Fossil Subduction Systems and the Evolution of Plate Tectonics

地球目前正通过全球板块构造（GPT）对流，这是迄今为止所有已知行星体中唯一的一种机制。在GPT中，地球坚硬的外壳，岩石圈，被分解成板块，沉入地球内部，这个过程被称为俯冲。我们知道，GPT在生命的出现和可持续性中发挥了重要作用，并对金属矿床的焦点施加了控制。然而，我们不知道GPT是如何以及何时开始的，它是否在发病后进化，如果是，又是如何进化的？为了回答这些基本问题，俯冲带——GPT的主要表现形式——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，这使得对其岩石记录的直接研究变得困难。此外，即使是最古老的活动俯冲带也相对年轻，仅在我们认为的GPT活动时期的最后10%才开始。作为替代方案，我的研究计划旨在破译以洲际缝合带蛇绿岩形式保存在陆地上的古代灭绝俯冲系统的地质记录。这些化石俯冲系统暴露在山脉中，因此可以对它们的岩石记录进行直接的地质调查。这些档案还涵盖了地球历史的很大一部分，可能可以追溯到GPT的开始。因此，这项研究计划的具体目标是限制导致俯冲带开始的过程，并调查这些过程是否自GPT开始以来演变。为此，我的研究小组将研究三个年龄对比的化石俯冲系统：Ungava半岛，Nunavik的20亿年前的蛇绿岩，纽芬兰和quacimbec的阿巴拉契亚山脉的4.85亿年前的蛇绿岩，以及青藏高原的1.7 - 1.3亿年前的蛇绿岩。我课题组将利用我们最近的科学突破，对下板块初始埋藏时间进行约束，以揭示俯冲开始的时间和方式。事实上，一些蛇绿岩下面是非常重要的岩石，称为变质底，代表了俯冲带成核时第一批沉入地幔的物质的残余物。通过对这些变质底中石榴石生长的定年，我们可以限定俯冲开始的时间；通过与上覆蛇绿岩年龄的比较，我们可以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导致俯冲起始的地球动力学背景和机制，将有助于限制GPT构造何时以及为何在地球上开始，如果它已经进化，以及如何进化。这些问题需要得到回答，以便我们了解我们的行星是如何运作的，是什么使它如此独特，GPT和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.