Plate tectonics is the framework through which we understand the large-scale Phanerozoic history of Earth. The question of when and how plate tectonics began remains the subject of debate, in no small part because through subduction, plate tectonics destroys much of the evidence of its earlier activity. Estimates of the onset of plate tectonics vary from the Hadean (Hopkins et al., 2008), to the Archean (Brown, 2006), to the Neoproterozoic (Stern 2005, 2008). There is no rock record from the Hadean, and only a limited rock record from the Archean. Thus, it is unlikely that we will determine whether any deformation recorded during this time period was part of a globally connected plate boundary system or a regional, transient event. Spherule beds are preserved within Archean age rocks in the Barberton greenstone belt, South Africa (Lowe and Byerly, 1986; Lowe et al., 1989) and the Pilbara craton, Australia (Glikson et al., 2016). Archean spherule beds formed from the distal ejecta of large bolide impacts. These beds contain important clues regarding their formation—the thickness of the beds can be used to estimate the size of the impactor (Johnson and Melosh, 2012). Lowe et al. (2014) described four additional spherule beds and placed their formation at the same time as the first major episode of orogeny and crustal deformation in the Barberton greenstone belt (3.26–3.23 Ga). Lowe et al. further suggested that these impacts may have been the trigger that initiated the modern plate tectonic regime. A new contribution by O’Neill et al. (2020, page 174 in this issue) uses the characteristics of these recently described spherule beds to constrain the size and velocity of the impactors that formed them, extending the Archean impact record. They then use the Archean impact record as input to geodynamic models to test Lowe et al.’s hypothesis that these impacts could have initiated a modern style of plate tectonics. Lowe et al. (2014) were not the first to postulate that the Archean greenstone belts record plate tectonic activity. There are multiple lines of evidence that plate tectonics may have been operating in the Archean, including apparent polar wander curves (O’Neill et al., 2007), felsic volcanism consistent with melting of a waterrich source, and isotopic systematics similar to modern-day arcs (Hugh Smithies et al., 2018; O’Neill et al., 2018). The absence of clearly identified fold-and-thrust belts, tectonic mélanges, or ophiolites in the Archaean rock record casts doubts on the subduction interpretation (e.g., Stern, 2005; Moyen and van Hunen, 2012). Geodynamic calculations have become an important hypothesis-testing tool when combined with the Precambrian geological record (c.f., van Hunen and Moyen, 2012: O’Neill et al., 2018; Stern and Gerya, 2018). These models are based on basic laws of physics, including the conservation of mass and energy, as well as Newton’s second law, sometimes misleadingly described as the conservation of momentum. To reduce the number of variables and create a set of equations that can be solved, a set of constitutive equations are required. For mantle convection, stress and strain rate are related through the viscosity. The viscosity of silicate minerals depends on temperature, pressure, composition, stress, grain size, water content, and history (c.f., King, 2016). Our understanding of viscosity is limited, in part due to the challenge of measuring the viscosity of silicate minerals at high pressures and temperatures, and the reality that the strain-rates achievable in such laboratory measurements must be extrapolated by orders of magnitude to mantle conditions. Geodynamic calculations are built upon solid physical principles; the calculations are limited in so far as our understanding of mantle viscosity is limited, and the appropriateness of the initial and boundary condition choices. Several modes of surface behavior are recognized in geodynamic models. In stagnant-lid convection, the lithosphere is immobile with surface heat flow limited by conduction. In mobile-lid convection, the lithosphere is part of the convecting system, cooling as it advects along the surface, and sinking back into the warmer mantle. All other factors being equal, a stagnantlid planet will have a hotter mantle than a mobile-lid planet. The transition from stagnant-lid to mobile-lid tectonics in geodynamic modeling has enriched our understanding of plate tectonics on Earth. As the mantle becomes hotter than at present day, many models show that platelike behavior becomes episodic, with alternating periods of mobile-lid and stagnant-lid behavior (c.f., van Hunen and Moyen, 2012). An implication of these models is that evidence for plate tectonics may appear and disappear in the geological record, and subduction may repeatedly fail (O’Neill et al., 2018). The primary force driving plate tectonics is the negative buoyancy in subducted slabs (Forsyth and Uyeda, 1975). It is unclear what additional processes could produce the large forces necessary to initiate subduction on a pre–plate tectonic planet. To identify and test candidate processes, researchers have modeled the arrival of large plumes under the lithosphere (Gerya et al., 2015), magmatic weakening and volcanic loading (Moore and Webb, 2013; Nakagawa and Tackley, 2014), and bolide impacts (O’Neill et al., 2017). In many cases, subduction is transient, with the mantle reverting to a stagnant-lid state after a relatively brief time interval. In a previous study, O’Neill et al. (2017) examined the effect of bolide impacts on a Hadean Earth, finding that the thermal anomalies produced by extremely large impacting bolides (>∼700 km in diameter) induce mantle upwellings that are capable of driving transient subduction events. These transient events terminate because the hotter Hadean mantle had a lower viscosity than the present-day mantle, reducing the coupling between the mantle and lithosphere (O’Neill et al., 2007), and the higher temperatures weakened the core of the subducting slab, resulting in necking and breakoff that reduces the slab pull force on the plate (van Hunen and Moyen, 2012). The present O’Neill et al. study (2020) addresses whether the estimated size and frequency of Mesoarchean impacts could have initiated subduction, and whether these events could have developed into a globally connected plate boundary network that continued without interruption

中文翻译：

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撞击会影响全球构造吗？

板块构造是我们了解地球大尺度显生宙历史的框架。板块构造何时以及如何开始的问题仍然是争论的主题，这在很大程度上是因为通过俯冲，板块构造破坏了其早期活动的大部分证据。对板块构造开始的估计从冥界（Hopkins et al., 2008）到太古代（Brown, 2006），再到新元古代（Stern 2005, 2008）。冥界没有岩石记录，太古代只有有限的岩石记录。因此，我们不太可能确定在此期间记录的任何变形是全球连接的板块边界系统的一部分还是区域性瞬态事件。球体床保存在南非巴伯顿绿岩带的太古代岩石中（Lowe 和 Byerly，1986 年；Lowe 等人，1989 年）和澳大利亚皮尔巴拉克拉通（Glikson 等人，2016 年）。太古宙小球床由大型火流星撞击的远端抛射物形成。这些床包含有关其形成的重要线索——床的厚度可用于估计撞击器的大小（Johnson 和 Melosh，2012）。洛等人。(2014) 描述了四个额外的球粒床，并将它们的形成与巴伯顿绿岩带 (3.26–3.23 Ga) 中第一次造山运动和地壳变形的主要事件同时进行。洛等人。进一步表明，这些影响可能是引发现代板块构造制度的触发因素。奥尼尔等人的新贡献。(2020, 本期第 174 页）使用这些最近描述的球粒床的特征来限制形成它们的撞击物的大小和速度，从而扩展了太古代撞击记录。然后，他们使用太古代撞击记录作为地球动力学模型的输入来测试 Lowe 等人的假设，即这些撞击可能引发了现代板块构造。洛等人。(2014) 并不是第一个假设太古代绿岩带记录板块构造活动的人。有多种证据表明板块构造可能在太古代发生过，包括明显的极地漂移曲线（O'Neill 等，2007）、与富水源融化一致的长英质火山作用，以及类似于现代的同位素系统学——日弧（Hugh Smithies 等人，2018 年；O'Neill 等人，2018 年）。太古代岩石记录中没有明确识别的褶皱冲断带、构造混杂岩或蛇绿岩，这使人们对俯冲解释产生怀疑（例如，Stern，2005；Moyen 和 van Hunen，2012）。当与前寒武纪地质记录相结合时，地球动力学计算已成为重要的假设检验工具（参见，van Hunen 和 Moyen，2012 年：O'Neill 等人，2018 年；Stern 和 Gerya，2018 年）。这些模型基于物理基本定律，包括质量和能量守恒，以及牛顿第二定律，有时会误导性地将其描述为动量守恒。为了减少变量的数量并创建一组可以求解的方程，需要一组本构方程。对于地幔对流，应力和应变率通过粘度相关。硅酸盐矿物的粘度取决于温度、压力、成分、应力、粒度、含水量和历史（cf, King, 2016）。我们对粘度的理解是有限的，部分原因是在高压和高温下测量硅酸盐矿物的粘度存在挑战，而且在这种实验室测量中可实现的应变率必须按数量级外推到地幔条件。地球动力学计算建立在坚实的物理原理之上；就我们对地幔粘度的理解以及初始和边界条件选择的适当性而言，计算是有限的。在地球动力学模型中识别出几种表面行为模式。在停滞盖对流中，岩石圈是不动的，表面热流受传导限制。在移动盖对流中，岩石圈是对流系统的一部分，在沿地表平流时冷却，然后沉回到较温暖的地幔中。在所有其他因素相同的情况下，停滞行星的地幔将比移动盖行星更热。地球动力学建模中从静止盖构造到移动盖构造的转变丰富了我们对地球板块构造的理解。随着地幔变得比现在更热，许多模型表明板状行为变得偶发，活动盖和停滞盖行为交替出现（参见 van Hunen 和 Moyen，2012）。这些模型的一个含义是，板块构造的证据可能会在地质记录中出现和消失，并且俯冲可能会反复失败（O'Neill 等，2018）。驱动板块构造的主要力量是俯冲板块中的负浮力（Forsyth 和 Uyeda，1975）。目前尚不清楚哪些额外的过程可以产生在板块前构造行星上引发俯冲所需的巨大力量。为了识别和测试候选过程，研究人员模拟了岩石圈下大羽流的到达（Gerya 等人，2015 年）、岩浆减弱和火山载荷（Moore 和 Webb，2013 年；Nakagawa 和 Tackley，2014 年）以及火流星撞击（奥尼尔等人，2017 年）。在许多情况下，俯冲是短暂的，地幔在相对较短的时间间隔后恢复到停滞状态。在之前的一项研究中，奥尼尔等人。(2017) 检查了火流星撞击冥王星地球的影响，发现由极大撞击火流星产生的热异常 (> 直径约 700 公里）诱发地幔上涌，能够驱动瞬时俯冲事件。这些瞬态事件终止是因为较热的冥王地幔的粘度低于现今的地幔，减少了地幔和岩石圈之间的耦合（O'Neill 等，2007），并且较高的温度削弱了俯冲板块的核心，导致颈缩和折断，从而降低了板上的板坯拉力（van Hunen 和 Moyen，2012 年）。现在的奥尼尔等人。研究（2020 年）探讨了中太古代影响的估计规模和频率是否会引发俯冲，以及这些事件是否会发展成一个全球连接的板块边界网络，不间断地继续 这些瞬态事件终止是因为较热的冥王地幔的粘度低于现今的地幔，减少了地幔和岩石圈之间的耦合（O'Neill 等，2007），并且较高的温度削弱了俯冲板块的核心，导致颈缩和折断，从而降低了板上的板坯拉力（van Hunen 和 Moyen，2012 年）。现在的奥尼尔等人。研究（2020 年）探讨了中太古代影响的估计规模和频率是否会引发俯冲，以及这些事件是否会发展成一个全球连接的板块边界网络，不间断地继续 这些瞬态事件终止是因为较热的冥王地幔的粘度低于现今的地幔，减少了地幔和岩石圈之间的耦合（O'Neill 等，2007），并且较高的温度削弱了俯冲板块的核心，导致颈缩和折断，从而降低了板上的板坯拉力（van Hunen 和 Moyen，2012 年）。现在的奥尼尔等人。研究（2020 年）探讨了中太古代影响的估计规模和频率是否会引发俯冲，以及这些事件是否会发展成一个全球连接的板块边界网络，不间断地继续 较高的温度削弱了俯冲板块的核心，导致颈缩和断裂，从而降低板块上的板块拉力（van Huen and Moyen，2012）。现在的奥尼尔等人。研究（2020 年）探讨了中太古代影响的估计规模和频率是否会引发俯冲，以及这些事件是否会发展成一个全球连接的板块边界网络，不间断地继续 较高的温度削弱了俯冲板块的核心，导致颈缩和断裂，从而降低板块上的板块拉力（van Huen and Moyen，2012）。现在的奥尼尔等人。研究（2020 年）探讨了中太古代影响的估计规模和频率是否会引发俯冲，以及这些事件是否会发展成一个全球连接的板块边界网络，不间断地继续