Fossil Subduction Systems and the Evolution of Plate Tectonics

地球目前正在通过全球板块构造(GPT)进行对流，这是迄今为止在所有已知行星中独一无二的一种体制。在GPT中，地球坚硬的外壳，即岩石圈，被分解成板块，然后下沉到地球内部，这个过程被称为俯冲。我们知道，GPT在生命的出现和持续发展中发挥了重要作用，并对金属作为矿床的焦点起到了控制作用。然而，我们不知道GPT是如何以及何时开始的，它是否从开始就演变了，如果是的话，又是如何演变的？要回答这些基本问题，作为GPT的主要表现形式的俯冲带是首要目标。然而，目前地球上所有活跃的俯冲带都在海平面以下，这使得直接研究它们的岩石记录变得困难。此外，即使是最古老的活动俯冲带也是相对年轻的，只在我们认为GPT活动的最后10%开始。作为另一种选择，我的研究计划旨在破译在陆地上作为洲际缝合带蛇绿岩保存下来的古老的、已灭绝的俯冲系统的地质记录。这些化石俯冲系统暴露在山脉中，因此完全可以对其岩石记录进行直接地质调查。这些档案还涵盖了地球历史的很大一部分，可能最早可以追溯到GPT开始。因此，这项研究计划的具体目标是限制导致俯冲带启动的过程，并调查这些过程是否自GPT开始以来演变。为了做到这一点，我的研究小组将研究三个年龄截然不同的俯冲化石系统：昂加瓦半岛20亿年前的努纳维克蛇绿岩，纽芬兰和魁北克阿巴拉契亚山脉4.85亿年前的蛇绿岩，以及青藏高原1.7亿至1.3亿年前的蛇绿岩。我的研究小组将利用我们最近的科学突破，限制最初下部板块埋藏的时间，以揭示俯冲开始的时间和模式。事实上，一些蛇绿岩被非常重要的岩石所覆盖，这些岩石被称为变质底岩，代表了俯冲带成核时第一批物质在地幔中沉没的残留物。结果表明，我们可以通过测定变质海底石榴石的生长年龄来确定俯冲开始的时间，并通过与上覆蛇绿岩年龄的对比来推断俯冲开始的机制。记录全球所有时代的化石俯冲系统中编码的导致俯冲开始的地球动力学环境和机制，将有助于限制地球上GPT构造开始的时间和原因，如果它已经演化，以及如何演化。我们需要回答这些问题，以便了解我们的星球是如何运作的，是什么让它如此独特，GPT与生命的联系有多紧密，以及是什么控制了矿产资源的分布。

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.