 485 million years old ophiolites of the Appalachian mountains in Newfoundland and Québec, and the 170 to 130 million years old ophiolites of the Tibetan Plateau. My research group will take advantage of our recent scientific breakthrough and constrain the timing of initial lower plate burial in order to reveal the timing and mode of subduction initiation. Indeed, some ophiolites are underlain by very important rocks termed metamorphic soles, representing remnants of the very first material that sank in the mantle when the subduction zone nucleated. We showed that we can constrain the timing of subduction initiation by dating the growth of garnet in these metamorphic soles, and infer a mechanism for subduction initiation by comparing with the age of the overlying ophiolite. Documenting the geodynamic settings and mechanisms leading to subduction initiation encoded in fossil subduction systems of all ages across the globe will contribute to constraining when and why GPT tectonics began on Earth, if it has evolved, and how. Such questions need to be answered in order for us to understand how our planets work, what makes it so unique, how strongly are GPT and life linked, and what controlled the distribution of mineral resources.